9708114 Schmitt Many plant species exhibit characteristic morphological responses to crowding and vegetation shade, such as stem elongation and reduced branching. This "shade avoidance response" is cued by a reduction in the ratio of red to far-red wavelengths (R:FR) in light transmitted or reflected from green vegetation; this R:FR signal is perceived by light-sensing molecules called phytochromes. It has been hypothesized that phytochrome-mediated shade avoidance responses are beneficial to plants by enabling them to anticipate and avoid competition for light in dense stands. Recent results from this project strongly support this hypothesis. In a field experiment, stem elongation increased the reproduction of the common annual Impatiens capensis in dense stands, but plants induced to display the shade avoidance response at low density suffered reduced fitness relative to nonelongated plants, apparently due to an unmeasured physiological cost. The goals of the proposed research are to examine the genetic basis of the shade avoidance response and potential mechanisms for selection on the response in the field, and to test for physiological, ecological, and genetic costs of the response. Replicated inbred lines differing in the magnitude of morphological response to R:FR will be planted into a natural population at manipulated densities and group compositions to study the level at which selection acts on shade avoidance traits. Elongated and nonelongated seedlings will be grown under different levels of water and nutrients, to determine the conditions under which shade avoidance traits may be costly. This research will integrate genetic, physiological, and population biology approaches to the study of phytochrome-mediated shade avoidance responses in plants. It will provide important insights into the role of these responses in natural populations. The results will also provide an ecological and evolutionary context for current research by plant molecular biolog ists and developmental biologists on the underlying genetic basis for plant responses to light. Since most crops are grown in dense stands, this study will also provide insight into possible selection strategies for improvement of agricultural yields.
