This blog is a notebook of thoughts on gene expression in avocado, avocado germplasm, avocado climatic adaptation, and the future of avocado in its native forests, orchards, and home gardens.
This format helps me to understand the operation of genes in avocado, and, to a limited extent gives me an insight into the fast evolving techniques for locating genes and inferring their purpose and interactions.
Wild and domestic genetic resources are disappearing at the same time as ‘gene jockeying’ makes important advances. Can gene editing and insertion of genes from other species simply ‘re-mould’ avocado without recourse to conserved germplasm?
Or is this concept flawed by presumptive thinking?
As this blog is written by an amateur, it is bound to have errors. Errors of fact, errors of understanding, errors of ommission.
Corrections are welcomed!
Opinions are my own, and therefore eclectic and idiosyncratic.
Some avocado fruit, such as Mexicola, Topa Topa, and Rincon, have smooth skin. The odd one, such as Lula, has ‘almost’ smooth skin. Most other have skin with varying degrees of ‘roughness’.
Botanically, the bumps on avocado skin are tubercles, as are the bumps on the skin of other fruit, such as lychee and gourds. Avocado tubercles are usually described by terms such as warty, rough, bumpy, or pebbled (further qualified by words such as low, high, slightly, somewhat, moderately, highly, very). The diverse terms used to describe avocado skin surface topography struggle to include two major components – the number of tubercles per unit area of skin (tubercle density) and height/width (‘prominence’). Minor components of skin topography include coalescing of tubercles into ridges; apparently non-tuberculate sharply defined creases; and disorganized reticulation.
The question are: what are tubercles? Why do they exist? How do they arise? This article draws together investigations in other fruit (mainly) which provide, to a degree, plausible initial explanations.
The article was prompted by this Twitter post by Sonia Rios, Subtropical Horticulture Farm Advisor, University of California, showing a ‘sectorial chimera’ in a Hass avocado fruit. A mutation at a very early stage of fruit development causes a zone or sector of ‘smooth’ skin within the otherwise normal ‘rough’ Hass skin.
Chimeras in avocados– organisms that are composed of cells of more than one genotype… you can tell by the smooth embossed section of the fruit.🥑 pic.twitter.com/2JP10C7cnz
smooth skin section on the left side of a normal ‘bumpy’ Hass fruit.
As most mutations are ‘loss of function’, blocking or diluting the ‘proper’ operation of a gene, it is fair to suppose this mutated area of skin is due to the direct or indirect blocking of some signal to the skin that ordinarily results in the warty tubercles. Mutations usually happen in the cells of the actively dividing meristem tissues. The various causes of mutations causing chimeras and bud sports are extensively listed in Foster and Aranzana (2018) .
Chimeras in the avocado Skin
According to Foster and Aranzana (2018), angiosperms meristems consist of (generally) 2 outer layers of cells, the tunica, draped over an inner layer, the corpus. The first, outer, layer of the tunica gives rise to the epidermis. The second layer gives rise to sub-epidermal tissues.
Foster and Aranzana (2018) propose color mutations in one sector of a flower are due to a mutation in gene expression in a mericlinal (pole to pole) sectorial chimera affecting either one or both of the tunicate layers. This is illustrated by the pink petal in the advanced flower bud of a white azalea cultivar which serendipitously appeared when I was writing this article. Both the solid color flower (on the left) and the petal mutant are probable L1 mutants ( Foster and Aranzana, 2018).
Mericlinal sectorial chimara in a white azalea.
This principle may also apply a gene (or genes) responsible for the smooth skin sectorial mericlinal chimera in the above photo. This might imply that ‘pebbled’, ‘bumpy’, ‘warty’ or ‘rough’ skin topography in Hass and other avocados with various degrees of tubercule expression is controlled by a single gene, perhaps additively expressed in the outer tunica.
But, anatomically, what is a tubercle anyway, and how does it arise? This is a suprisingly complex question to find a plausible answer to.
Does stomata patterning cause or coincide with tubercle formation?
According to Schroeder (1950), in avocado, the epidermis is made up of a single layer of cells. A second layer, the hypodermis, either one or two cells thick, is usually present immediately under the epidermis. These cells have a natural polymer, cutin, laid down in the cell walls. Under that are parenchyma cells. Stomata are formed by a pair of crescent-shaped epidermal cells (the guard cells) flanking an open pore in the epidermis.
In avocado, stomata have immediately underneath them a mass of loosely packed cells with abundant intercellular spaces. This is believed to make gas exchange maximally efficient.
Schroeder notes that in round relatively smooth skinned avocados the uniform expansion of the skin surface of the growing fruit results in relatively evenly spaced stomata. But in rough skinned genotypes the stomata are “restricted to the elevated mounds of tissues…varieties such as Hass and Dickinson have stomata concentrated in groups on the elevated surfaces of the rind”.
Curiously, Everett et al (2001) – contra Schroeder – found in ‘firm’ green avocados that lenticels (identifiable “as small yellow dots”) “form all over the fruit surface, not just on the lumps.” I examined new season spring-set Hass, Fuerte, 3 seedlings of unknown parentage and a ‘smooth’ skin cultivar in late autumn and found stomata (yellow or cream dots) co-incide to the highest degree with elevated skin relief. If we use a ‘mountain and valley’ analogy, stomata (presumably aka lenticels) are almost entirely confined to mountain slopes and are relatively rare on valley floors.
stomata grouped on Hass tubercles
But in immature fruit there is almost no ‘valley floor space’ and the toe of one ‘mountain’ abuts the toe of adjacent ‘mountains’. This changes as the fruit increases in size. The valley floors that ’emerge’ are easier to observe. (This is best seen in natural light where the shadows cast by surface relief across the fruit surface make topography easy to observe.)
Late-hung last season Hass fruit observed at this stage have lower relief, that is, ‘flattened’ mountains and wider valley. Creasing, unrelated to stomata patterning, becomes either more developed, or more apparent.
The case for tubercle formation being related to tissues associated with stomata looks promising. So what is the importance of clustered stomata?
In leaves, Dow et al (2014) define the “proper spacing” of stomata (on Arabidopsis leaves) as less than 5% of stomata in clusters. This spacing (scaling with increasing stomata number) achieves maximum water vapor conductance, maximum gas exchange and consequent net carbon assimilation, as well as maximal stomatal responsiveness to increases in CO2 concentration.
In contrast, the authors found that genotypes with greater than 19% of stomata in clusters had (> 19% clustering) did not. Genotypes with clustering also had reduced net carbon assimilation and “impaired” stomatal response to increasing CO2 concentrations, and impaired stomatal responses. Water use efficiency was largely unaffected by clustering.
A mutation in a gene or genes controlling the stomatal patterning process could cause clustering.The clustering phenomenon occurs when the protodermal cells in the epidermis which produce stomatal guard cells fail to divide asymmetrically. Asymmetrical division creates cells between guard cells, spacing out stomata. Symmetrical division creates two abutting guard cells with no intervening spacing cell. Clustering also occurs where the pattern of guard cell placement has an error, and the two adjacent protoderm cells create guard cells on sections of cell wall immediately facing each other (see figure 1 Dow et al). This is a simplistic explanation, and for detail, see Ohashi-Ito and Bergmann (2006).
If the pattern of stomata placement map to tubercles, then, so long as the tissues under the stomata are raised or swollen, this may explain tubercles. Everett et al (2001) found that the loose gas-exchange tissues under stomata have the ability to swell with water after heavy rain. The water is not absorbed through the stomata themselves, it is water absorbed by the roots and pushed by vascular turgor pressure into the fruit. As the loose sub-stomatal cells become turgid with water, they act somewhat as a shock-absorber. About two hours after the rain the cells return to normal. This temporary cell expansion could hardly explain tubercles – especially as they are already laid down at the earliest time of fruit development, regardless of whether it is dry or wet at the time.
In addition, the number of stomata per square centimeter varies widely between cultivars. Hass, a genotype with prominent tubercles, has an average of 69 stomata per square centimeter, and Topa Topa, which has smooth skin, has about 442 (Schroeder (1950)). This is the opposite of what you would expect if stomata patterning was responsible for tubercles.
And as Schroeder also points out, for the most part, stomata on avocado fruit skin are level with the epidermal layer.
Schroeder showed that a cork cambium and corky tissues develop at the stylar end of the avocado fruit (commonly called the fruit base, as opposed to neck) in some genotypes. This is associated with breakdown and suberisation of the tissues under the stomata. So, while there can be swelling associated with corky sub-stomatal tissue, it is only at a late stage of fruit maturity.
Stomata heavily concentrated around the remnants of the flower style
Finally, stomata are highly concentrated at the stylar end, and in conditions where these suberise at maturity, they appear to coalesce into a general corkiness of the base. The corkiness in no way resembles tubercles, and is localised to the base. In spite of the high local concentration of stomata, the base is no more tuberculated than the rest of the skin – and sometimes is less so, especially in the immediate vicinity of the stylar scar.
That’s it for the ‘stomatal tissue patterning’ hypothesis for tubercle development – or so it seems.
Rathod et al (2019) investigated the cause of both the tubercles and the ridges characteristic of the fruit of bitter gourd, Momordica charantia. They found that both phenomena were under single gene control. One gene, Tb, controls tubercles, and another, Cr, controls whether ridges are discontinuous, rather than continuous.
In cucumber, Cucumis sativa, a functionally identical gene (isofunctional homologue) also causing tubercles is labelled Tu (CsTu).
The Tb gene.
Crosses of smooth-skinned and ‘warty’ (tuberculate) genotypes of bitter gourd showed tuberculate to be dominant over smooth (Kole et al 2012). Similarly, the discontinuous ridge character state (Cr) was found to be dominant over continuous ridges (cr).
