Energy balance implications of floral traits involved in pollinator attraction and water balance


For most angiosperms, flowers are critical to reproduction because they increase rates of outcrossing. Flowers are highly variable in numerous traits, including size, shape, and color. Most of this variation is thought to have arisen because of selection by pollinators. Yet nonpollinator selection is increasingly being recognized as contributing to floral trait evolution. Nonpollinator agents of selection that often oppose pol- linator selection include the physiological and resource costs of producing and maintaining flowers. Here, I (1) summarize recent studies on the macroevolution of floral physiological traits and (2) apply an energy bal- ance model to examine how two pollination traits (flower color and flower size) can interact with hydraulic traits to influence flower physiology. These modeling results show that under certain conditions flower color variation can overwhelm the effects of floral transpiration and flower size variation on flower temperature. Using a novel data set of flower size from the California flora, I show that the range of flower size most com- mon in the California flora is the range in which complex, nonlinear dynamics in flower energy balance occur. These results suggest that floral traits under selection by biotic agents can have large implications for flower physiology, and only some of these potentially deleterious effects can be offset by flower hydraulic traits. These complex interactions between pollination traits (flower size and color) and physiological traits (surface conductance to water vapor) suggest that a more unified framework for understanding the evolution of floral form and function would simultaneously consider the interaction between physiological traits and traits un- der biotic selection.

International Journal of Plant Sciences