Schmitt 9806858 Many plant species exhibit characteristic morphological and physiological responses to crowding and vegetation shade. This "shade avoidance" response is cued by a reduction in the ratio of red to far-red wavelengths (R:FR) in light transmitted through or reflected from green vegetation. This R:FR signal is perceived by light-sensing molecules known as phytochromes. It has been hypothesized that phytochrome-mediated shade avoidance responses are beneficial to plants because it enables them to anticipate and avoid competition for light in dense stands. Recent results from a related project strongly support this hypothesis. In a field experiment, phytochrome-mediated stem elongation increased the reproduction of the native annual Impatiens capensis (jewelweed or touch-me-not) in dense stands, but plants induced to display the shade avoidance response at low density suffered reduced fitness relative to non-elongated plants, apparently due to an unmeasured physiological cost. The objectives of the proposed research are to investigate the possible ecological, physiological, and genetic mechanisms underlying this cost, and to quantify the frequency and pattern of natural selection on shade avoidance traits within a natural population. A specific goal is to determine whether the shade avoidance response results in increased damage from ultraviolet-B radiation due to a lack of protective pigments, and to test for genetic variation in morphological responses or sensitivity to UV-B. To address these questions, elongated and non-elongated seedlings from replicated inbred lines will be produced by manipulation of the R:FR cue, and planted into a series of treatments in a natural population and an experimental garden. The results of this project will provide important information about the role of phytochrome-mediated shade avoidance responses in natural plant populations, and will provide an ecological and evolutionary context for current research on the molecular, genetic, developmental, and physiological mechanisms underlying plant responses to light. It will also help to inform selection strategies for crop improvement and provide an ecological context for recent proposals to increase yield by genetically engineering crops in which shade avoidance responses are blocked. The results will also be valuable for understanding how wild plants will respond to increasing UV radiation levels during the current period of rapid global change.