A Tb – like gene in avocado
An avocado isofunctional homolog of this gene would explain the surface topography of avocado fruit. It is also permissive of some association with the gene/s associated with stomata patterning (if so, this would be pleasantly tidy).
The variation in expression of tubercle height in avocado plants might be the result of the Tb gene being additive. A cross between a tuberculate cultivar and a cultivar that has a smooth skin as a result of a mutation that prevents Tb expression (tb mutant) fits nicely with this scenario, and Sonia Rios’s presentation of a smooth sector chimera in a tuberculate avocado fruit provides convincing evidence.
Selfed Hass seedlings show both markedly tuberculate progeny, and progeny with nearly smooth skin (Bergh and Whitsell 1973). If Tb was dominant in avocado, all the progeny would have had markedly tuberculate skin.
This result would be explained if Hass was heterozygous for Tb (Tb/tb). Absent modification by other genes, Tb/tb x Tb/tb ‘should’ give one Tb homozygote with markedly rough skinned seedling and one tb homozygote with apparently smooth skin to every two heterozygotes with intermediate skin pebbling.
In contrast to Hass, Fuerte has relatively small and low tubercles. But Bergh and Whitsell (1974) record that “many” selfed Fuerte seedling have “warty” skin, which suggests Fuerte, too, is a heterozygote for this character.
But why does Hass express the Tb gene so strongly if it is a heterozygote with only one active copy of the gene?
There may be other genes that further modify Tb expression.
How the Tb gene might work in avocado
Yang et al (2014) discovered that in cucumber, the Tu gene codes for a transcription factor (a C2H2 zinc finger protein) that ‘probably’ promotes cytokinin biosynthesis in the tissue that ultimately became tubercles. This transcription factor is further enhanced by another gene, Tsl, which affects the ultimate size of tubercles.
Tubercle size – the Ts1 gene
The specific gene (Ts1) listed in Wang et al 2020 (supplementary file 1) that specifies cucumber tubercle size codes for an oleosin protein, and is a promoter of gene expression. There are low levels of Ts1 expression in cucumbers without warts, or with small warts, and high levels of expression in those cultivars and lines with greater tubercle size (Yang et al 2019).
This heritable variation in tubercle size due to expression of the Tsl gene has a strong appeal as a plausible cause of variability in tubercle size in heterozygote avocado cultivars.
Some avocado cultivars, and Maluma is a good example, are extremely tuberculate. This is exaggerated at the immature stage.
Left, seedling showing low tubercle expression, right Maluma immature fruit with ‘hyper-expression’
This kind of ‘hyper-expression’ may well be due to even further epigenetic influences that enhance tubercle size.
Within-fruit localised tubercle over-expression
Avocado fruit – or at least pyriform avocado fruit – are asymmetrical. If you stand an avocado up on its base it may look symmetrical, but in fact the morphological ‘true’ base point of the fruit, at the floral scar (stylar scar, strictly speaking), is shifted up the side of the fruit, away from the apparent base (Cummings and Schroeder, 1942; see Fig. 12). The stylar scar, the ‘true’ halfway point, is marked with a yellow pin in the photo of the late season Hass fruit below. In other words, one side of the fruit is ‘smeared around the bottom of the fruit, visually at the expense of the other half.
The ‘apparent’ short (left hand) side of this Hass is has about the same perimeter distance as the other side.
The fruit stalk is attached offset from the apex in many pyriform cultivars, and viewed from the bottom of the stalk to the apparent base, this creates an apparent ‘short side’. The apparent short side meridial sector has significantly greater tubercle expression than the rest of the fruit surface, especially the apparent ‘long’ side. The ‘long’ side has rather flattened tubercles, which becomes more marked as the fruit enlarges and matures.
But the perimeter of the botanically accurate sides, measured from the pedicel core to the stylar scar, are almost identical in the above illustrated mature Hass fruit. The more heavily tuberculated ‘short side has a perimeter of 124 mm, and the ‘long’ side is 127 mm. Of course this is only one measurement and one cultivar.
For clues as to why side of the fruit might be more heavily tuberculated, we can look to the internal morphology of the fruit, which is well described in Cummings and Schroeder, 1942. In short, one half, the apparent short side has more and better vasculature, and is in contact with the seed as it develops. The apparent short side is more important to the plant for this very reason – it feeds the seed, it is the pipeline to the tree.
In contrast, the apparent ‘long’ side had fewer ‘pipes’, thinner ‘pipes’, and its vasculature terminates at the floral scar (or ends blindly), and doesn’t enter the seed at all (Cummings and Schroeder, 1942).
It is plausible that both endogenous hormones from the tree and locally produced phytohormones are delivered more quickly and more efficiently to the apparent short side, increasing signal concentration, and ‘pumping up’ tubercles locally. In the meantime, the infrastructurally depauperate apparent long side is short-changed in phytohormone supply and has reduced tubercle expression as a result.
This leaves still open the questions of which tissues initiate tubercle formation, and what exactly is a tubercle anyway.
What are tubercles?
Yang et al (2014) showed that in cucumber, tubercle initiation starts two days before flowering and continues until thirteen days after flowering. Tubercles develop due to localised rapid cell division. But it isn’t epidermal cells that develop into tubercles, spine cells do. Tubercles develop directly below the layer of spine cells. See Yang et al‘s (2014) paper for details.
Site of gene expression
Yang et al‘s (2014) work confirmed that in the crucial tubercle initiation stage 2 days before flowering, the cucumber Tu gene was not expressed in the epidermal tissues, nor in the tubercle tissues, but only in the basal cell layers of the spine. However, Ts1 which interacts with the Tu gene is expressed in tubercles, epidermis and pulp adjacent to the epidermis (Yang et al 2019). This implies signalling by a phytohormone. Cytokinin, involved in cell proliferation and differentiation is a good candidate.
The authors observed that the smooth skinned cucumber lines had small, fine spines (really a specialised form of trichome), whereas the ‘warty’ lines had large robust spines.
But avocado fruit don’t appear to have spines, remnant or not, as far as my x20 eyeglass allows me to see (although Wang et al 2020 mention what they call a “micro-trichome”(mict) mutant that shows no visible trichomes on fruit). Schroeder (1950), who examined the skin morphology in detail, makes no mention of hairs, spines or structures atop of tubercles. Everett et al describes lenticels, but makes no mention of spines.
Avocado don’t seem to have spines, and are therefore absent the layer of spine basal tissue that proliferates to form tubercles. But they do have areas of tissue proliferation on the skin. These areas of initially undifferentiated cells are the meristemoid tissues that are triggered to go on to form stomata in a patterned manner.
A speculative mechanism of tubercle origin in avocado
Mutations in the stomata precursor cells cause poorly controlled cell division, forming, in some cases ‘epidermal ‘tumors’ of guard cells (see Fig.2 (C) of Ohashi-Ito and Bergmann (2006) for an example in Arabidopsis leaf. The pattern , or map of the way stomata ‘should’ appear is also disturbed.
It is plausible that in avocado a similar series of mutations in cell proliferation and mapping might result in a clustered pattern of stomata whose underlaying tissues and adjacent cells have receptors sensitive to cytokinins (and maybe auxins). The cytokinins then cause proliferation of cells, and therefore tubercles. At some point the complex hierarchy of gene feedback then down-regulates promoters and cytokinin levels fall away, and tubercle growth stops.
What selective advantage do tuberculate fruit have over non tuberculate fruit?
There might be biomechanical advantages if tubercles ‘flatten’ with swelling fruit. Sudden increases in turgor after heavy rain can cause fruit skin cracking. A combination of cutin deposits in the epidermis (perhaps for tensile strength), sclerenchyma for skin thickness, and tubercles for flexibility might prevent premature cracking and fruit drop. The seed can then reach full maturity with maximal reserves in the cotyledons.
But avocado seeds are usually viable a long before full natural tree ripeness, so this seems a weak argument.
So far, I see no satisfactory explanation.
References
Bergh B. O., and Whitsell R. H. 1973. Self -Pollinated Hass Seedlings. California Avocado Society 1973 Yearbook, 57: 118 -156
Bergh B. O., and Whitsell R. H. 1974. Self -Pollinated Fuerte Seedlings. California Avocado Society 1974 Yearbook, 58: 128-134
Cummings, K, and and Schroeder C. A. 1942. Anatomy of the Avocado Fruit. California Avocado Society 1942 Yearbook 27: 56-64
Dow, G.J., Berry, J.A. and Bergmann, D.C. (2014), The physiological importance of developmental mechanisms that enforce proper stomatal spacing in Arabidopsis thaliana. New Phytol, 201: 1205-1217. https://doi.org/10.1111/nph.12586
Everett, K. R., Hallett, I. C., Yearsley, N., Lallu, N., Rees-George, J., Pak, H. 2001. Lenticel Damage. NZ Avocado Growers Association Annual Research Report Vol. 1 2001
Foster, T. and Aranzana, M. 2018. Attention sports fans! The far-reaching contributions of bud sport mutants to horticulture and plant biology Horticulture Research 2018 5:44 Doi 10.1038/s41438-018-0062-x
Huang, X. M., Yuan, W. Q., Wang, H. C., Li, J. G., Huang, H. B., Shi, L., & Jinhua, Y. (2004). Linking cracking resistance and fruit desiccation rate to pericarp structure in litchi (Litchi chinensis Sonn.). Journal of Horticultural Science and Biotechnology, 79, 897-905. doi: 10.1080/14620316.2004.11511863
Kole C., Bode A. O., Kole1 P., Ra V. K., Bajpai A., Backiyarani S., Singh J., Elanchezhian R. and Abbott A. G. 2012. The first genetic map and positions of major fruit trait loci of bitter melon (Momordica charantia). J. Plant Sci. Mol. Breed., 1(1): 1-6. Doi: 10.7243/2050-2389-1-1.
