A Possible Bullate Leaf Mutation in Avocado

by Laurie Meadows
First published 12 July 2022

        leaf mutation in an avocado seedling

An unusual seedling avocado was noted in July 2022. It was distinguish by regular fine bubbling of the leaf surface, the relatively small and narrow leaves, and the small plant size relative to other seedlings that germinated at the same time. Seedlings with a mutation that causes twisting and distortion of the leaf surface and margins occur from time to time, but this is the first occurence I have seen of a leaf with a regular pattern of small bubbles on the upper surface, as distinct from mutants with wrinkles, creasing and twisting of the whole leaf.

I find weak plants such as this seedling difficult to keep alive. Government funded pure science seems a thing of the past, so obscure avocado mutations with no apparent commercial importance attract zero academic interest. Therefore, this observation, ephemeral although it will be, may be not only the first, but also the only record of this phenomenon in avocado.

Rugose leaf surface mutations

Some plants, such as some caladium cultivars, have various degrees of creased, furrowed, 'wrinkled' and distorted leaves. The botanical term 'rugose' (and rugate) has been applied to this leaf surface form. Rugose is derived from the latin word 'rugosus', which means wrinkled. Although there are other botanical latin words which include 'wrinkled' as a descriptor, 'rugose' has been applied to plants with quite long furrows, creases and 'pinching' constrictions, often with wavy or distorted leaf margins, as well as to plants with leaves that have a regular pattern of 'bubbles or 'blisters' covering the entire leaf. The leaf may or may not have marginal undulations, or even somewhat downward rolled edges.

The caladium leaf below is from figure 1 of Cao et al 2017. It shows typical rugose (American sense) 'pinching', crumpling, wrinkling (= US 'puckering'), with undulate leaf margins and some terminal twisting. The mutant avocado plant on the right shows a similar, but less exaggerated rugosity (the mechanical strength of the leathery avocado leaf is probably higher than that of the relatively soft caladium leaf). In caladium, at least, this broad irregular 'rugosity' is controlled by a single dominant gene. Caladium varieties with the recessive allele have flat leaves.

      mutation caladium and avocado leaves

Rugose vs bullate - which is correct?

In general, plants whose leaves are bubbled, blistered or puckered - as this avocado seedlings leaves are - are described as being 'bullate', from the latin word bullatus, inflated or bubbled. Botanically, 'bullate' specifies leaves that are bubbled, blistered or puckered. But the leaf of the primula, for example, is described by one botanical source as 'rugose', and by another source as 'bullate'. Which is correct? Botanical descriptor words explaining the meaning of both 'bullate' and 'rugose' commonly include the descriptor 'puckered'. Different understandings of the meaning of the word 'puckered' may be the key to explaining the apparent contradiction.

According to the Missouri Botanical Garden 'Grammatical Dictionary of Botanical Latin', in America the word 'puckered' is not generally understood as meaning bubbled or blistered (although there are exceptions). In American english puckered means: 'wrinkled, furrowed, drawn together so as to crimp, contracted into folds or corrugations'. In the rest of the english-speaking world, 'puckered' means bubbled, drawn together into small folds, creased. The regularly puckered fabric called seersucker is an example of the concept. So a leaf described as 'puckered' would usually mean something like creased or wrinkled to an American, but to the majority of the world's english speakers it would include 'bubbled' as well as corrugated. An American might describe a bubbled leaf as 'rugulose' - which means 'finely' rugose. It still doesn't explicitly capture the 'blister/bubble' concept.

In general, both bullate and rugose leaf forms are caused by a mismatch between the growth rate of the leaf lamina and the veins.

In bullate leaves, the growth of the lamina is a little faster than the veins, and the upper leaf surface is forced to inflate slightly because it is trapped between the veins, thus causing a 'blistered' or 'bubbled' leaf surface. As the venation pattern is regular, the series of inter-vein inflations are also regular. If the entire leaf is bubbled, then the term 'bullate', not 'rugose' is fully appropriate in my (unschooled) opinion. Botanist might disagree.

In rugose leaves, the entire leaf may be unevenly (sometimes quite exaggeratedly) distorted with various degrees of waves, wrinkles, corrugations, and areas of pinching. These features might affect the whole leaf, of just some sectors. Good examples are the rugose-leaved ornamental caladium varieties. It seems to me that vein-constrained, evenly and finely rugose leaves (sometimes called 'rugulose'), as in the leaves of Rosa rugosa are more accurately described as bullate.

Bullate leaf surface mutation

Bullate avocado and
        polyanthus leaf

This avocado seedling is an example of a mutation causing a bullate leaf form. A gene causing the bullate mutation has been investigated in soyabean (Bernard et al 1975).
"Several years ago at Urbana, plants with abnormal leaves were noticed among segregates of crosses involving T217 (a Korean introduction) and ' Clark'.
The leaf was quite rugose, much like one infected with mosaic virus, but with characteristic circular bumps or a blister-like upper surface. The term "bullate", used in plant taxonomy to denote having a blistered or puckered surface, seems to be appropriate for this trait.

Results obtained at Urbana gave evidence that bullate leaf was controlled by two genes with both recessive alleles (here designated lb1 and lb2) necessary for its expression."
Bernard and Rode, USDA, 1975
Bernard et al (1975) found that the heterozygote plants showed a slight bullate expression, sufficient to identify plants carrying the bullate gene with a high degree of accuracy. This suggests a dose-dependent expression, perhaps of a plant hormone or of a transcription factor. If a single recessive gene controlled the bullate leaf form in avocado, then one in four selfed seedlings would be expected to have bullate leaves. Unfortunately, it is very hard to control pollination in avocado, so it is not realistic to attempt it here (given time and energy constraints). The parent tree is a chance seedling, and there is no trace of bullate form in its leaves. As the fruit are black and pyriform, the parent is very likely to be either Hass or Maluma. I have only grown a relatively few Hass seedlings and haven't observed this mutation. It is not impossible that this is a new dominant mutation. The question will likely remain moot.

        parent avocado leaf  Avocado bullate leaf

Leaf of the parent plant (magnified).                                                                               Seedling with bullate upper leaf surface

Bernard, R. L.; Cremeens, C. R.; Rode, M. W.; and Wax, L. M. 1975. 'Research Notes: U.S. Regional Soybean Laboratory and University of Illinois, Urbana-Champaign'
Soybean Genetics Newsletter: Vol. 2, Article 13.
Available at: http://lib.dr.iastate.edu/soybeangenetics/vol2/iss1/13

Zhe Cao , Shunzhao Sui, Qian Yang and Zhanao Deng. 2016. Inheritance of Rugose Leaf in Caladium and Genetic Relationships with Leaf Shape, Main Vein Color, and Leaf Spotting
J. AMER. SOC. HORT. SCI. 141(5):527–534. 2016. doi: 10.21273/JASHS03854-16

Zhe Cao, Shunzhao Sui, Qian Yang, Zhanao Deng, 2017. A single gene controls leaf background color in caladium (Araceae) and is tightly linked to genes for leaf main vein color, spotting and rugosity,
Horticulture Research
, Volume 4, 2017, 16067, https://doi.org/10.1038/hortres.2016.67