Changes in photosynthesis and antioxidant defenses of Picea asperata seedlings to enhanced ultraviolet-B and to nitrogen supply

The paper mainly studied the effects of ultraviolet-B (UV-B) radiation, nitrogen, and their combination on photosynthesis and antioxidant defenses of Picea asperata seedlings. The experimental design included two levels of UV-B treatments (ambient UV-B, 11.02 KJ m(-2) day(-1); enhanced UV-B, 14.33 KJ m(-2) day(-1)) and two nitrogen levels (0; 20 g m(-2) a(-1) N) - to determine whether the adverse effects of UV-B are eased by supplemental nitrogen. Enhanced UV-B significantly inhibited plant growth, net photosynthetic rate (A), stomatal conductance to water vapor (Gs), transpiration rate and photosynthetic pigment, and increased intercellular CO2 concentration, UV-B absorbing compounds, proline content, malondialdehyde (MDA) content, and activity of antioxidant enzymes (peroxidase (POD), superoxide dimutase, and glutathione reductase). Enhanced UV-B also reduced needle DW and increased hydrogen peroxide (H2O2) content and the rate of superoxide radical (O-2(-)) production only under supplemental nitrogen. On the other hand, supplemental nitrogen increased plant growth, A, Gs, chlorophyll content and activity of antioxidant enzymes (POD, ascorbate peroxidase, and catalase), and reduced MDA content, H2O2 content, and the rate of O-2(-) production only under ambient UV-B, whereas supplemental nitrogen reduced activity of antioxidant enzymes under enhanced UV-B. Carotenoids content, proline content, and UV-B absorbing compounds increased under supplemental nitrogen. Moreover, significant UV-B x nitrogen interaction was found on plant height, basal diameter, A, chlorophyll a, activity of antioxidant enzymes, H2O2, MDA, and proline content. These results implied that supplemental nitrogen was favorable for photosynthesis and antioxidant defenses of P.asperata seedlings under ambient UV-B. However, supplemental nitrogen made the plants more sensitive to enhanced UV-B, although some antioxidant indexes increased