Ohashi-Ito, K., and Bergmann, D. C. (2006). Arabidopsis FAMA controls the final proliferation/differentiation switch during stomatal development. Plant Cell 18, 2493–2505. doi: 10.1105/tpc.106.046136
Rathod, V., Behera, T. K., Gaikwad, A. B., and Hussain, Z. 2019. Genetic analysis and tagging of gene controlling fruit tubercles and fruit ridgeness pattern in bitter gourd using SSR markers. Indian J. Genet., 79(4) 749-755 (2019) DOI: 10.31742/IJGPB.79.4.14
Schroeder, C. A. 1950. The Structure of the Skin or Rind of the avocado. California Avocado Society 1950 Yearbook 34: 169-176
Wang, Y., Bo, K., Gu, X. et al. Molecularly tagged genes and quantitative trait loci in cucumber with recommendations for QTL nomenclature. Hortic Res7, 3 (2020). https://doi.org/10.1038/s41438-019-0226-3
Yang, S., Wen, C., Liu, B., Cai, Y., Xue, S., Bartholomew, E.S., Dong, M., Jian, C., Xu, S., Wang, T., Qi, W., Pang, J., Ma, D., Liu, X. and Ren, H. 2019. A CsTu–TS1 regulatory module promotes fruit tubercule formation in cucumber. Plant Biotechnol J, 17: 289-301. https://doi.org/10.1111/pbi.12977
Yang X, Zhang W., He H., Nie J., Bie B., Zhao J., Ren G., Li Y., Zhang D., Pan J., and Cai, R. 2014. Tuberculate fruit gene Tu encodes a C2H2 zinc finger protein that is required for the warty fruit phenotype in cucumber (Cucumis sativus L.). Plant J. 2014 Jun;78(6):1034-46. doi: 10.1111/tpj.12531.
These sclerified areas on these avocado seed are unusual. Seeds of the genus Persea don’t usually have sclereids associated with the seed. Two other genera in the Lauraceae family (Endiandra and Beilschmeidia) do have an inner sclerified layer, but it is continuous, not scattered (Little et al, 2009).
These hard longitudinal encrustations on the surface of the seed cotyledons are possibly lignified ‘stone cells’, or sclereids. Gritty ‘stone cells’ appear in (for instance) pear flesh, and are due to lignified cells (see Xue, C. et al, 2019)
Generally, the outer epidermal layer of cotyledons have thickened cell walls. Just why localised areas of this seed should over-express lignification (if that is indeed what is happening here) is a mystery to me.
Another seedling had several small horizontal lignified ridges, within a vertical swathe of light-colored seed coat.
Genes involved in lignification
The final step in the enzyme driven cascade of steps in the biosynthesis of lignin is the polymerization of monolignols into lignin, catalysed by either peroxidase (POD) or laccase (LAC) enzymes. A number of POD and LAC genes have been identified in plants. When some have been artificially suppressed, the cell walls are not as thick, and lower quantities of lignin are deposited.
Conversely, perhaps over-expression of POD or LAC gene/genes in some seed-coat epidermal cells causes this hard, stone-like sclerification of the tissue. These genes may or may not also be responsible for the localised tissue enlargements – either by increased cell volume or simply by increased cell number.
Gene modulation
MicroRNAs (miRs) are small noncoding RNAs that help modulate gene expression. The MicroRNA known as MiR397 has been found to target LAC, and overexpression of MiR397 increases grain size in rice by (in effect) suppressing LAC. Perhaps local underexpression of MiR397 results in hyper-lignification.
However, lignin biosynthesis is affected by many plant hormones.
According to Zhang et al., 2015, auxin reduces vessel lignification of the stoney peach endocarp. Once again, it is plausible that local overexpression of auxin levels might have increased lignification in this avocado seed. Overexpression of cytokin levels may have a similar effect.
The number of cell layers forming the stony endocarp of plum is very much larger than the cell layers in the endocarp of the plum variety ‘Stoneless’ (see fig. 6 in Callahan et al 2015). If the principle that stone tissues are much thicker than ‘non-stone tissues’ applies to avocado, then this principle, in conjunction with lignification, may fully explain this mutant seed.
Callahan et al (2015) found that endocarp started to form around 10 days after pollination, and there was more stone tissue in the (substantially stoneless) ‘stoneless’ plum cultivar when post-pollination temperatures happened to be warm. Perhaps warm temperatures also influence sclerified tissue expression in some avocado genotypes.
The seed coat of the seed with vertical sclerifications of the cotyledon was incomplete and ‘torn’ at the site of the abnormality. Whether genetic of physical is moot. A gene that affects seed coat morphology is APETALA2. Defects in AP2 activation affects seed coat development in Arabidopsis. ap2 mutant Arabidopsis seeds are more irregular-shaped, and lack the distinctive outer seed coat epidermal cellular structure of normal wild type seeds (Jofuku et al, 2005). As APETALA2 is evolutionarily conserved, it is likely to play a similar role in avocado, but how (and if) mutants might influence avocado seed coat is an open question.
Jofuku K. D., Omidyar, P. K., Gee, Z. Okamuro. 2005. Control of seed mass and seed yield by the floral homeotic gene APETALA2 Proceedings of the National Academy of Sciences Feb 2005, 102 (8) 3117-3122; DOI: 10.1073/pnas.0409893102
Little, S.A., Stockey, R.A. & Penner, B. 2009. Anatomy and development of fruits of Lauraceae from the Middle Eocene Princeton chert. American Journal of Botany 96(3), 637–651. DOI 10.3732/ajb.0800318
Xue, C., Yao, J.‐L., Qin, M.‐F., Zhang, M.‐Y., Allan, A.C., Wang, D.‐F. and Wu, J. (2019), PbrmiR397a regulates lignification during stone cell development in pear fruit. Plant Biotechnol J, 17: 103-117. https://doi.org/10.1111/pbi.12950
Zhang, W., Li, Y., Shi, M., Hu, H., Hua, B., Yang, A. and Liu, Y. (2015) Immunohistochemical localization of endogenous IAA in peach (Prunus persica L.) fruit during development. Korean J. Hortic. Sci. Technol. 33, 317–325.
The ‘traditional’ economic model for fruit production and sale has changed dramatically in recent decades, but the change has not registered in the public consciousness.
Growers used to decide what variety of fruit tree to plant, according to their own ideas about the market trends, disease resistance, climatic adaption, and so on. Trees could be bought from any wholesale nursery, as many or as few as the orchardist wanted.
When the crop started to come in, the grower would either pack their own crop, or send bulk bins of fruit to a packhouse. The choice of packhouse was theirs alone.
This ‘chain of choices’ starts with the existence a desirable fruit variety, whether ‘desirable’ is viewed from the growers point of view (maybe resistant to an important disease), or from the consumers point of view (better flavor – always in demand).
It is relatively uncommon for growers to breed their own new fruit variety. Most developed countries pay institutions, universities, or government departments to do the specialist work of developing new fruit varieties as a ‘public good’. The economic fruit of these programs enhances the wealth of the nation by increasing production, overcoming newly emerging disease problems, and increasing export competitiveness. It also enriches the knowledge base of the experts who spend a lifetime doing this work – making them more valuable to society.
‘Free rider’ problem solved
But from the earliest days, there has been the problem of the ‘free rider’. A new variety was anybodies to do what they wanted with as soon as the first tree was sold. There was no law to prevent it. Indeed, it was not uncommon for a new variety to be renamed by a nursery, in the hope of deceiving buyers that they also had a new variety, supposedly ‘similar to’ the variety in question! The only available strategy for the breeder and nursery wholesaler was to heavily promote the virtues of their new fruit variety, and put as big a volume of trees on the market as possible – before competitor nurseries had a chance to ‘muscle in’ – with the inevitable crash in prices due to oversupply.
The free rider problem was solved through countries taking up a form of patenting, which guaranteed the breeder exclusive (and legally enforceable) ownership rights to the variety, and giving the breeder the right to exclusively license a nursery (or nurseries) to propagate and sell the plants.
Plant Variety rights, patenting and UPOV
Plant patenting and the ‘International Union for the Protection of New Varieties of Plants’ (UPOV) system has been around for a while. Plant Varieties Rights (and in the US plant patents) allow the plant breeder to exclusively license one or more nurseries of their choice to propagate their newly bred variety, in return for collecting the breeders royalty payable on every tree sold. Importantly, there is no restriction on the number of fruit trees the nursery can sell. And often the promotion and marketing of the trees is largely in the hands of the fruit tree wholesaler, not the breeder (unless the breeder has also created a trademark under which to sell the plant).
Unrestricted plant sales meant an important new variety, ‘Gala’ apple, for example, was in high demand, soon widely planted, then overplanted, fruit oversupplied the market, prices fell, and growers income with it.
Something had to be done to improve profitability all along the chain. The solution was cooperative vertical integration to choke supply and drive up prices.
Higher prices aren’t a bad thing when low prices act as a disincentive to investment in plant breeding and orcharding. And packers and marketers might earn a lower percentage on commodity fruit versus niche fruit.
Higher prices for a particular variety of fruit have to be supported by some easily appreciated quality. It has to live up to the marketing promise.
It is easier to create and live up to superior fruit when there are only a few varieties in existence, as is the case with kiwifruit. But even here, the value intrinsic to a delicious unique fruit can easily be destroyed by overproduction and erratic marketing.
