Evaluating the effects of UV-B and UV-A irradiances on plant pigments, photosynthesis and growth in Glycine max L

Increasing penetration of UV-B radiation to the earth's surface resulting from stratospheric ozone depletion is an important environmental concern, but plant response to UV-B irradiation has been difficult to assess. The UV-A irradiance has not been specifically measured or controlled previously. The experimental UV-A was controlled in a series of three glasshouse experiments conducted under high photosynthetic photon flux (midday PPF ≥ 1200 μmol m-2 s-1). Low (LT) and High (HT) daily UV-BBE irradiances (10.7; 14.1 kJ m-2) were utilized in two experiments, whereas treatments with different UV-BBE:UV-A ratios (< 0.5, for total UV-A) were examined in the third. UV-B and UV-A irradiation effects were determined for pigment, photosynthesis, and growth variables in 3 isolines o f two soybean cultivars, Clark and Harosoy. UV-B irradiation consistently induced UV-B absorbing compound and photosynthetic pigment accumulation, especially carotenoids, and increase in the Chl-a/b ratio, and a stomatal limitation to photosynthesis. UV-B absorbing compounds were correlated with total carotenoids (UV-B irradiated plants) and with biomass (all plants). Many variables exhibited an interaction between spectral quality and quantity, interpreted as moderation of UV-B irradiation resembled open-quotes shadeclose quotes responses, the involvement of phytochrome was suggested, and the interaction was explained as the net response induced by phytochrome and the UV-A/Blue photoreceptor. LT responses often were greater than those for HT, and differences in responses for the LT and its UV-A Control were often more clearly separable, providing an explanation for the common failure to verify a dose-related response in UV-B studies. This was further supported by response of pigments to three treatments with different UV-BBE:UV-A ratios