Susceptibility to UV damage in Impatiens capensis (Balsaminaceae): testing for opportunity costs to shade-avoidance and population differentiation.

Plastic increases in leaf secondary compounds may be an adaptive strategy that reduces the damaging effects of high-energy, ultraviolet radiation (UV). Here, we examine (1) the relationship between fitness and anthocyanin and flavonoid concentrations in experimental, UV environments, (2) the effects of UV on Impatiens capensis plants derived from woodland and clearing sites, and (3) whether susceptibility to UV damage is reduced by exposure to high ratios of red : far-red wavelengths (R : FR), which also stimulate the production of leaf compounds. Seedlings from each site were exposed to either high R : FR typical of sunlight or low R : FR characteristic of foliar shade, after which plants were moved into ambient UV or UV-removal treatments. Ultraviolet radiation stimulated the production of anthocyanins and flavonoids. However, higher anthocyanin concentrations were associated with lower biomass in the UV environment. Relative to the clearing population, reproductive output of the woodland population was more detrimentally affected by exposure to UV, despite its higher concentration of anthocyanins. Increased anthocyanin production may therefore be a stress response rather than an adaptive one. The greater tolerance of the clearing population to UV suggests that populations with an evolutionary history of UV exposure evolve mechanisms to limit damage. The R : FR pretreatments did not influence susceptibility to UV damage.