As a result, New Zealand -bred gold kiwifruit copied the ‘club’ model used by the US apple industry. It works like this:
The 6 principles of creating a club fruit – the kiwifruit example
First, create a single marketing body controlled by the growers themselves that controls almost all kiwifruit exports. In this way the fruit growers prevents exporters undercutting each other to land export supply contracts – because undercutting has to come at the expense of the price paid to the fruit growers.
Second, obtain public taxpayer subsidies to (initially, at least) pay for study of problems in kiwifruit culture, as well as breeding and germplasm conservation.
Third, use the UPOV testing and description requirements to objectively identify your newly bred variety as unique, and distinguishable from all other varieties currently available.
Fourth, and this is the heart of the ‘club’ system, only release a pre-determined limited number of plants; and, most importantly, under a restrictive contract. This ‘chokes’ supply to the consumer, presumably to an economically optimal ‘set point’ where sufficiently good prices don’t choke off too much demand.
Fifth, trademark and brand the supply chain and the fruit to build consumer brand recognition of an exceptional fruit – and a reliable fruit supplier. The reliability of the branded marketer and their fruit varieties merge into one. Zespri generically market any gold kiwifruit they create as ‘Zespri Gold’. (This is standard practice in the global fruit industry. The ‘Dole’ brand, an intellectual property of the well-known grower and marketer of banana, pineapple and papaya is one example among many.)
Sixth. Finally, the protected varieties must be vigorously defended from use by anyone who is not in the club. Big money can be at stake. At least as bad, if we take a long term view, the reputation of the ‘club fruit’ brand may be undermined if illegal growers market inferior fruit.
Protecting the Interests of the members of the club
The members of the ‘club’ tightly co-ordinate markets and market volumes, usually with strategic counter-hemisphere planting. The idea is that as seasonal supply from the southern hemisphere tapers off, supply from the new season northern hemisphere takes over – ensuring a steady year around supply to global customers. The brand on the piece of fruit is always the same, but it may have been grown and exported from different countries throughout the year. Quality is the hallmark of success, and the grower club members often have to agree on minimum maturity standards (among others), so that unripe fruit doesn’t erode consumer trust in the brand.
The profits to the grower and marketer for a ‘niche fruit’ where there are few varieties in existence can be very large. As a result, there can be fierce demand for the ‘right’ to grow highly profitable ‘niche’ varieties with large money-making upside and loss-limited downside. For example, growers are prepared to pay 400,000 NZD a hectare for the license to be allowed to grow Zespri gold kiwifruit. Presumably, the PVR per-plant royalty is on top of that.
Of course, in this kiwifruit example, it is a growers cooperative (Zespri) that in effect sets the price per kilo, promotes and brands the fruit, and pioneers new export markets. In turn, while packhouses have to meet standards set by Zespri, they can probably charge a bit more on the back of the more valuable fruit. For the breeders part (funded by Zespri), they have to agree not to release a new variety that betters the existing one until the license holders have had a chance to make big bucks from their expensive license. (I believe the growers are given 10 years, under ordinary circumstances.)
Any licensed grower who sells propagative material (in breach of their contract) is in double jeopardy – they can be taken to court for breach of plant varieties rights, and also for a specific breach of the contract terms relating to not selling or distributing plant material. In one recent case in New Zealand, the court imposed a fine of around 1.5 million NZD on each charge. In this case, the grower was a New Zealander, and he illegally exported the plants to China. However, for both political and legal reasons, little can be done to mitigate the financial and reputational losses Zespri has now incurred – the Zespri G3 variety (branded as SunGold) is already being grown and marketed in China.
The problem is likely to be transitional – China hasn’t had an effective plant rights protection system until recently. It has inefficiencies and inequities in the current system of administration of breeders rights. The main ones are that rights have to be obtained region by region – creating expensive duplication – and the structure of fines that can be imposed in the courts is so low that the cost of taking a legal action to protect plant rights often outweighs any monetary compensation the courts may order.
China is very advanced in some areas of plant breeding, particularly in genetic modification. China has a large overrepresentation of university graduates in the science and technology areas, and it is almost certain that CRISPR techniques will be used to accelerate plant breeding.
As a result, in my opinion, China is likely to reform its plant rights administration, and join the club system with enthusiasm and energy. It will be interesting to see which fruit species China selects from its vast array of indigenous germplasm, let alone introduced germplasm. It will even more be interesting to to see the club system expand into new territory – both into China and China into other countries.
But regardless of country, I suspect that the club system only exist for high value fruit that consumers themselves judge have special appeal worth paying a bit more money for. What ‘special appeal’ would persuade consumers to pay more for an avocado?
The avocado as a club fruit
The dominant Hass avocado variety is astonishingly hard to supersede. It is very productive, relatively tolerant of heat and cold, has good leaf retention in windy conditions, and above all, has that rich ‘nutty’ Hass taste. First patented in 1935 (Plant Patent Number 139), its patent expired long ago, and any nursery can propagate as many trees as they want. (Ironically, Rudolf Hass’s patent was widely violated, and although he had an agreement with his licensed nursery for a 25% royalty, he made very little money from his invention.)
Sure, Hass-like varieties have been developed, and they have value to the grower for specific growth habit and/or seasonal production features. Lamb Hass, for example, matures later than Hass, and extends its season. Lamb is also very upright, making tree management somewhat easier. There are other early and late Hass-like selections which will be released by the University of California breeding program in due time. But from the consumers point of view, they look like Hass, taste like Hass, and are likely to be mistaken for Hass. So where does this leave the ‘club’ concept for avocados?
Consumers won’t pay more for an avocado variety with improvements that only benefit the grower (such as more compact plant form). But they might pay more for a ‘connoisseur’ variety that genuinely tastes better than Hass. How feasible is it to develop a connoisseur commercial avocado?
The connoisseur avocado
Former University of California avocado breeder Bob Bergh considered Sharwil to be the best tasting avocado, thereby elevating it above ‘the rest’ to the rarefied status of ‘simply the best’. But Sharwil fruits very poorly.
Contra Bergh, some American avocado afficionados believe Jan Boyce is the best tasting avocado. But it is said to be an undesirably big tree with an unacceptably small fruit.
So there is a gap in the market for a connoisseur avocado that has acceptable sized fruit, bears well, and stores and ships well. This gap may now be filled by the new variety ‘Greystar’, a late fruiting sharwil seedling selected by the Grey family, who run a family orcharding business in Gisborne, New Zealand.
Greystar has sharwils exceptional flavor, but unlike sharwil, greystar crops regularly. All the other minimum requirements from a consumers point of view are ticked – the seed is an acceptable size relative to the flesh, there are no obvious fibers, the flesh is dense, smooth, creamy and resists discoloration when cut. (As yet there is no published information on attributes important to growers, such as storage ability, cropping cycles, resistance to wind damage, climatic adaptibility etc.)
In line with the ‘club’ concept, Greystar has been granted breeders rights under the UPOV convention (PVR 32964), a market agent (MG) appointed, Southern hemisphere nurseries licensed in Australia and New Zealand, at least, with nurseries in South Africa and USA (northern hemisphere) also showing interest. Aspiring growers will have to sign a contract not to propagate the plants, and perhaps even not to sow seeds (although that contravenes the UPOV convention under which PVR rights were granted). It is likely that fruit will have to be sold through the marketing agent, MG marketing. Whether or not tree planting will be restricted to a pre-determined number is unclear at this point, as is any club membership fee.
It seems to me that the UPOV system, in some cases, is devolving to be a means of authoritatively describing the new variety. After that, and superceding UPOV conditions, contract law takes over. Why? Because contract law is almost infinite in the conditions it can demand whereas UPOV is limited to ownership of the variety, and rights to royalty from propagating nurseries.
But the club contract conditions still have to be attractive to all parties.
I don’t know whether or not the Greystar contract provides for a fee per kilogram going back to the breeders, but these or similar provisions are certainly the new trend. This certainly gives a needed boost to commercial avocado breeders given the well known difficulties in breeding this fruit.
Given the increasing rate of understanding of plant processes due to application of genomic studies, the future of avocado breeding may belong to superb tasting club fruit controlled by triumvirates of select breeders, nurseries, and marketers.
One of the ‘knock-on’ effects of molecular manipulation techniques is the possibility of taking a dominant cultivar – Hass – and altering gene expression in a novel way. For example, New Zealand researchers have taken G3 kiwifruit plants and altered gene expression so that it is no longer a vigorous vine, but grows as a bush. The only significant barrier to ‘shrinking’ Hass trees is the notorious difficulty of retrieving avocado plants from a mass of tissue cultured undifferentiated tissue in the lab – a critical step in retrieving CRISPR – altered plants.
NZ Avocado Ltd, the NZ avocado industry body, has received government grants to help start a project to work on problems of tissue culture of avocado varieties – including varieties covered by Plant Variety Rights. All this is in the context of whether or not a New Zealand breeding program can be justified. I am sure they are very well aware of the American experience of tens of thousands of trees yielding only a few cultivars, not counting the multi-decadal nature of the enterprise.
The future for club avocado?
There are hints of the future for ‘club avocados’ here. It may be a future analogous to ‘pimping up’ production automobiles. Add different colored skin here. Increase phytonutrients such as lutein or α-Tocopherol. Change the oil profile there. Change the chemicals responsible for perception of flavor. Change the branch architecture. And so, onward.
But whether these ‘tweaks’ add enough value either to the consumer or the grower to justify a higher fruit or plant price is debatable. As is the question of whether the ‘front end’ cost of making the tweaks is recoverable from hoped for higher priced fruit, or savings in growing costs.
Perhaps Hass, Empress and Emperor of avocado, will finally have to hand over the Mexo-Guatem portion of the avocado empire due to CRISPR.
But perhaps not.
