Theoretical treatments of the evolution of adaptive phenotypic plasticity commonly invoke a cost of plasticity as the force that limits further evolution of a particular plastic response. To date, there is little empirical support for the existence of such costs. One way in which a particular plastic response could be costly is if it disrupts the expression of another trait or the plasticity of another trait. We are examining the possibility that compromises between plasticity in leaf morphology and in flower morphology in a single species limit the evolution of adaptive plasticity in both traits. The perennial violet, Viola septemloba exhibits seasonal leaf and flower dimorphism in response to day length. Individuals produce primarily entire leaves during the fall and winter and switch to the production of deeply lobed leaves during the spring and summer. Lobed leaves are able to maintain average mesophyll temperatures nearly 3 degrees C lower than entire leaves during the hot summer. Thus heterophylly appears to be adaptive plasticity in response to seasonal environmental variation. Individuals of this species also exhibit seasonal variation in flower morphology. The production of open (chasmogamous) flowers occurs simultaneously with increasing production of lobed leaves during the late winter, and the production of closed (cleistogamous), obligately selfed flowers during the mid-summer and fall corresponds to increasing production of entire leaves. We have raised replicates of inbred lines of V. septemloba under long and short days and demonstrated heritable variation in response of leaf and flower morphology to day length. Preliminary results also suggest a genetic covariance between the plastic responses of the two traits, which could underlie a cost of plasticity. We will illustrate how simultaneous consideration of adaptive plasticity in multiple traits can contribute to understanding the existence and mechanistic basis of costs that limit the evolution of adaptive plasticity.

Key words: cleistogamy, cost of plasticity, heterophylly, phenotypic plasticity, Viola septemloba