Phenotypic responses of the twining vine Ipomoea purpurea (Convolvulaceae) to physical support availability in sun and shade

Vines depend on external support to prevent shading by neighbouring plants. Hence, it is important to determine whether shading enhances the phenotypic responses of vines to support availability. I evaluated the consequences of support availability (a vertical stake) on shoot and leaf traits of the morning glory Ipomoea purpurea (Convolvulaceae) under full sunlight and extreme shade. It was hypothesised that phenotypic responses of vines to support availability should be greater in the shade. In addition, to investigate possible constraints to such phenotypic responses, the correlations among phenotypic traits and the plasticity of such correlations were evaluated. The phenotypic variation of the main stem length and of the number of branches was consistent with the hypothesis, i.e. greater responses to support availability in the shade. In contrast, both internode length and leaf area (two traits that showed a significant and positive correlation) decreased in the sun and increased in the shade with support availability. Petiole length decreased with support in the sun but had no response in the shade. On the other hand, the number of significant trait correlations found in plants in the sun and supported plants was higher than those of shade and non-supported plants, respectively. Several of the correlations were significantly sensitive to the environment. Flowering only occurred in the sun treatment. Whereas no shoot or leaf trait was significantly correlated with flower number in supported plants, both petiole length and shoot biomass showed a significant correlation with such estimate of plant fitness in non-supported plants.