If the benefit of Hass and Lamb is as widely recognized ‘mainstream’ commodity avocados, relatively cheap, why would consumers – in an economically constrained circumstance – pay more?
Most Americans (for example) can’t meet a significant unexpected expense without going even deeper into debt. Covid-19 is teaching some what post-war ‘baby boomers’ grew up with – frugality. And the post-war generation is retiring and finding the make-do practices of childhood have new relevance. Retirees are once again watching their money, not spending.
So Hass and Lamb may be here to stay as best value for money.
‘Pimped’ avos may appeal to millenials, and so there is the market. Millenials feel they will never own a house, carry a crippling student debt, so might as well spend, spend, spend. You can add in the rich if you like, but the rich are still a tiny percent of the population.
The rise of the mutants
The twist in the ‘pimped avo’ tale is the unpatentability of mutations when they spread to the next generation and beyond.
Yes, you can patent a plant showing a novel mutation or gene expression. But seedlings of the patented plant that carry the genetic feature can’t be patented unless they are essentially identical to the patented parent. And this will never be the case with the very heterozygous avocado.
Avocado seedlings pop up all the time, in orchards, in compost bins, and also because people sow the seed from their store bought fruit.
CRISPR enhanced avocados simply add to the existing gene pool of useful mutations and over or under expression of gene products. So that is a public good, just as conserving avocado germplasm is a public good.
The nett result will be more avocado foundlings with valuable mixes of attributes – including CRISPR directed changes.
Some interesting seedlings may ultimately go to ‘club fruit’ situations, some may become home garden fruit, and re-vitalize the struggling small avocado nurseries – ‘cut out of the action’ by club fruit and exclusive nursery licensing.
The family farm re-visited
The Famous-in-New-Zealand pomologist Dr. Don McKenzie worked with Japanese breeders to develop apple varieties meeting their cultural requirements for big perfect apples (they were purchased as expensive gifts). Reading his reports of apple growing in Japan, I was surprised that very small family farms could survive.
Small Japanese family farms can make a good living partly based on government subsidies, and partly on ability to produce very high value perfect fruit. The ultimate niche market.
While Japan’s respect-culture supports this market segment, it may be that a small family farm could be supported by the millenial segment – but only if they control the price of their newly found superior avocado by keeping it out of the club.
This is counterintuitive. My idea is that CRISPR will ultimately identify all the chemical components and ratios that result in the famous ‘Hass taste’. Multiple CRISPR interventions could in principle then be used to re-jig existing avocados whose trees are of special merit to growers. Of course, this will take time and a huge technological effort.
But in the long run it may cause a kind of ‘selective sweep’ where the ‘Hass taste factor’ is completely normal in all cultivars. Or the ‘Greystar taste factor’, if that becomes the new benchmark.
We have reached a point where molecular techniques can uniquely identify any plant variety (a view resisted by UPOV for some reason). Successful legal actions for illegal possession and use of proprietary plant varieties, from strawberries to kiwifruit, have underscored the asset-value and inviolability of privately owned plant varieties.
Therefore, it is enough to take tissue samples of a new variety and hold them in a time-stamped and tamper-proof manner to establish origination and ownership. Why bother with patenting or UPOV compliance? They are just proof-of ownership systems when plants are being sold. But if the model is to hold privately and never release, then patenting and UPOV have no relevance. Administration costs and time delays (10 to 15 years is normal) are eliminated.
Perhaps, if customers are beating your door down for your amazing tasting avocado, you should collapse the breeder-marketer-distributor chain down into your own orchard store plus internet shipping. You then control everything, from price to quality, to marketing shtick. But especially price. High as you can squeeze your customers.
Anyway, the fact you choose not to buy plant variety rights in your own country doesn’t prevent you from obtaining plant variety right in other countries (if you think it is a world-beater).
It’s not the model for everyone, but it might work for quite a while.
And it doesn’t have to be ‘either proprietary or club’. An orchard (‘ranch’ in west-Americanese) could include a commodity-Hass/Lamb backbone, an expensive-but-hopefully-profitable ‘club’ planting, and an unreleased proprietary cultivar for gate and local high-end and millenial restaurant sales.
Whatever the future of the ‘club fruit’ concept for avocado, it will happen in slow motion.
This fruit taste good (for the most part), but the tree is unproductive and the fruit has bitterness under the skin and intrusive fibers.
The fruit illustrated above fell naturally from the tree on the 3rd of December 2019. It was ripe on the 8th. So roughly a week to ripen.
This fruit is nearly as good as Hass at this time of year (early Summer), but is in a different flavor class. It has a “nice” flavor, creamy tasting, oil is there but it doesn’t “taste” oily. There is no ‘nutty’ note, unlike Hass. It has a lingering creamy taste in the mouth after eating. The flesh just under the skin is distinctly bitter. This flesh has to be removed for a pleasant eating experience.
This alone disqualifies it from further interest.
The flesh is pale in color, firm, smooth, and the leathery skin peels easily.
It has relatively few fibers, but they are long and unacceptably intrusive.
This fruit is 220 grams. The seed size I rate about medium.
Its parentage is unknown.
The tree is upright, with deep green foliage.
It has no flowers at all this year, and will probably be top-worked.
Arabidopsis thaliana, not much more than a ‘steekin little weed’, has proved to be a towering giant in revealing gene structure, function and location in plants.
Genes tend to be conserved over time, even as speciation takes place. It’s been shown to be reasonable to suppose that if a gene found in Arabidopsis does a certain thing with an observable effect, then the same effect observed in a different species may be caused by a gene very similar to that already described in Arabidopsis.
Genes that are common to two (or more) species, and assumed to be from a lineage continuous (up each fork) from the branching point of a common ancestor of the two species were/are termed ‘orthologs’ of each other. But if the gene functioned differently in the 2 species (influenced by, for example, the ‘epigenetic’ effects of other mutated genes in one or both the species) it wasn’t/isn’t of primary importance.
The same gene appearing in two different species, but with no ‘line of descent’, were/are called ‘paralogs’. (In other words, a gene duplicated ‘anew’ in the two species, independently.)
When genomes of Arabidopsis, Vitis, Populus, Solanum lycopericon and now Persea are compared, you can see same or related genes across genera/species. Are they retained from a common ancestor in the distant past and thus orthologs? Arisen anew and thus paralogs? Who knows?
Who the hell cares? What do the genes do? How does the expression of these genes differ (if they do)? What other genes affect (or even effect) the expression of the gene in the taxon of interest (in this case, Persea)? This is what is important and interesting.
So I am going with Roy Jensen’s concept (outlined below) and use the term ‘homolog’ and the appropriate adjective (if determinable), unless referring to speciation events and whole genome duplication events in deep time.
“Genomic biology needs to get beyond semantic issues. It needs to focus on defining those sequence-structure-function relationships that are necessary for understanding both the structural origins of biological function and the molecular bases for the divergence of biological function. So, those of us who study the relationships among sequence, structure, and function should discontinue the use of ‘ortholog’ and ‘paralog’, unless we want to focus on the speciation and gene duplication events that produced functional diversity in homologs.
But, unlike Petsko [2], we believe that genomic biologists need to describe, compare, and contrast sequence-structure-function relationships not only for a complete group of homologs but also for subsets of homologs that share particular attributes. Based on our experiences, genomic biologists need words to describe ‘homologs encoded by different genomes’ and ‘homologs that have different functions’.
To accomplish these needs, we suggest the following adjectives to describe homologs: ‘Isofunctional’ homologs exhibit the same function(s); ‘heterofunctional’ homologs exhibit different functions; ‘isospecic’ homologs are found in the same species; and ‘heterospecic’ homologs are in different species.“
Jensen RA. Orthologs and paralogs – we need to get it right. Genome Biol. 2001;2(8):INTERACTIONS1002. doi:10.1186/gb-2001-2-8-interactions1002
Avocado flower ‘petals’ are comprised of 2 whorls, each of three similar-sized petal-like structures called tepals. No one can decide if avocado ‘petals’ are actually sepals or really sepal-like petals – thus the compromise name ‘tepal’.
According to a University of California avocado expert, in some small-fruited species in the genus Persea the subtending tepal whorl is quite a bit smaller than the overlaying whorl. As a result, the flowers in these species look like they have 3, not 6 tepals.
But the avocado almost always has 6.
A Hashimoto flower with a size-reduced tepal whorl
On 9th November 2018, towards the end of the avocado flowering (for most cultivars) I noticed a few flowers in a Hashimoto tree that had one whorl so greatly reduced in size that at first glance I mistook it for a three-tepal abnormality.
Left, normal flower, right 4 tepal mutant
Checking other cultivars, I found a few flowers with 4 tepals. These were mainly in Hass seedlings, Pinkerton, and Gwen. Today, a year later, I noticed several 4 tepaled flowers in a Maluma tree.
4 tepals, equal size, viewed from under the flower
In all cases, the 4 – tepaled flowers have a single whorl. There are no subtended tepals of any size.
8 tepaled avocado flowers – left a ‘Mexican’ type, right, Maluma
Several other Maluma flowers have 8 tepals, as does a seedling expressing many characters typical of the Mexican ecotype.
I haven’t seen an explanation of these phenomena in avocado.
In the case of the reduced whorl resulting in an apparent 3 – tepal flower, maybe a point mutation resulting in a loss-of-function meristem mutation (ptl) in PETAL LOSS (PTL) gene or affectors as discussed by Quon et al 2017 in Arabidopsis?
The 4 tepal flower could perhaps be the result of a different gene action. Quon et al 2017 cite Lampugnani et al., 2013 to note that “growth-induced distortions to auxin accumulation internal to the sepal whorl” may disrupt petal initiation. Several genes control auxin transport and accumulation and when these are compromised auxin levels in meristem tissues may be abnormally low. Assuming, of course, that Persea has homologous genes.
