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Negative associations between individual life‐history traits of an organism are referred to as life‐history trade‐offs (Stearns 1992; Zera et al. 1998). The existence of costly trade‐offs is thought to have favored the evolution of phenotypic plasticity as a mechanism through which organisms can account for environmental heterogeneity while modulating costs and benefits incurred by fixed allocation to competing functions (Bradshaw 1965; Stearns 1992; Sultan 1995; Dudley and Schmitt 1996; Pigliucci 1996). In plants, the ability to modify stem elongation in response to environmental cues appears to be a classic form of adaptive phenotypic plasticity (Sultan 1995; Gedroc et al. 1996). Plastic increases or decreases in stem elongation rate in plants can be triggered by light‐quality and light‐quantity cues (Smith 1982; Ballaré et al. 1989, 1990; Rajapakse and Kelly 1995; Petit and Thompson 1997) and exposure to mechanical stimuli (e.g., wind) (Gartner 1991; Jaffe and Forbes 1993; Emery et al. 1994; Cipollini 1997b). Increased stem elongation in response to light cues can enable plants to escape shading by neighbors and capitalize on heterogenous resources, while reduced elongation in response to mechanical cues can enable plants to resist mechanical damage (Ballaré et al. 1991, 1995; Jaffe and Forbes 1993).