Friday, February 8, 2013

The Question of Environmentally-Induced Variation

In previous posts I may have given the impression that Portulaca are morphologically-unstable, chameleon-like plants, whose frequent mutations allow them to occupy a huge range of habitat types. This was never my intention.

Salinity clearly does influence the morphology of some species or subspecies. But salinity-adapted (or salinity-modified) populations are generally made up of individuals that have consistent and stable characters in that situation. If a population of a different variety or subspecies is encountered at the same location, it too is generally made up of individuals with consistent characters. I have raised the possibility of "intergrades" or "intermediates" in some situations, but I have never been able to prove their existence with any certainty. If they do occur they are probably rare and may well turn out to be distinct taxa in their own right!

I should also state that it is rare to find more than one variant of a Portulaca species at any given location.

Whenever variants of P. oleracea have been grown side by side in cultivation, they have for the most part retained the characters that made them distinct in habitat. Surprisingly, even plants that came from very saline habitats (and their offspring) have often retained enough of their characters in cultivation to suggest that their characters are genetically fixed and unique. If the differences that define a taxa are consistent and can be duplicated, this raises the possibility that these populations may be undescribed varieties, subspecies, or even species.

A variant may still be in the process of evolving, in which case it may be difficult to separate from an already described taxon. This "incomplete evolution" may also provide a reason why many halophytes do not die when they are grown in non-saline soil and provided with rainwater. Some highly specialized halophytes may, on the other hand, become etiolated and pine away if transplanted into a non-saline environment. Others may be less specialized, so will be capable of making the sudden transition to non-saline conditions. These plants may show only limited changes to their physical attributes after changes occur in their environment. The physical modifications that allowed them to cope with salinity may even be retained for many generations in their offspring. In some desert areas, this kind of adaption may give the variant a distinct advantage over any others, as it will be able to survive the transition between the less saline wet season and the more saline dry season. On the other hand, plants that are salinity specialists run the risk of dying out completely during a prolonged wet season, unless they have other strategies in place, such as seed dormancy.

Environmental factors like climate, salinity, and soils (in the long term), along with light, moisture, temperature, and nutrient levels (in the short term), definitely influence the size and vigour of many plant parts. In Portulaca, an increase in one or more of the short-term environmental factors can induce the formation of larger leaves and stems, as well as longer and more numerous branches and roots. However my observations of cultivated specimens suggest that although such changes do occur, the changes are consistent and occur within a range of what can be expected for each of the variants. Furthermore, flower size in Australian Portulaca does not appear to be greatly influenced by short or long term environmental factors. A long period of heat, moisture and sunlight, may increase flower size in some species, such as P. australis and P. intraterranea, but this is something that requires further study.

In the wild, most Australian species will only flower in bright, warm, well-drained situations, and most species occur on nutrient-deprived, gravelly soils. The exception of course is P. oleracea, but apart from an increase in leaf size and branch length, the addition of extra light, moisture and nutrients does not appear to influence the size of the flowers in that species to any significant degree  Likewise, an increase or decrease in any of these environmental factors does not appear to influence the length and number of stamens in any of the species I have studied.  I doubt too that the seed texture will change simply due to a sudden change in the environment. Seed size may of course be reduced by drought, disease or predation, or if the temperature suddenly falls below 10 degrees Celsius for a prolonged period of time. In these cases the seed has most likely been aborted. (Disease symptoms cannot be used as a tool to differentiate taxa!)

I have however noted a significant change in flower size and leaf shape due to changes in the environment in some exotic species, in particular those from the Americas. For instance, P. cryptopetala will often produce tiny flowers about 5 mm diameter with few stamens early in the season, or if plants are positioned in cool conditions or in heavy shade. The flower size increases to about 30 mm diameter as the plants mature in full sunlight. The stamens also become more numerous. Furthermore, the leaves on juvenile plants are often ovate-oblanceolate, but become linear-lanceolate and channeled with somewhat acute tips as the plant matures.

In cool, shaded situations, the exotic species P. pilosa and P. grandiflora can have smaller flowers and develop long, sparse and spindly branches and leaves. But I have not seen any shade-induced variation in these species that was unexpected. The plants can still be easily identified as "shade forms" of their respective species. The same can be said for our native P. filifolia, which is sometimes found in the partial shade of trees and bushes.

Most Australian species will simply fail to grow or flower if positioned in heavy shade. So it may be difficult to determine the influence of shade on the morphology of many native species, except perhaps to say that heavy shade kills them!

In plant populations there is always, as a general rule, more variation evident in the size and shape of the vegetative parts than the flowering parts. The Linnaean System is largely based on this premise. A botanist friend tells me that "Molecular biologists call the gene sequences involved in reproduction "highly conserved"." If anyone is interested, the concepts of "conserved sequences" and "conserved genes" are outlined here.

The apparent "widespread morphological variation" in some of our native Portulaca species is likely masking the existence of a number of distinct taxa. It would be a shame if any potential taxa are not being recognized. A lack of clearly defined, clearly delineated variation in a species or genus can potentially lower the biodiversity in many ecosystems. In the case of rare taxa that remain unrecognized and undescribed, the risk of undocumented extinction events is ever present.

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