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

1. Genetic variation, plasticity and genotype x environment interactio n have been studied in four populations of Arabidopsis thaliana expose d to a set of treatments including one 'optimal' and three 'stressful' conditions. 2. We found strong phenotypic plasticity for all nine tra its measured, highly significant genetic variation among populations f or all traits except for two of those directly related to reproductive fitness and genetic variation for the plasticity of phenological, veg etative, early size and one fitness-related character. 3. Flowering ti me and life span formed a distinct covariance set clearly separating t he populations into late, intermediate and early flowering (an ecologi cally important aspect of A. thaliana phenology). Uni- and multivariat e analyses of variance revealed that more within-treatment genetic var iance for traits tended to be expressed under 'stressful' conditions t han in the 'optimal' treatment. 4. However, we suggest that the freque ncy of occurrence of the environments might be more important than the ir 'stressfulness' in determining genetic and phenotypic changes. Comp arison of the components of phenotypic variance measured in a series o f unordered treatments with those components measured under several or dered treatments (from a parallel study) revealed that the amount of g enetic variation for plasticity tends to be higher in the set of unord ered treatments.