| Experiments were conducted under open-top-chambers conditions to assess the photosynthetic responses of wheat plants (Triticum aestivum L., YangMai16) to supplemental UV-B radiation (10%-10.9% higher then control group, T1) and enhanced ozone ［(100±9) nmol·mol-1, T2］, separately and in combination (combination treatment, T3), making use of LCpro+ Portable Photosynthesis System and DIVING-PAM Fluorometer to determine gas exchange and chlorophyll fluorescence parameters. Results indicated that Pn, Gs, Tr, Pm andIk of T1, T2 and T3 treatments decreased significantly compared to CK (control group, natural air and UV-B radiant intensity condition), while there were no differences between T3 and T1 or T2 or both in major growth stages. UV-B fiercely inhibited the stomatal conductance and transpiration of plants, while T1 stimulated stomata opening and transpiration in jointing stage. Dark respiration (Rd) of T1 was increased, while no significance difference was found between T2 and CK or T3 and CK in most stages.T1 and T2 reduced Fv/Fm value only in booting stage, while T3 was significant lower than CK except jointing stage. qP value declined significantly in treatments of T1, T2 and T3 as Compared to CK, with decreasing amplitude occurring in the order T3>T1>T2. NPQ, Y(NPQ), Y(NO) value of T1, T2 and T3 treatments increased significantly compared to CK, with maximum increasing amplitude occurring in the order T3>T1>T2, of which NPQ of T1 and T2 turned to decrease since filling stage, and T3 turned to decrease since flowering stage to a greater degree than T1 and T2. T1, T2 and T3 also caused significance reduction in Y(Ⅱ), with reducing amplitude occurring in the order T3>T1>T2. Obviously, supplemental UV-B radiation and enhanced ozone caused a significant decrease in gas exchange capacity, maximum photochemical capacity and photosynthetic activity of winter wheat, and the photoprotective mechanism was damage, leading to greater proportion of excitation energy dissipated in the form of non-regulated heat and fluorescence. The photosystems of winter wheat were damaged by both excess energy and UV-B or excess energy and O3, or excess energy, UV-B and O3together. UV-B and O3 in combination enhanced the negative effects on photo-protective mechanisms and excitation energy distribution in PSⅡ compared to UV-B or O3 alone, while the interactive effects were less than addition. |
