Managing oxalis obtusa seed pods? (Rick Buell)

Christiaan van Schalkwyk christiaans.garden@gmail.com
Mon, 09 Feb 2015 13:13:35 PST
On 2015-02-09 08:11 PM, Tim Eck wrote:
> Is heterostyly the only barrier to self-fertilization?  If so, a lot could
> be accomplished with a camel hair brush.

I've been real quiet lately, and debated whether I should chime in on 
this topic, alas I think my poorer part won, so here is a lengthy reply...

(South African) Oxalis species employ a system of tristyli, that is 
there are three whorls of styles, two of them bearing anthers and the 
third the stigma (5 to a whorl). If the uppermost whorl contains the 
stigma, it is referred to as a longistylar form, mediostylar for the 
middle and brevistylar if at the bottom. Not all Oxalis species seem to 
employ tristyli, but it is common in the bulbous forms. (Most Oxalis 
from the America's are either weedy, i.e. without a bulb, or have more 
or less a rhizomatous underground structure. Most (all) South African 
Oxalis have true bulbs, and tristyli is the rule for sexual 
reproduction. (I presume that there might be exceptions, as always)

Only flowers with two different stylar forms can be pollinated to form 
seeds. Thus if you have two different clones of the same species, but 
both are of the same stylar form, they are sterile towards each other. 
Often only one stylar form is available (for example because only one 
plant/bulb is collected or because one stylar form exhibits better 
characteristics from a cultivator's point of view (i.e. Bigger, 
brighter, taller flowers), or the one stylar form is more vigorous than 
the other and the weaker does not get distributed). Personally I think 
this is an excellent state of affairs, because it precludes seed forming 
which could otherwise turn quite a few species real weedy real quick. ( 
I am still battling to get rid of O. haedulipes. This one is " a local" 
and managed to get into nearly every pot of plants I have. Luckily the 
bulbs are quite distinctive and can be culled during transplanting). 
Also think of the pest O. pes-caprae have become in some parts of the 
world, and that only by vegetative reproduction. Think of the havoc if 
it could set seeds as well).

Some research has suggested that the pollen size of the different stylar 
morphs differ, whether it has been proven conclusively, I do not know. 
Whatever the case, heterostyli might be the only barrier to self 
fertilization, but alas, a camel hair brush is not going to break 
through it that easily.

To complicate matters, the basic number of chromosomes vary widely 
between species, with 5, 6, 7, 8, 9 10, 11, and 12 being possibilities. 
A multitude of ploidy levels are available, sometimes quite restrictive 
in a particular species, but, as in the case of O. obtusa, about 
anything is possible(from 2n to 8n plus?). Even plants from the same 
locality can display different ploidy levels and are thus incompatible. 
I once saw three very divergent forms of obtusa growing intermingled 
with each other without any obvious hybrids inbetween. Salter mentions 
more than 10 species growing and flowering together in nature, without 
any hybrids seen.

Hybrids between species are unknown. Maybe because of lack of effort and 
interrest, or maybe because there are too many factors inhibiting 
pollination. Even when my collection exceeded 80 different accessions of 
O. obtusa (and about 400 for Oxalis in total), natural seed set were 
rare, and even hand pollination yielded poor results (bad ploidy 
selections, I think). The only other Oxalis seedlings I've seen (other 
than obtusa, haedulipes and that pesky corniculata), are from O flava 
var fabifolia and O. helicoides. I've heard of O. hirta forming seeds, 
but it does not grow well for me (actually I do not think I've got any 
left).

Nhu mentioned that Oxalis seeds are very short lived. More than 60% of 
the South African species does not have endospermous seed. The need to 
either fall in a suitable place or be planted within 24hours to grow. 
The tiny seedling with two lobes and a small root gets ejected from the 
seedpod, it starts to grow immediately and it forms true leaves within 
two days.

O. obtusa does have endospermous seed, and it flowers close to the end 
of the growing season. I suspect they might last longer, and may even 
germinate the next season, but I have not investigated it or read about 
it. Maybe a little experiment is called for....

I think this is enough rambling for one night

Happy Gardening (or snow shoveling?)

Christiaan

South Africa




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