| As a rambling and climbing berry tree which originated in the forest, grape’s photosynthesis is sensitive to the changes of temperature and light. The occurrence frequency of extreme weather like high temperature and strong light is creasing as the global climate warms. In nature, grape sunburn happens widely under high temperature and strong light which have severely affected grape growth and development. Therefore, it is important to investigate the effects of high temperature and strong light on the operation of photosynthetic apparatus and find measures to alleviate the depression of photosynthesis.In this study, one-year-old grapevine seedlings(Vitis vinifera L. cv. Cabernet Sauvignon)were selected to investigate the diurnal variations of photosynthetic parameters, the factors of photosynthetic mid-depression in summer and the combined effects of high temperature and strong light on the photosystemⅡ(PSⅡ)photoactivity, the photosynthetic carbon assimilation and the photoprotection mechanisms in grapevine leaves; the function of horizontal canopy to alleviate the heat stress in the field was also investigated.1. The temperature and vapor pressure deficit(VPD) increased and the stomatal conductance(Gs) decreased clearly with the light intensity raised in the morning of a sunny summer day.The net photosynthetic rate(Pn) and apparent quantum yield(AQY) decreased significantly and the amplitudes of the J and I steps of the OJIP transients were clearly increased at midday,suggesting that both the donor side and the accept side of PSⅡwere inhibited, and an obvious photosynthetic ‘midday depression’ phenomenon occurred. Along with the decrease of Pn at midday, the de-epoxidation level of xanthophyll pigment pool and non-photochemical quenching(NPQ) increased markedly showing that the xanthophyll cycle in grapevine leaves was enhanced to dissipate excess excitation energy. However, with temperature and light intensity increased, NPQ could not deal with all the excess energy, resulting in the severe photoinhibition at midday.2. The effects of different degrees heat stress(35, 40 and 45 ℃) on the PSⅡactivity were different. Treatment at 35°C did not significantly influence PS Ⅱ activity. The maximal photochemical efficiency(Fv/Fm) decreased and H2O2 content increased clearly in leaves treated with 40 and 42.5 ℃. The donor side and accept side of PSⅡwere damaged with the temperature increased, and the damage degree of the donor side was more severe than that of the acceptor side. After 2 h recovery, the PSⅡ reaction center and donor side could recovered significantly, but the accept side recovered less, indicating that short-term heat stress inhibited PSⅡactivity, the recovery degree of the accept side was relative less when the stress was removed.3. The effects of different light intensity on PSⅡ activity of grapevine leaves under high temperature stress were different. Treatment of dark(0 μmol m-2 s-1) and strong light(1 600μmol m-2 s-1) with heat stress(40 ℃) decreased Fv/Fm, density of reaction centers(RC/CSm)and quantum yield for electron transport(φEo) significantly; treatment of low light(200 μmol m-2 s-1) with heat stress alleviated the photoinhibition caused by heat stress. Besides, treatment of dark with heat stress caused a significant increase of the relative fluorescence intensity of K point(VK) and trapped energy flux per reaction center(TRo/RC), and there was no significant recovery after treatment of dark with heat stress. The experiments showed that when it was treated with heat stress, dark treatment inhibited both PS Ⅱ activity and recovery, light treatment participated in the photoinhibiton and recovery through alleviating excess reduction of QA.4. The combined effects of high temperature and strong light on photosynthetic carbon assimilation were analyzed. Both heat stress(40 ℃, 600 μmol m-2 s-1) and strong light stress(25℃, 1600 μmol m-2 s-1) decreased Pn because of the non-stomatal limitation. There were differences in the effects of different stresses on photosynthetic carbon assimilation.Carboxylation efficiency(CE) decreased more in leaves treated with heat stress than that treated with strong light stress, and further decreased in leaves treated with combined stress.Compared with the control, the electron flux for photosynthetic carbon reduction(Je(PCR))and electron flux used for photorespiratory carbon oxidation(Je(PCO)) in leaves treated with heat stress and combined stress decreased significantly, however, Je(PCO) increased in leaves treated with strong light stress. These results suggested that heat stress and combined stress decreased Pn by inhibiting the carboxylation and oxygenation, however, the decrease of Pn was alleviated by accelerating the oxygenation in leaves treated with strong light stress.5. PSⅡphotodamage occurred in Lin- or DTT-treated leaves under high temperature stress or strong light stress, besides, there was little recovery in Lin- treated leaves when stresses were removed. Compared with pretreatment, Fv/Fm in Lin-treated leaves decreased 55.5% under high temperature and 40.8% under strong light, PSⅡ excitation pressure(1-q P) increased140.7% and 118% respectively. Fv/Fm in DTT-treated leaves decreased 23.4% under high temperature and 41.6% under strong light, PSⅡ excitation pressure(1-q P) increased 41.7%and 145% respectively. The results showed that more severe photodamage occurred in Lin-treated leaves under heat stress than that under strong light stress, meanwhile,xanthophylls cycle played a more important role in protecting PSⅡunder strong light.6. There was no significant difference in Pn between vertical canopy and horizontal canopy in the weather without high temperature, however, Pn and Gs of horizontal canopy leaves were significantly higher than that of vertical canopy leaves at 10:00 on a high temperature day.Fv/Fm, efficiency of electron move beyond QA(Ψo) and φEo in horizontal canopy leaves were greater than those in vertical canopy leaves in hot weather. The grape stem flow rates and the climate diurnal change regulation were analyzed to find that the climate factors were closely related to the stem flow rates. The stem flow rates of the horizontal canopy were significantly higher than that that of vertical canopy, which might be related with the less decrease of Gs in horizontal canopy leaves in hot weather. The distribution of carbon and nitrogen isotope results showed that 13 C and 15 N were less kept in stem and more were moved to the fruit of the horizontal canopy. All the results indicated that the horizontal canopy can alleviate the heat stress by improving the distribution of absorbed energy and move more carbon and nitrogen to the fruit. |
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