Phenotypic Variation and Plasticity in the Colonizing Species Xanthium Strumarium L. (Noogoora Burr)

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Abstract
Levels of genotypic (.sigma.G2) and environmentally induced (.sigma.E2) variation for 15 quantitative characters were estimated in populations of the 4 naturalized races of X. strumarium in Australia. Estimates of .sigma.G2 indicated that populations of X. strumarium were often genetically variable for quantitative traits. For the majority of the characters studied, .sigma.E2 was a larger component of the total phenotypic variation than was .sigma.G2, indicating that phenotypic plasticity is the major mode of adaptation of this species to variable and varying environments. Few significant differences were found among the races, or among populations within a race, in either .sigma.G2 or .sigma.G2. Marked differences in colonizing ability of the 4 races of X. strumarium are probably not due to differences in phenotypic plasticity (individual buffering) or genotypic variation (populational buffering) but to differences in such factors as their reproductive strategies and photoperiodic requirements for flowering.

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