The genetic architecture of plasticity to density in Impatiens capensis

Plant responses to crowding may be mediated by resource availability and/or by a specific environmental cue, the ratio of red:far red wavelengths (R:F R) perceived by phytochrome. This study examined the contribution of phytoc hrome-mediated photomorphogenesis to genetic variation in plastic responses to density in the annual plant Impatiens capensis. Inbred lines derived fr om open and woodland populations were grown under low density, high density , and high density with selective removal of FR wavelengths to block phytoc hrome-mediated perception of neighbor proximity. Genetic variation in plast icity to density and to the R:FR cue was detected for several traits. Plant s grown at high density displayed increased internode elongation; decreased branch, flower, and node production; increased meristem dormancy; and decr eased leaf area and specific leaf weight compared to plants grown at low de nsity. Stem elongation responses to density were suppressed when phytochrom e perception was blocked at high density. For these phytochrome-mediated tr aits, a genotype's plasticity to density was strongly correlated with its r esponse to R:FR. Phytochrome-mediated traits were tightly correlated with o ne another, regardless of the density environment. However, the responses t o density of meristem allocation to branching and leaf traits were less str ongly phytochrome-mediated. These traits differed in patterns of plasticity , and their genetic correlations often differed across environments. In par ticular, genetic trade-offs involving meristem allocation to branching were expressed only at low density. The observed density dependence of phenotyp ic and genetic correlations implies that indirect selection and the potenti al for correlated response to selection will depend upon the competitive en vironment. Thus, the differential sensitivity of characters to the R:FR cue can influence the evolution of integrated plastic responses to density.