PHENOTYPIC PLASTICITY IN THE ANNUAL WEED POLYGONUM AVICULARE

Reaction norms of fourteen life history and morphological traits were investigated in four tetra- and two hexaploid genotypes of the annual weed species complex, Polygonum aviculare. The plants were cultivated in six treatments consisting of factorial combinations of three pot si zes and two fertility levels. All characters, except life span, were p lastic but the relative importance of genotype (G), treatment (T) and interaction (G x T) to total variance was strongly trait-specific. Con sistent genetic differentiation, not correlated with ploidy level, was found in metamer size and life history: genotypes originating from tr ampled sites had smaller metamers and shorter shoots while those origi nating from sites with a short growing season, due to weeding activiti es, had a shorter life span, an earlier flowering date and a higher bi omass allocation to reproduction compared to genotypes from less distu rbed sites. Significant variation was found in reaction norms for all characters, including a lower amount of plasticity in metamer size in genotypes with numerous metamers and a lower amount of plasticity in t otal weight in short-lived genotypes. This suggested that variation in phenotypic plasticity reflected developmental constraints imposed by contrasting life span and metamer size in different genotypes. There w as no evidence for niche differentiation along the soil resource gradi ent, suggesting that the species is comprised of ''general purpose'' g enotypes with respect to soil fertility. It is concluded that the Poly gonum aviculare complex has evolved a ''dual'' adaptive strategy i.e. a combination of genetic polymorphism and high phenotypic plasticity.