Phytochrome-mediated stem elongation in response to crowding and vegetation shade is hypothesized to be a form
of adaptive phenotypic plasticity. The increased stem elongation is thought to allow the plants to place their leaves above their neighbors, increasing light interception. Light that has passed through a canopy of leaves has a reduced red to farred ratio (R : FR) (Smith 1982). Via the phytochrome family of photoreceptors, plants are able to detect this change in light quality and respond morphologically (Smith 1982). A typical shade avoidance response is characterized by dramatic stem elongation, reduced branching, and a redistribution of leaves to the top of the canopy (Smith 1982; Geber 1989; Weiner et al. 1990; Ballare´ et al. 1991; Weiner and Thomas 1992; Schmitt and Wulff 1993). Even before plants are directly shaded, light reflected off neighboring plants in a developing 1 Current address: Department of Plant Biology, 502 Life Sciences Building, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A.; e-mail
[email protected]. 2 Author for correspondence and reprints. Manuscript received September 1998; revised manuscript received February 1999.
dense canopy is sufficiently lowered in R : FR to elicit an elongation response (Ballare´ et al. 1987, 1990); thus, important functions of phytochrome may be to sense future competition for light and to trigger morphological and physiological responses to avoid future shading (Casal and Smith 1989; Schmitt and Wulff 1993). Recent studies have demonstrated that elongated plants have higher fitness in high-density conditions and nonelongated plants have higher fitness in lowdensity conditions, as predicted by the shade avoidance hypothesis (Schmitt et al. 1995; Dudley and Schmitt 1996). But while such studies provided strong support for the shade avoidance hypothesis, they did not explicitly test hypotheses about the costs and benefits of the stem elongation response that cause the fitness differences.