The AUXIN1 gene (AUX1) has variants in Arabidopsis whose partial- loss-of-function mutant form arose as a point mutation (e.g. aux1-7). If there is an homolog in avocado, and a point mutation arose (the most likely explanation), it might result in sepal loss. The loss of 2 tepals in a whorl may result in the creation of a 4 tepal whorl.
The 8 tepal flower is a bit more difficult to explain.
Note: I am grateful to Chris Sayer, Californian avocado grower, for documenting climate effects on avocado cropping via Twitter.
We are on track to retain more and more energy in our climate system
Increasing carbon dioxide emissions from burning fossil fuels will increase the energy retained in our climate system by at least an additional 4.5 Watts per square meter by 2100. This suggests a rise in global average temperature of around 2.5 to 2.7o C.
CO2 emissions grew 2% and reached a record high of 37 billion tonnes of CO2 in 2018, and are expected to be at least as high in 2019. There is still no sign of a peak in global emissions, says @gcarbonproject at #ClimateAction science event pic.twitter.com/kTy3LoGBXb
As ocean surface temperatures rise they release more water vapor. As air temperatures rise water vapor levels rise (every 1 degree centigrade allows air to hold about 7% more moisture). This results in heavier downpours. In large land masses such as South America and USA, coastal areas and high mountain chains are hit hardest. Smaller land masses surrounded by oceans are also affected.
In large continental masses the difference between high temperatures inland and lower temperatures on the coast can cause strong, drying winds; and as inland temperatures rise with climate change, so do wind speeds.
The Santa Ana winds have stopped, humidity has returned to normal levels, but the potential for damage still exists. Fruit exposed by defoliation could sunburn. Luckily cool temperatures forecast for the next 10 days. pic.twitter.com/8xT585wTjH
Strong winds can snap off or shred mature avocado leaves. Chris Sayer’s rough estimate is a loss of around 5% on these 6 year old Lamb Hass trees.
Rising temperatures perturb high atmospheric circulation patterns, causing large pulses of cold air to extend out from Arctic and Antarctic poles. These can bring unusually intense cold periods and even affect subtropical jet stream patterns, causing warmer and drier conditions.
These are examples of some of the more obvious effects. What other knock-on effects that may occur in the interconnected complexity of the weather system are harder to predict.
What does it mean for avocado growers?
In some areas, higher temperatures may affect the ability of avocado flowers to set. In other areas cultivars that do poorly due to a higher heat requirement for flower set may begin to bear well. If marine coastal fogs decrease in spring, avocado pollination may be enhanced. Initial trends suggest reduction in summer, but it is highly complex and uncertain. Winners and losers. Growers in the more challenging dry Mediterranean climate areas will likely to struggle with sunburn damage to foliage and trunks.
What Santa Ana winds can do to young #avocado trees. Thankfully this type of damage is not widespread in the orchard and the trees will recover. Glad the event did not last longer. 9% humidity is tough on avocados. pic.twitter.com/XSIzXJVmmc
Damage to tender new avocado foliage caused by dry anabatic winds in California, USA.
As mentioned, strong drying winds are likely to increase in strength. Autumn winds can break branches carrying heavy fruit loads. Stronger winter winds can blow off both leaves and immature fruit.
Increases in climate variablity may mean successive challenges within a year – frosts, damaging storms, waterlogged soils, then drought and dessicating winds.
Increased evapotranspiration may ultimately lead to salination, especially where irrigation water already contains undesirable salt levels. Extended periods of drought may reduce the level of natural aquifers.
Managing the challenges
Most weather-related problems associated with avocado culture are already being managed.
What staking should help prevent: this branch had 30 or 40 pounds of #avocados on it and snapped when the winds picked up this afternoon. This would have been 100 pounds or more at maturity. pic.twitter.com/bH1QUSQqF5
Heavy terminal clusters of avocado fruit in Lamb Hass may snap branches in wind storms – but higher productivity per tree might more than offset the loss.
We put a lot of effort into staking up the LH this season. No control group, so I don't what impact it had, but I think we saved a lot of fruit.
Staking, hedgerowing, and other innovations cost time and materials, but long term these investments may pay off in both monetary and peace of mind terms. (LH = ‘Lamb Hass’).
Wind damage can be mitigated with shelterbelts, staking, hedgerowing.
Frost damage can be mitigated with sprinklers.
Covercrops don't hold up well to traffic, so when I can get wood chips for the roads in our #avocado orchard, I take them. pic.twitter.com/MwVPOGTE7h
Salinity can be managed with salt-tolerant rootstocks and irrigation leaching. New technology can help guide appropriate decisions.
With more warm, dry days ahead, we're touching up any exposed limbs on the young #avocado trees with kaolin clay. (The white coating helps protect against sunburn and heat stress. ) pic.twitter.com/dbIhopYAUP
Rainstorms can be managed with improved subsoil drainage, planting on mounds, ditching, and soil structural improvements via deep rooting cover crops.
Meanwhile, our orchard 12 miles inland. 8 degrees hotter, rockier soil, older irrigation system, older trees. Our faith in the future of avocados is very dependent on knowing our specifics. 3/3 pic.twitter.com/pdlPfj3zNs
At this point, it seems likely that future shifts in duration and intensity of adverse climatic events can continue to be dealt with by existing and new management practices. High temperatures in Spring that exceed a critical threshold for fruit set, and summer temperatures sufficient to damage fruit seem to be the only exception.
Is there really any need to genetically ‘improve’ avocado as a crop to meet higher wind energy, higher global average temperatures, and more intense rainfall effects?
Breeding avocado is more expensive than breeding other crops
Avocado is very difficult to ‘breed’ in the classical sense of controlled pollinations. Worse, time from seed to fruit ranges from about 5 years to 15 years. It is an expensive and difficult crop to breed, even with the aid of new genomic techniques. New characters could be added to meet climate change (wood tensile strength was mentioned, but characters affecting tree size, trunk caliper, and so on could be considered). Each new character added requires generations of breeding cycles, and sometimes very very large populations to find rare genetic recombinations.
Solving problems with management techniques is much quicker than waiting for plant breeding results.
And it seems to me the only problem that can’t be solved by management is heat stress at flowering and summer fruit maturity.
The answers to managing the heat stress problem
Heat stress at flowering may prevent fruit set. Summer fruit can be severely damaged in inland heatwaves. Regular and prolonged whole-tree heat stress is likely to damage important physiological processes. It is ‘game over’ at that point. Growers must choose:
relocate to a suitable climate
grow a different crop
fund a cross-species genetic modification or gene editing program that allows flower set under high heat conditions AND confers physiological resistance to whole-tree heat stress
Interesting trends in per capita avocado use in the United States, US avocado production, and imports.
"If USDA’s forecast is realized, imports in 2018/19 will represent 93 percent of the domestic avocado supply."
If climate and soils are unsuitable in one country, large vertically integrated growing/shipping corporations can develop large orcharding operations in countries with a more suitable climate
Relocating internally
The only growers that are likely to relocate (within a country, or to a different country entirely) are corporate growers. Relocating is expensive. Only corporate growers have the capital (or can attract it). Inevitably there is a delay of 4 or so years before the first useful crop volumes flow from new plantings. Corporates can wait years for a return on investment.
Climate preview? We saw 110.7 degrees on Friday. Very hot for #avocados. But with young trees, good irrigation system, and good soil, they have come through with only trivial damage. @savortooth 1/3 pic.twitter.com/2Dms44jVP4
Smaller family-based farmers have less financial flexibility. In some hotter inland areas away from ameliorating coastal ocean effects, avocado orcharding will have to be abandoned. Chris Sayer notes there is already a trend for the industry to relocate from San Diego to the cooler San Lois Obiso, about 430 kms north.
California agriculture is not like the rest of the country. Young #avocado trees in the foreground, #cilantro across the street, replacing strawberries. pic.twitter.com/9TaSsmZFdW
Strawberries crop the year of planting, cilantro in 6 weeks or so, but avocado takes 5 or 6 years for good crop levels. But flipping’ between annual crops is also challenging. Different annual crops require different equipment, different skills.
Most medium to smaller growing enterprises are far more likely to change crop, give up growing (especially if their land is being encroached by suburbia), or grow a different (probably short-cycle) crop.
In short, moving counties or regions is an effective heat stress mitigating measure, and requires no further plant breeding effort. But is moving countries an effective strategy?
Moving Countries to mitigate climate risk
Moving countries is only a realistic option for corporates. As noted above, USA domestic production will likely be imported. It might make economic sense to in effect ‘outsource’ US avocado production to another group of countries. This pre-supposes US corporates will be allowed to buy land in those countries. While money talks, USA and other western countries have had a long history of de facto colonization of horticultural crops important in the US and European market. Banana and pineapple production in South America and the Philippines are examples. Countries are increasingly demanding meaningful local ownership or share of their own land-based productive industries. As the COVID 19 emergency has illustrated, countries have to consider control over food and strategic assets, whether carbohydrates or hydrocarbons.
What to do?
Stay and ‘fight’ the heat? Move the orchards ever north and coastal to try to stay ahead of it? Re-locate to another country? Let’s look at the ‘stay and fight’ option from the point of view of ‘re-engineering’ avocados to tolerate increasing heat.
Avocado nodal groups already carry climate resilience
Avocados of the tropical group (so-called ‘West Indian) grow and fruit well in spite of tropical temperatures. ‘Guatemalan’ avocado nodal group generally fruit well in the warm temperate+ to subtropical zone of the South American Andes. Many of the more northerly ‘Mexican’ node tolerate cooler conditions, including some frost resistance.
