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

摘要

Increasing penetration of UV-B radiation (0.290-0.320 {dollar}mu{dollar}m) 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. Although the determination of UV-B radiation effects on plants should rely on equivalent UV-A (0.320-0.400 {dollar}mu{dollar}m) radiation from the experimental UV-B Treatments (T) and UV-A Controls (C), the UV-A irradiance has not been specifically measured or controlled previously. Significant differences in lamp UV-A irradiances associated with two daily biologically effective UV-B irradiances (UV-B{dollar}sb{lcub}rm BE{rcub},{dollar} see below), and with filter photodegration, were found in both T and C; these were maintained when the natural solar UV-A transmitted into the glasshouse {dollar}(lambdage 0.350){dollar} was considered, even in summer. The experimental UV-A was controlled in a series of three glasshouse experiments conducted under high photosynthetic photon flux (mid-day PPF {dollar}ge{dollar} 1200 {dollar}mu{dollar}mol m{dollar}sp{lcub}-2{rcub}{dollar} s{dollar}sp{lcub}-1{rcub}).{dollar} Low (LT) and High (HT) daily UV-B{dollar}sb{lcub}rm BE{rcub}{dollar} irradiances (10.7; 14.1 kJ m{dollar}sp{lcub}-2{rcub}){dollar} were utilized in two experiments, whereas treatments with different UV-B{dollar}sb{lcub}rm BE{rcub}{dollar}:UV-A ratios {dollar}(<{dollar}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 normal, chlorophyll-deficient, and flavonoid-deficient isolines of two soybean cultivars, Clark and Harosoy. UV-B irradiation consistently induced UV-B absorbing compound and photosynthetic pigment accumulation, especially carotenoids, an increase in the Chl-a/b ratio, and a stomatal limitation to photosynthesis. Although no direct relationship between UV-B absorbing compounds and photosynthetic response was found, these 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 effects by UV-A irradiation. Since some responses to UV-B irradiation resembled "shade" 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-B{dollar}sb{lcub}rm BE{rcub}{dollar}:UV-A ratios. A change in the experimental protocol for glasshouse studies was recommended, whereby the UV-B{dollar}sb{lcub}rm BE{rcub}{dollar} irradiance is varied against a standard, high UV-A and ambient visible light background, and the UV-A Control is replaced with a treatment having a low UV-B{dollar}sb{lcub}rm BE{rcub}{dollar}:UV-A ratio representative of field values.