The adaptive significance of ontogenetic changes in physiology: a test in Avena barbata.

Physiological changes with ontogeny are common in plants. Although ontogenetic changes are hypothesized to improve plant function, their adaptive significance has rarely been tested. Here, we estimated phenotypic selection on ontogenetic change in photosynthesis (A) and stomatal conductance (g(s)) of Avena barbata. We tested whether ontogenetic changes in A and g(s) increased fitness in wet and dry soil environments. To determine whether evolution in response to this selection would be constrained, we estimated the heritability of ontogenetic change in physiology, as well as cross-environment genetic correlations. Ontogenetic change in A, but not g(s), was adaptive in the wet soil environment; plants that maintained or increased A from the prereproductive to the reproductive phase had higher fitness. In the dry soil environment, ontogenetic change in A and g(s) was adaptively neutral. We detected significant genetic variation for ontogenetic change in A and g(s), but no cross-environment genetic correlations, suggesting that the evolution of these traits would not be genetically constrained. We demonstrate that ontogenetic changes in physiological traits can increase fitness but the adaptive value of these changes varies among traits and environments.