REACTION NORMS OF ARABIDOPSIS .2. RESPONSE TO STRESS AND UNORDERED ENVIRONMENTAL VARIATION

1. Genetic variation, plasticity and genotype x environment interaction have been studied in four populations of Arabidopsis thaliana exposed to a set of treatments including one 'optimal' and three 'stressful' conditions. 2. We found strong phenotypic plasticity for all nine traits measured, highly significant genetic variation among populations for all traits except for two of those directly related to reproductive fitness and genetic variation for the plasticity of phenological, vegetative, early size and one fitness-related character. 3. Flowering time and life span formed a distinct covariance set clearly separating the populations into late, intermediate and early flowering (an ecologically important aspect of A. thaliana phenology). Uni- and multivariate analyses of variance revealed that more within-treatment genetic variance for traits tended to be expressed under 'stressful' conditions than in the 'optimal' treatment. 4. However, we suggest that the frequency of occurrence of the environments might be more important than their 'stressfulness' in determining genetic and phenotypic changes. Comparison of the components of phenotypic variance measured in a series of unordered treatments with those components measured under several ordered treatments (from a parallel study) revealed that the amount of genetic variation for plasticity tends to be higher in the set of unordered treatments.