The driver of this wide band of climate tolerance seems to me to be a once contiguous wild avocado population experiencing wide temperature differences with both latitude and altitude. (More complex local climate effects within parts of the broad range may also have played a part.)
Localised population genetic diversity has probably been fragmented and thus ‘pruned’ by human deforestation from shifting ‘slash and burn’ agriculture, and more recently by forest clearing for coffee growing. The case of the cultivar Nabal illustrates the point:
“The parent tree was accidently destroyed in June 1917, by a laborer who was planting coffee. It stood among coffee bushes in the Finca ‘Santa Lucia,’ 7a Calle Poniente, near the Alameda de Santa Lucia, Antigua, Guatemala. “
Wilson Popenoe, 1917
If Popenoe had not collected budwood and shipped to the USA over a hundred years ago, the Nabal cultivar would not exist today. And neither would cv. Reed, as Reed is a seedling of Nabal.
‘Guatemala’ is a relatively small geographic area, and possibly has greater depletion of diversity than its much larger neighbour. It seems to me, the greatest amount of avocado genetic diversity is likely to be in local trees in isolated villages, farmlets, and wild places throughout Mexico. And as some parts of Mexico experience dry heat, perhaps some genetic adaptation has occurred.
Genes for heat tolerance can’t be seen
Heat tolerance is probably mediated by a number of physiological processes regulated by a number of interacting genes. ‘Classical’ avocado breeding is primarily guided by easily measurable physical attributes – fruit size, flesh quality, fruit storage quality, crop load, tree size and so on. Avocados historically have been selected in climates well suited to avocado culture and production. The ability of the plant to tolerate heat isn’t revealed until plantings are tried in marginal climatic zones.
Problems in breeding for heat tolerance
If tolerance to heat has evolved in the hottest part of avocado’s natural range, then that is where germplasm should be sought. Not in commercial orchards, but farmlets, villages, towns, wild places.
Historically, the primary selection filter was fruit quality, coupled with reliable productivity. Tolerance to heat and cold was a bonus. But as the ‘pedigree’ of avocado in commercial use is narrow (excluding tropical avocados) there may not be any marked heat tolerance within this rather small subset of avocado diversity. (This is not to say that there is no useful selectable variability in existing germplasm collections. Avocado cultivars are very heterogenous, and their flowering system tends to maintain that heterogeneity.)
I don’t suppose anyone has ever attempted to characterize what ‘heat tolerance’ in avocado is composed of (in terms of which genes affecting which physiological processes, and in what manner). It is certainly possible to do this, and a similar process has been carried out by Prof. Neena Mitter at the University of Queensland to identify genes involved in Phytopthora resilience. Prof. Mitter’s work identified 17 genes involved in a variety of physiological processes apparently responsible for the Phytopthora resilience phenomenon. ‘Heat tolerance’ may not involve as many genes, but is likely to be multigenic.
If genes are identified, then the process of constructing and patenting gene-markers can begin. An expensive process, and probably an expensive product.
Plant exploration is also expensive, and in the case of invisible characters, difficult. I also doubt anyone has gone in search of ‘heat tolerant’ avocado plants in Mexico.
If an apparently heat tolerant seedling can be found, the fruit will likely be inferior – small, big seed, probably thin skinned. Transferring the wild genes from putative ‘heat tolerant’ individuals into commercial cultivars would be a long job.
Why breed for heat tolerance?
Why bother? Who would do it? Who would benefit?
Outside the tropical avocados, at least, this question implies that breeding for heat tolerance, whether by conventional or genetic manipulation techniques, would only happen if Hass variety was threatened on a relatively wide scale.
In such an ’emergency’, I would hope ‘science diplomacy’ would steer the task, with a cooperative effort for the common good, and the tab picked up by everyone.
Temperatures that would threaten Hass avocado
Temperatures above 35 °C (95 °F) have been quoted as adversely affecting fruit set. I am not sure if this is due to warm dry air (perhaps plus wind) desiccating the flowers in the crucial late spring period, or if this is a temperature level that triggers abiotic stress gene up-regulation. This might result in flower or fruitlet drop, or maybe early-stage pollen or ovule damage or death.
Will temperatures rise above 35 °C in the critical spring period?
If so, will such effects be localised to more inland and lower latitude areas of the Mediterranean-climate avocado producers (USA, Israel, Spain, North Africa)?
Will the speed of climate warming outpace the speed of breeding more heat tolerant avocado cultivars?
We don’t have answers. Yet.
Increasing general resilience
Avocado is not an important food in the well-fed West. Objectively, at only 2% protein and about 20% oil, it hardly stands out nutritionally. In contrast, for example, low THC hemp seed has much more protein (around 25%) and contains about 30% oil (which contains exceptionally high levels of the 2 essential fatty acids linoleic acid and alpha-linolenic acid), and unlike avocado, hemp is relatively drought tolerant. But avocado – with corn and beans – is an important food for poor Mexican folk. The trees barely need cultivation, are long-lived and fairly reliable producers.
Avocado as a business
Avocado growers are ‘in it’ to make money, a return on capital. So the laws of economics apply.
It’s only worth doing so long as the monetary returns exceed all costs by a margin that competes with other possible uses of that capital.
Avocados are a commodity. They used to be an exotic fruit used by top Los Angeles hotels and clubs. The per fruit price matched the mystique at that time. Now, returns are dictated by competitive factors such as first to market, last to market, reliably high oil content, preferred size (often larger for the restaurant trade) and event-driven production (in the USA, Superbowl).
I have a friend who grows and sells finger limes for $50/lb. But I do not believe she is ever able to sell her whole crop at that or any price. It's not the price you get for the first pound that matters… it's price for the average pound.
An excellent illustration of price versus demand vs total market size vs annual cash flow
Most USA Hass come from Mexico these days. But drought in Mexico and reducing artesian water resources may change everything. The climate crisis relentlessly wears on. It is real, the situation will worsen. Supply from Mexico may reduce, as it has in USA: but demand may also reduce as avocado prices creep up. Consumer willingness to pay well for premium produce may reduce as economic pressures come on. We’ll see.
The rich become richer, but there aren’t enough of them. They are irrelevant to produce growers. What is needed is an ever increasing pool of middle class, if avocado prices are to sustain growers.
Avocado will always have a place in Western domestic economies, even in times of economic constraint. It is less certain that avocado breeding will have a place if avocado become a low value fruit with no industry ‘fat’ to subsidize fruit research.
Therefore, the avocado ‘resilience system’ should be strengthened as a matter of general food security. Security is the essence of a social good, and its delivery is the responsibility of government.
In the case of securing maximal resilience in the avocado system all governments should cooperate in funding avocado germplasm conservation everywhere (resilience depends on multiple redundancies), but most particularly in the natural range.
The accent should shift from selecting existing cultivars with ‘good’ fruit or fruiting to collecting a range of diversity, including diverse tree forms. Mutants, particularly dwarfing mutations, should also be curated widely.
Universities and other research organisations should continue to cooperate to better understand and explore the gene-environment nature of avocado.
Breeders, both private and public, should generally share information to aid insights into avocado breeding. Non-commercial cultivars should also be shared (and phytosanitary requirements rigorously observed).
If avocado fruit are a public good, then the consuming public should contribute a small amount of each avocado purchase price to an international, consensus-driven Trust tasked with maintaining avocado diversity in perpetuity for the common good.
Ironically, a somewhat similar arrangement has been put in place for conserving germplasm of coffee, a tree whose industrial culture is responsible for destroying a part of avocado diversity forever.
Perhaps coffee drinkers should pay ‘reparations’ for the damage they have done. The money could then go to a community fund to enduringly avocado germplasm for the common good.
One of the better ‘home garden advantage’ avocados. From time to time a home garden seedling will be superior to Hass in the early part of the Hass season. And that is the case here.
Picked 18 September, it was ripe on the 25th. So roughly a week.
This fruit is better than Hass at this time of year (early Spring), oily, rich, but not as nutty as Hass.
The flesh is between soft/firm, and the very thin skin peels OK (ish).
It has some fibers, albeit they are not particularly intrusive. Like many thin skinned-green fruit, it develops black spotting of the exterior skin when ripe. These spots don’t intrude into the flesh, at least in this season, they are cosmetic.
The fruit (so far) vary quite a bit in size, from about 150 grams to about 290 grams. The seed size varies from small (as illustrated above) to large in larger fruit sizes (below).
It is a Gwen seedling. The pollen parent, as usual, is unknown.
Like Gwen itself, the tree is ‘temperamental’, soaking up nitrogen without greening up satisfactorily. To be fair, I can’t discount the beginning of Phytopthora infection. But it does have the virtue of not being excessively vigorous.
It seems to be an ‘A’ flowering type, and at this point it has a long flowering period, starting with a modest winter flowering, and, like Carmen Hass, setting a few winter-set fruit.
It remains to be seen whether or not it will fruit well in the longer term.
Laurie Meadows, the author, Pinkerton, the avocado.
Whether we are consciously aware of it or not, I believe that we act in what we believe to be our own best interest, all the time.
So it must be in my best interest to allocate time write on this subject.
I am a ‘non-scientist’, retired, a home gardener on a ‘lifestyle block’, and my interest in avocados is a subset of a life-long general interest in food plants, particularly fruit and nuts.
I started out with the idea of growing and selling fruit of some of the ‘old school’ and some of the new avocado varieties (to cover some of the costs of my hobby when on a pension income), but in the end I decided it was too much work for too little reward – especially as the few good areas for avos here have increasingly been affected with phytopthora.
Anyway, ‘gardening’ for me is pretty much planting the tree and doing very little else thereafter. The results – or lack thereof – reflect my negligent style. Not a good attitude for a commercial avocado grower!
