Abstract
Pistia stratiotes is an aquatic macrophyte that grows in temporary-ponds in the southern Pantanal, Brazil. It reproduces both sexually and asexually and is usually observed forming dense mats on the water surface, a condition favored by the plant's vegetative reproduction coupled with an ability for rapid growth. In this study we examined the effect of densely crowded conditions on the production of reproductive and vegetative structures. In addition, we verified whether there is a trade-off between clonal growth and investment in sexual reproductive structures, and whether there is an allocation pattern with plant size. Individual plant biomass and the number of the rosettes producing sexual reproductive structures and vegetative growth structures both increased with density. Increase in plant size resulted in increased proportional allocation to sexual reproductive structures and vegetative growth structures. Allocation of biomass to reproduction did not occur at the expense of clonal growth. Thus, the density response appears as a increase of rosettes producing sexual reproductive structures and vegetative growth structures. Therefore, long leaves and stolons may be adaptive under densely crowded conditions where competition for light is intense. An important aspect in the study of trade-offs is the size-dependency of the allocation patterns .Usually, larger plants produce more biomass. Therefore, larger plants can allocate more biomass to both vegetative and sexual reproduction than smaller plants and thus show a positive correlation between both traits rather than the expected negative one.
Highlights
The concept of trade-offs between traits is crucial to development of a theory of life history evolution (Stearns 1989)
(1) Does densely crowded conditions affect length of leaves and stolon, biomass of leaves and reproductive strutctures (RS)? (2) Do trade-offs occur between sexual reproduction and clonal growth? (3) Do allocation patterns vary with plant size?
They are consistent with Abrahamson (1975) prediction that clonal plants should shift to sexual reproduction when density becomes high and the survival probability of further ramets produced locally becomes low
Summary
The concept of trade-offs between traits is crucial to development of a theory of life history evolution (Stearns 1989). There is considerable evidence that plant populations differ with respect to the proportion of biomass allocated to reproductive structures in individual plants. Reproduction in water plants consists of both sexual and asexual mechanisms and possesses three allocation options: growth, vegetative propagation and sexual reproduction. Several studies have focused measuring the partitioning of biomass to different plant structures, and variations in reproductive effort have been considered as functions of differences in density (Abrahamson 1975, Thompson and Beattie 1981). Changes in relative allocation to vegetative propagation and sexual reproduction are sometimes associated with changes in plant size caused by varying plant density (Hartnett 1990, Méndez and Obeso 1993). It reproduces rapidly through stoloniferous offshoots and is generally considered an aquatic weed (Sculthorpe 1967), with a world-wide distribution. Three specifc questions were addressed. (1) Does densely crowded conditions affect length of leaves and stolon, biomass of leaves and reproductive strutctures (RS)? (2) Do trade-offs occur between sexual reproduction and clonal growth? (3) Do allocation patterns vary with plant size?
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