Spinescence in the pea genus Astragalus, part 1
(writing in progress)
I have known spinescent peas (Fabaceae) for more than half a century. This started in the Fynbos Biome in South Africa, where various spp. of e.g. Aspalathus (typically ‘fireweeds’ in nutrient-poor, fire-prone, semi-sclerophyllous vegetation) are foliar-spinescent.
And of course I well know various spinescent mimosas such as Vachellia, with their stipular or epidermal spines.
However, it was for the first time in my life that I recently encountered a new configuration of spinescence in legumes - and indeed an unusual configuration of spines for any lineage of plants.
I refer to a spinescent rhachis, and what is more, in some cases, a spinescent rhachis that persists as a functional spine after the pinnae are shed.
Now that I have encountered this form of spinescence, it seems so obvious that plants would have this configuration of physical defence available to them. But I have never seen it before as far as I can recall.
Astragalus is the largest genus of plants on Earth, containing perhaps up to 3000 spp. in Eurasia, North and East Africa, and the Americas. Iran alone has about 850 spp. In the checklist of spinescent plants of Israel (), there are seven spp., all of which I show below.
Astragalus sharpens the mind about the nature of spinescence. This is because it is so novel from the perspective of a plant ecologist based in fynbos, kwongan, or any other type of vegetation in Australia or New Zealand.
I have only just begun my learning curve with this genus, and it may turn out that there are several different forms of spinescence in Astragalus. The form for which I have evidence is the spinescent rhachis, and even this one form has already proven to be somewhat complicated/multi-facetted.
As far as I have been able to establish so far, all spp. of Astragalus have pinnate leaves. However, some (perhaps several hundred spp. in total) have converted the tip of the rhachis into a spine. Since the rhachis is part of the leaf, this would, at first glance, seem to qualify as a foliar spine.
For a start, I find this phenomenon new to me: the rhachis-tip as spine.
I have encountered many plants with foliar spines, and the configurations are varied. But in general I have not regarded the typically pinnate leaves of peas (Fabaceae) as candidates for foliar spinescence. This is because it is in the nature of the typical pea leaf that it is soft, non-sclerophyllous, and defended by toxins rather than lignification and ‘pungency’.
Yet here, in Astragalus, we find that counterintuitive combination, the pinnate-pea leaf made spiny, without lignification (sclerophylly) of the leaf-blades themselves. Because the rhachis functions mainly as a structural support (any photosynthesis in the rhachis being a subsidiary function), it is ‘pre-adapted’ to be rigid even in those plants lacking any sclerophylly, such as the kinds of plants living in ecosystems capable of supporting dense populations of herbivores.
In Australia, there is noteworthy genetic plasticity in the pea genus Bossiaea, with several different types of spines in various spp. And Bossiaea does occupy some relatively nutrient-rich, relatively fire-free habitats such as the margins of salinas, where it has converted the twig-tips into green spines. However, even Bossiaea has not come up with the idea of a spinescent rhachis.
Part of the explanation for the unusual evolution of a spinescent-tipped rhachis in Astragalus seems to be the overall form of the shrub, which tends to be hummock-like in partial convergence with e.g. Australian hummock grasses (Triodia). The hummock form, combined with the hedgehog-like armature of spinescent rhachises projecting from the foliar surface, makes for an effective defence against herbivores in phrygana- or batha-type vegetation. This is the sort of vegetation that has been grazed to within an inch of its life for thousands of years in the overgrazed semi-arid ‘pastures’ of the Middle East.
So here we have a syndrome of spinescence quite different from any we see in Australasia, and associated with the ungulate fauna of the Northern Hemisphere.
In my comparisons of the Australian flora with that of the Fynbos Biome in South Africa, I have noticed that the peas show foliar spinescence even where few other plants show foliar spinescence. So, the fact that Astragalus, being a genus of peas, is foliar-spinescent is not that surprising to me.
However, it is the nature and derivation of these spines, plus the hummock form of the spinescent spp. of Astragalus, that is new to me. This provides some sort of parallel with my comments, in another Post, re the plumbaginaceous genus Acantholimon, which seems convergent with Astragalus in Israel in its combination of a cushion form and foliar-spinescence.
Please note that there are cushion-like plants in Australia. However, but as far as I know these are not foliar-spinescent. They thus differ from Acantholimon and Astragalus in e.g. Israel. In the case of New Zealand, one possible parallel lies in the apiaceous genus Aciphylla, with which I am not familiar.
There are hummock plants in Australia, but they tend to be pyrophilic grasses with foliar-spinescence, not pyrophobic peas with foliar spinescence as in the case of Astragalus in Israel. And there are foliar-spinescent peas in Australia, but these do not have the rhachis as the spinescent plant-part as in Astragalus in Israel.
Given that the rhachis is a kind of leaf-stem, and that it is a relatively non-photosynthetic part of the leaf, is a spinescent rhachis truly part of the leaf, functionally, or should it rather be regarded as a kind of spinescent stem? I am unsure.
I have observed in photos that the spinescent part of the rhachis, in some spp. of Astragalus, is non-green, much like a stipular spine in acacias. Furthermore, in some spp. of Astragalus there seems to be a pattern in which the rhachis is initially green, but persists in non-green form after the pinnae are shed.
Normally in peas and other legumes, when a pinnate or bipinnate leaf is shed, the rhachis is shed as well as its pinnae or pinnules. However, in some spp. of Astragalus, the shedding seems confined to the pinnae, the rhachis remaining as a spine functionally similar to a non-green stipular spine.