The non-monetary reward, really, is that it is quite interesting fiddling around with edible plants, and because I am a home gardener I can grow any ‘uneconomic’ plant I want.
As a result, we have a smattering of pecans, bags of macadamias, a variety of cherimoya, and so on. Usually bird pecked or possum-damaged, but it doesn’t matter that much, especially when my partner is a compulsive bottler and jam-maker.
I came to realize that ‘avocados’ are more than ‘just Hass’, so they excite my ‘huh, that’s interesting’ gland. Which, as my off-sider points out, tends to bouts of oversecretion.
Over the years a small number of avocado trees resulting from random seeds originally planted as rootstocks have fruited. Self sown seedlings have also popped up and fruited. It is always fun to see what the fruit are like, because the home gardener is not shackled by the tight requirements of commercial cultivars.
The home gardener doesn’t give a toss if the fruit are small, big, thin skinned, subject to scarring, doesn’t keep, is a funny shape, the wrong color, crops only modestly etc.
But now that the average size home garden is not much bigger than a sheet of A4 paper, the home gardener does care a great deal about how big the tree is.
So, in retirement, I am throwing in a few seeds of whatever interesting (non restricted) avocado cultivars I can find in the hope – and it is a very faint hope indeed – that a small avocado tree with a small ‘footprint’ can be found for todays micro-garden. But where to find interesting seed-parent trees?
Sadly, the ‘official’ New Zealand avocado variety collection (curated with 50% taxpayer funding) was mostly chainsawed in the late 1990’s. Wish I knew their plan before the deed was done. Still, as my mom-in-law used to say “if wishes were horses, beggars would ride”.
I am not an avocado breeder. Avocados are all but impossible to ‘breed’ in the conventional sense of controlled pollination, large numbers of progeny, re-selecting and re-crossing and so forth. And as seedlings take 5 to 20+ years to fruit (if they fruit at all) planting a few seeds late in life seems a little futile.
But it is interesting and fun.
And in the unlikely event a small tree was found, I would be very happy to have contributed to the dietary quality of the average householder. And if the tree was really small, it might fruit in a large pot, and become part of a mobile orchard. After all, dwarf apples and peaches will fruit in pots, so why not a dwarf avocado?
Will a dwarf avocado come from commercial programs? Doesn’t seem like it right now. Dwarf avocados were once bred in Mexico, but now seem to have fallen off the radar. Perhaps they weren’t ‘commercial’.
There is some hope. First, a small side trip into the driver of modern fruit breeding and the introduction of contract law instruments is required.
A major problem for commercial growers is that avocados are now reaching the point of being a commodity. Commercial success comes from being a low-cost producer, and not too far from your market. These conditions are found in only a limited number of countries with ideal climates, cheap labor, political stability, plentiful water, and good geographic location. So what’s the ‘fix’ for the rest of us who don’t have these advantages?
Well, in the kiwifruit, stonefruit, and pipfruit industries the unstoppable trend is to breed unique and desirable fruit varieties that are exclusively licensed to a small ‘club’ of limited number of growers worldwide. The idea is to limit supply and keep up the prices for these elite varieties. And they are branded and promoted as a being ‘a cut above the rest’.
The trees don’t usually make it to the home gardener unless they fall dramatically from commercial favor. If the ‘club’ no longer uses them, they may (or may not) allow them to be released under plant patent law.
Before the so-called ‘club fruit’ concept caught on, home gardeners used to receive the commercial ‘cast-off’ varieties of the horticultural industry. That now seems to be coming to an end.
As a result we only have the ‘old school’ avocado varieties that were available decades ago – if they still exist. That seedling growing from the compost in your back yard may be the only source of a new avocado variety for the home garden – as long as it has some fault that disqualifies it from becoming a commercial cultivar! And chance seedlings do have a small history of making ‘the big time’.
After all, today’s commercially dominant Hass variety grew from an ungrafted rootstock tree grown from seed of the Lyon cultivar. Lyon, very likely the seed parent of Hass, was in turn a home garden tree growing in Mrs.Lyon’s front yard in – yes – Hollywood!
The irony is that as commercial cultivars improve – in part maybe due to application of gene technology – every home gardener who plants an avocado seed has a better and better chance of coming across a good quality avocado. Whether it is small enough for the backyard is a different matter.
As human induced climate change pumps more and more energy into the weather system we may have much higher energy winds. This may drive an effort to develop smaller trees capable of withstanding wind damage.
Marker assisted selection might accelerate the hunt for smaller trees with leaves that don’t snap or shred in high winds.
Eventually, such trees, even with higher-priced ‘elite’ fruit designations will be producing fruit for the retail market. Chance seedlings will be more likely to be be smaller – suiting the home gardener.
So there you are, my motive is fun. It’s a tremendous lot of fun cheer-leading from the sidelines of a commercially-funded ‘science-game’ in which the average person barely understands the rules, and can only ever be a spectator.
Trying to gain a level of understanding of the rules of the ‘molecular game’ is also a major reason for this blog, even when the molecular work is of no direct importance to my interests.
But from a home gardeners perspective it’s fun to throw a few seeds in the ground and see what you might get. E v e n t u a l l y…!
Avocado seedlings can pop up almost anywhere in the garden.
We had one grow near our incinerator, another came up in the compost heap, then there was the one by the deck, the seedling tree right outside the kitchen window. All these trees fruited (eventually). We named the fruiting seedlings ‘incinerator’, ‘compost’, ‘deck’ and ‘kitchen’ – thereby displaying a keen sense of geo-location, rather than a lack of imagination.
As we have a small rural ‘lifestyle block’ with some patches of soil suitable for avocado, we were able to plant 10 or so cultivars, as well as planting a few seedlings for later grafting (mostly from commercial fruit, mostly Reed, some Hashimoto, a few probably Fuerte).
My grafting skills are rather poor, and some of these few ‘rootstock seedlings’ grew on to produce fruit. I left a rootstock stem ungrafted on five of the rootstocks I did successfully graft. Although these trees are currently very badly affected by Phytopthora, several have had some flowers and even produced a pathetic sized fruit.
As anyone with a group of larger avocado trees will confirm, seedlings germinate endlessly from fruit fallen under the canopy. Like most people, I spray them out. But the odd one escapes attention.
Almost all self-sown seedlings are commercially useless – as are deliberately sown seedlings (whether sown by plant breeders or a home gardener.) Almost all, but not all seedling are useless. Self sown escapee seedlings in commercial avocado orchards have given us the commercial varieties Pinkerton, Maluma, and Reed.
A few avocado cultivars are more likely to give worthwhile seedlings – even if there is a fault (such as thin skin) which prevents them from being commercially acceptable. The best seed parent is Hass, and as it is almost the only cultivar grown (in Mediterranean and warm-temperate climates), most seedlings will have Hass as the female parent. Hass is self fertile – a big part of its commercial success – and so many chance Hass seedlings will be selfed.
Dr. Bob Bergh’s experience (Bergh 1976b) is that Hass is “probably the best of all progeny-tested parents”. Of 400 selfed seedlings, 18 were found worthy of selection for further study (1 in 22).
Since Bergh’s time at the University of California, the emphasis has shifted. Gwen, a seedling of Thille, has been found to be an outstanding maternal parent. Gwen and Gwen progeny (Harvest, Nobel, Marvel, Gem, 5 -552, Lamb Hass) have been used very extensively in the Californian breeding program.
But Gwen has never really gained much traction against Hass, the fruit are simply not available in New Zealand shops (and barely available in the USA) so the average random seedling is still likely to be a Hass seedling. And if it fruits it is likely to fruit at around the same time as Hass – and is highly likely to be inferior to Hass.
From a home garden perspective, the best seedling would be one that fruited in winter, and is good quality. Fuerte should be the number one choice for winter, but it is a large spreading tree, and is a very erratic bearer.
Fuerte seedlings, whether deliberate or accidental, are a waste of time and space. Selfed Fuerte seedlings were “markedly inferior” according to Bergh (1976b). He went on to comment that most [selfed] Fuerte seedlings “never set fruit, and most setters have fruits of deplorable size, shape and seed ratio”.
Mexican avocado cultivars have a much shorter time to maturity, are oily, and seedlings of Mexicola, in particular are quite precocious. Pity the tree is huge, and the cultivar no longer available (in New Zealand, at least).
In the long run, molecular techniques enabling selection for specific desirable characters will result in more and better commercial cultivars (hopefully, including season extension into winter).
As a result, in the distant future, home garden seedlings, whether chance or planted, will be much more likely to be desirable. But it’s a slow process, and it’s probably many decades away.
In the meantime, in the spirit of Dr. Berghs short reports on seedlings that ‘didn’t make it’, I will post photos and comments on random home garden seedling found here, both past and present.
Bergh, B. O. 1976(b). Factors Affecting Avocado Fruitfulness. In: J.W. Sauls, R.L. Phillips and L.K. Jackson (eds.) Proceedings of the First International Tropical Fruit Short Course: The Avocado. Pages 83-88. Gainesville: Fruit Crops Dept., Florida Cooperative Extension Service. Institute of Food and Agricultural Sciences, University of Florida
Bergh, B. O. 1967. Some late-maturing avocado seedlings of various parentage. California Avocado Society 1967 Yearbook 51: 131-158
Bergh B.O., and Whitsell R. H. 1973. Self -Pollinated Hass Seedlings. California Avocado Society 1973 Yearbook, 57: 118 -156
Bergh B. O., and Whitsell R. H. 1975. Self -Pollinated Fuerte Seedlings. California Avocado Society 1974 Yearbook, 58: 128-134
Avocado Gene Expression, Climate, and Society 
