Phenotypic plasticity is the ability of an individual to respond in the short term to environmental conditions. It is not genetically based. Species that are phenotypically plastic are often able to survive and reproduce under many environmental conditions. Plasticity can be used to take advantage of a sudden, temporary improvement in environmental conditions or to avoid stressful environments (Meerts, 1995; Sans and Masalles, 1997; Wulff et al., 1999; Weinig, 2000).
In general, weeds are thought to have a high degree of phenotypic plasticity, although there are relative few studies that clearly demonstrate this (D.R. Clements et al., unpublished observations). Pheno-typically plastic weeds are able to mature and reproduce under a broad range of environmental conditions. For example, showy crotalaria (Crotalaria spectablis) can reproduce in heavy shade even though it is substantially smaller in the shade (Patterson, 1982). This species can also produce seed under a range of temperatures in spite of decreased size and biomass. Barnyardgrass (Echinochloa crus-galli) showed extreme plasticity among six cohorts planted from March to September. While vegetative biomass was over 3000 g in early cohorts and less than 25 g in a later cohort, late cohorts still flowered - although the number of flowers/plant was reduced from 10,000 to less than 100. Although the allocation of resources to reproduction may be genetically controlled, it is influenced strongly by the environment (Bazzaz et al., 2000).
Other weeds exhibiting phenotypic variation are dandelion, jimsonweed (Datura stramonium) and velvetleaf (Clements et al., 2002). Two similar weeds, wild oat (Avena fatua) and slender wild oat (Avena barbata), rely on different mechanisms to respond to environmental heterogeneity (Marshall and Jain, 1968). Wild oat is more genetically variable, and this allows it to persist in many types of environments. Slender wild oat is more genetically uniform but is able to persist in a variety of habitats because it is phenotypically plastic. Wild oat is genetically variable and thus populations contain many genotypes, some of which will be able to survive and reproduce in the ambient environment. Thus genotypic variation and phenotypic plasticity are two mechanisms to deal with environmental stress and heterogeneity.
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