Effects of shading on photosynthesis and reactive oxygen metabolism in Vitis vinifera ‘Cabernet Sauvignon' leaves under ozone stress

Abstract:【Objective】A previous study showed that ozone stress decreased photosynthesis and destructed reactive oxygen mechanism in grapevine leaves. The highest diurnal ozone concentration usually happens at about 10:00—14:00, when plants are exposed to strong light as well as high ozone stress. Under such conditions, leaves of some grape varieties are damaged. To investigate the effects of shading on photosynthesis and reactive oxygen metabolism in grape leaves under ozone stress, photosynthesis and reactive oxygen mechanism in the leaves of Vitis vinifera‘Cabernet Sauvigon'grown in a open top chamber with a high ozone concentration(105 n L·L^-1) were measured under different shading treatments.【Methods】The potted grapevines were treated with T1(O3+full light), T2(O3+30% shading) and T3(O3+60% shading) on a sunny day with a highest diurnal light intensity of about(1 700±100) μmol·m-2·s-1, which is strong for grapevine. Five plants were used for each treatment, and the shading nets were placed 60 cm above the plants. After eight hours of treatments, photosynthetic parameters such as net photosynthetic rate(Pn), stomatal conductance(Gs), intercellular CO2concentration(Ci) were measured by a CIRAS-2 portable photo-synthesis system, and fluorescent parameters including number of active reaction centers per unit area(RC/CSm), the probability that a trapped exciton moves an electron into the electron transport chain beyond QA(ΨO), and non photochemical quenching(NPQ), conversion efficiency of primary light energy(Fv/Fm),quantum yield of PSⅡelectron transport(ΦPSⅡ), and the photochemical quenching(q P) were measured by a handy PEA plant efficiency analyzer and an FMS-2 chlorophyll fluorometer. Then the leaves were harvested for determining enzyme activities and photosynthetic pigments.【Results】Ozone stress induced decreases in the chlorophyll a, chlorophyll b, carotenoid and total chlorophyll contents and in chlorophyll a/b ratio. Shading reduced the decline in photosynthetic pigments caused by ozone stress(T1). Compared with T1, the chlorophyll a, chlorophyll b, chlorophyll a/b, carotenoids and total chlorophylls increased by18.2%, 0.8%, 16.9%, 17.5% and 12.9% in T2 treatment and by 32.5%, 6.5%, 24.9%, 33.3% and 24.9%in T3 treatment, respectively. Compared with the control, Pnand Gswere decreased by 87.1% and 79.1%under ozone stress, respectively, and Ciwas increased, which indicated that ozone stress reduced Pndue to non-stomatal limitation. Shading alleviated the negative effect on Pn. Compared with T1, T2 increased Pn and Gsby 8.2% and 65.5%, respectively, and T2 by 176.3% and 203.0%, respectively, while Ciin T2 and T3 were lower than in T1 but still higher than in the control. The results indicated that shading reduced the damage on photosynthesis and Gsin grape leaves caused by ozone stress through alleviating the nonstomatal limitation to photosynthesis. ΦPS Ⅱof grape leaves decreased under T1 treatment compared with the control. Shading treatments reduced the decline in ΦPS Ⅱcaused by ozone stress. This parameter increased by 113.6% and 136.0% in T2 and T3 treatments compared with T1, respectively. 1-q P and NPQ were increased by ozone stress, while shading reduced the increase in 1-q P and NPQ caused by ozone stress, and they were not significantly different between T2 and T3. Fv/Fmreflects the activity of PSⅡ, and was decreased by ozone stress. Shading alleviated the negative effect of ozone stress on Fv/Fm. RC/CSmandΨEowas decreased by 89.7% and 78.2% by ozone stress, respectively. Compared with T1, shading increased RC/CSmand ΨEo,which displayed an increase of 63.4% and 194.1% in T2 and T3 compared to T1 treatment, respectively. Ozone stress increased active oxygen and MDA contents in grape leaves. O2-production rate, and H2O2 and MDA contents increased by 53.7%, 17.2% and 19.1% compared with the control, respectively. T3 reduced active oxygen and MDA production, and the O2-production rate and H2O2 and MDA contents were significantly decreased by 19.8%, 9.2% and 12.0% compared to T1, respectively,suggesting shading treatments reduced the membrane lipid peroxidation in leaves of grape under ozone stress. Superoxide dismutase activity was increased by 4.5% under ozone stress, and the enzyme activity was increased by 30% and 60% under T2 and T3 treatments, respectively. Peroxidase and catalase activities were decreased by 87.9% and 76.2% respectively under ozone treatment, compared with which shading treatments increased the activities the enzymes. Compared with T1, peroxidase and catalase activities increased by 68.3% and 70% under T2 treatment, and by 146.7% and 130% under T3 treatment, respectively.【Conclusion】Ozone stress has negative effects on grape leaves, causing decreases in chlorophyll contents and net photosynthetic rate and increases in non-stomatal limitation, active oxygen concentration, and MDA content. Strong light intensifies the damage caused by ozone, and shading alleviates ozone damages by reducing the photoinhibition and active oxygen and MDA production under O3 stress. Based on the results, pergola trellis could alleviated the ozone stress.