Popular Science: Tug-of-war between parents and children: clonal spread boundaries
How do plants spread? Through seeds, of course - that is what most people would reply quite automatically. The truth, however, is that many plants also use much more effective mechanisms for flooding their surroundings with off-springs, e.g. stolons and rhizomes. (Charles University) Faculty of Science botanists have endeavoured to discover a clearly-set strategy behind this type of spread.
Underground shoots of clonal species constitute links between genetically identical individuals (ramets). Carex hirta, South Moravia. Photo by Jaroslav Vojta, Ph.D.Caption
Plants that spread through ramets are much more numerous than those that only spread through sexual reproduction, i.e. seeds, both in terms of the number of species (that is much larger than that of seed-spreading plants) and the quantity of produced biomass. This phenomenon - through which plants actually get round the benefits that sexual reproduction offers across all organisms - is seen as a big challenge by today's botanists. A satisfactory explanation of why this happens would no doubt earn its author an important place in the pantheon of modern botanical science, but although the world has already seen a number of theories, none of them seems to be strong enough to persuade the entire research community.
However, explaining the evolutionary and/or environmental reasons why plants spread clonally and describing this phenomenon as accurately as possible are two different ball games. Each plant's decision to grow and spread depends on the unique combination of local parameters, viz. resources availability and the presence of rivals in its immediate surroundings. As clonal plants produce a substantial quantity of biomass and tend to penetrate various man-influenced sites (dumps, abandoned fields) as the first colonizers, it is most desirable to have a model that describes the dynamics of such growth.
Clonal plants are everywhere. Phragmites australis clone fragment (location: fairly dry sediment formed in the upper stream of the Indus river; Ladakh, India, ca 3,500 m above sea level). Photo by Martin MacekCaption
Doing science based on unique cases is not easy, and Martin Weiser & Jan Smyčka of the Botanics Department, Faculty of Science, Charles University are well aware of that: Summarising their motivation to start their exploration efforts, Martin Weiser says: "Our key ambition was to create a clonal plant growth model for a situation where the immediate surroundings are completely homogeneous. The law that would describe such growth could then be used as Point Zero in describing real-life cases of clonal spread for each species in particular environments". The results of Weiser & Smyčka's work have been summarised in an article published in Proceedings of the Royal Society B, the Royal Society's premier international biological research journal.
And what was their initial concept all about? In clonal spread, a newly formed ramet is a desired result on one hand and a rival on the other, just like babies are for their animal (including human) parents. Babies compete with their parents for food that usually comes in scarce quantities. The main problem the parent plant needs to address when producing a baby plant is to find the optimum distance to which it should place its clonal off-spring. This distance, then, has an impact on the speed at which the plants grow on their respective sites.
The best distance between ramets (D) is one at which the sum of the (i) "shoot production costs" (S) and (ii) the loss caused by root system overlaps (C) is as low as possible. The space taken up by the respective root systems has been simplified to form the cap of a prolate spheroid whose depth and diameter are (b) and (a), respectively. The shapes and sizes of ellipsoids vary, depending on the particular situation (e.g. accessibility of nutrients). Martin Weiser
The plants' "decision-making" is affected by two factors. On one hand the off-spring should not be too far, because the investment in the shoot growth is not low. The longer the shoot, the higher the plant's investment. On the other hand, if the baby plant is too close to its mother, their root systems will compete to get hold of valuable nutrients. The result of such hidden fight can be quantified, and the "shared currency" which such numerically quantified value refers to is the absolute volume of nutrients to be won. The new model is expected to predict the optimum, i.e. the best win-win situation for the "mother" and the "baby". These two contrary pressures should result in a very general optimum strategy applicable to all types of plants (across all taxonomic groups) that spread through spacers, whether underground or above-ground ones. Obviously, the results in environments abounding in nutrients and those where the nutrient supply is low will be different.
Every hypothesis and every theoretical model needs to be tested by confronting it with reality. "We did not do any experiments to test the model; instead, we made use of two existing data sources: the well-tried Ellenberg Indicator Value Database that describes plants' demands for their surroundings and CLO-PLA, clonal spread database that describes the plant spread dynamics," says Martin Weiser, and goes on to say: "Then we correlated these two data sources".
And the conclusion drawn by the researchers? "Our neutral model has proven to apply well to plants that spread through above-ground stem derivatives. On the other hand, this model does not seem to be really substantive in describing the situation of plants that produce their own clones by means of underground shoots," says Martin Weiser. Why is that so? "Underground organs such as rhizomes play more roles than just mere spreading. For example, they are used as storage reservoirs for metabolites and also as bud sources. These additional functions seem to be taking the growth dynamics outside the predictions offered by our model," adds Mr. Weiser.
In conclusion, we can say that all researchers to model clonal plant growth mechanisms in the future can rely on the model designed by our botanists, and each deviation from this model will require a case-specific explanation to highlight e.g. some special features of the plant concerned or of its immediate surroundings. In other words, such explanations will be expected to describe phenomena that have been hidden from scientists so far.
Michal Andrle
Published:
Apr 21, 2015 10:50 AM
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