Physiological mechanism of population differentiation in shade‐avoidance responses between woodland and clearing genotypes of Impatiens capensis

Forest understory plants often respond less intensely to reduced ratios of red to far red (R : FR) light, an important signal of foliage shade, than conspecific or congeneric plants from open‐canopy sites. Reduced responsiveness to low R : FR in plants from closed‐canopy sites could be caused by two physiological mechanisms. First, closed‐canopy plants could have less sensitive shade‐avoidance responses to low R : FR. Second, the high irradiance response to FR (FR‐HIR), which allows seedling de‐etiolation under low R : FR, might be stronger or persist longer after de‐etiolation in closed‐canopy plants, thus counteracting shade‐avoidance responses to low R : FR. These hypotheses were tested using diodes that emit red and far‐red light to distinguish the responses to altered R : FR of genotypes of Impatiens capensis collected from a pair of open‐ and closed‐canopy populations that have previously been shown to differ in sensitivity to R : FR. Genotypes from the open‐canopy environment exhibited typical shade‐avoidance responses, elongating in response to supplemental FR. However, genotypes from the closed‐canopy environment responded to supplemental FR by elongating less than under ambient control conditions, indicating a persistent FR‐HIR. Thus, the observed population differentiation in response to low R : FR may be linked to population differences in FR‐HIR.