I do not know if such non-green, persistent rhachises, functioning as spines, are dead or alive. But in any species of Astragalus in which the main spinescent function of the rhachis is performed after the pinnae have fallen, I would tend to consider this as a form of nodal spinescence rather than foliar spinescence.
So, my current thinking is that the genus Astragalus does contain both foliar spines and nodal spines. However, as far as I know the seven Israeli spp. of spinscent Astragalus are all foliar-spinescent.
The following outlines the biogeography of genus Astragalus:
https://www.jstor.org/stable/4098954?seq=1#page_scan_tab_contents
The following shows the pinnate leaves of a non-spinescent species of Astragalus. The rhachis is not as bright a shade of green as the pinnae, but it is functionally part of the leaf.
https://www.google.com.au/search?q=astragalus+deinacanthus+spiny+rhachis&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjmh4nNgfniAhWJX30KHfLnAWIQ_AUIECgB&biw=1093&bih=514#imgrc=8WUkYAuCcL6vOM:&spf=1561066318726
The following paper describes Astragalus orientopersicus of Iran:
http://sciencepress.mnhn.fr/sites/default/files/articles/pdf/a2016n1a3.pdf
The following excerpt from the above paper, describing the leaves of A. orientopersicus, shows how unclear the typical botanical description is w.r.t. the origin and nature of the spines. Here we have confusion between the petiole and the rhachis, and the word ‘pungent’ is used instead in description of the pinnae, which I doubt are actually spinescent. In the abstract of the paper, the rhachis is clearly described as spiny, but in the body of the paper, i.e. in the full description, this simple fact gets garbled by ambiguous wording.
What the author of the above taxonomic paper does not seem to mention is a feature evident in the photos he himself uses in his paper: the green rhachis in this species tends to lose its pinna but remain attached to the plant as what is effectively a green spine.
The following sp. (Astragalus humillimus) of semi-arid Colorado has spinescent ‘petioles’, which seem likely to be the spinescent rhachis retained after the pinnae are shed.
https://cnhp.colostate.edu/rareplants/pdfs/23918.pdf
The following close-up of Astragalus humillimus of the USA shows the non-green rhachis acting as a spine, presumably after the pinnae have been shed and the originally greenish rhachis has lost its chlorophyll. I have not yet noticed this pattern of spinescence in any of the spinescent spp. of Astragalus in Israel (see below), and I would tend not to classify the spinescence of A. humillimus as foliar spinescence despite the fact that it is the rhachis that is spinescent and the fact that the rhachis is certainly part of the leaf.
https://cnhp.colostate.edu/rareplants/images.asp?id=23918&pid=2530
https://cnhp.colostate.edu/rareplants/guide_print.asp?id=23918
The following material all illustrates the various spinescent spp. of Astragalus occurring in Israel.
Astragalus angustifolius, which forms hummocks:
Astragalus angustifolius showing the spinescent rhachis of the pinnate leaves. In particular, please note that a) the spine of the rhachis hardly projects beyond the pinnae, and b) even the spine of the rhachis remains green. Hence I can catgorise this as foliar spinescence despite this being such a different configuration of spinescence from any of the other foliar spines that I’ve mentioned in the Australian flora. Astragalus has come up with foliar spines of a quite new kind, relative to its pea relatives in Australia, relative to all cushion- or hummock-like plants in Australia, and indeed relative to all plants in Australia. I associate this with the selective pressures exerted by dense populations of ungulates in the habitats of Astragalus.
https://www.west-crete.com/flowers/astragalus_angustifolius.htm
The photos of Astragalus bethlehemiticus in https://flora.org.il/en/plants/astbet/ show clearly enough that it is the long rhachis that ends in a spine.
Astragalus cephalotes: https://www.youtube.com/watch?v=92xnhr5oh3U seems not to have a growth-form similar to a cushion or hummock. It is listed in the Israeli website as having spinescent leaves, but I have yet to figure out exactly where the spines are.
Astragalus cephalotes:
Astragalus coluteoides:
http://flora.org.il/en/plants/ASTCOL/
Astragalus coluteoides:
http://portugal.inaturalist.org/taxa/736758-Astragalus-coluteoides
Astragalus coluteoides, showing the spinescent rhachis. Again, I have little hesitation in categorising this as foliar spinescence, because the spine itself remains basically green.
http://www.wildflowers.co.il/russian/picture.asp?ID=11179
Astragalus cruentiflorus:
http://flora.org.il/en/plants/ASTCRN/
Astragalus cruentiflorus, showing spinescent rhachis. In this case, the spiny tip of the rhachis does project some way beyond the pinnae, and it has lost its green colour. However, I would still be inclined to categorise this as foliar spinescence rather than nodal spinescence.
http://flora.org.il/en/plants/ASTCRN/
Astragalus cruentiflorus, showing spinescent tip of rhachis, close-up.
http://www.wildflowers.co.il/arabic/picture.asp?ID=3277
Astragalus cruentiflorus, showing hummock shape of shrub. I do not know of any Australian pea that adopts this growth-form, less still one that is foliar-spinescent, less still one that uses the rhachis tip as the spine.
I have yet to make out the exact nature of the spinescence in the Israeli species Astragalus deinacanthus:
https://flora.org.il/en/plants/ASTDEI/
The following illustrates Astragalus echinus:
https://flora.org.il/en/plants/ASTECH/
The following illustrates Astragaus gummifer:
https://www.turkiyebitkileri.com/en/photo-gallery/view-album/3028.html
(writing in progress)
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