| Photosynthesis is the key physiological basis for growth and development, yield and quality in pear trees. Environmental stresses are important limiting factors for photosynthesis. With the globe warming, strong light and high temperature severely restrict growth and photosynthesis of pear trees in summer, but less is known about their combined effects on photosynthesis. So, a research on the combined light and heat stresses on the operation of photosynthetic apparatus has a great value for understanding mechanism of plant response to environmental stresses and looking for practical measures to alleviate the depression of photosynthesis. In this thesis,1-year-old’Cuiguan’(Pyruspyrifolia Nakai’Cuiguan’) and’Wonhwang’(P. pyrifolia Nakai’Wonhwang’) pear cultivars on P. calleryana Decne were used to investigate the diurnal variations of photosynthetic apparatus operation and the mechanism of photosystem II reaction centre Dl protein turnover by measuring the chlorophyll fluorescence parameters, gas exchange parameters, Kinetic curves of chlorophyll fluorescence fast phase, photosynthetic light response curves and using western-blotting and QPCR technology. The main results were as follows:1. The differences of photosynthetic apparatus operation between high light adapted (the temperature and light intensity reached34-36℃and1400-1600μmol photon m-2s-1during day time in summer, respectively) and low light adapted (the temperature and light intensity reached32-34℃and400-600μmol photon m-2s-1during day time in summer, respectively)’Cuiguan’and’Wonhwang’pear cultivars were investigated under strong light and high temperature in summer. The results showed that maximal photochemical efficiency of PSII (Fv/Fm), electron transport rate (ETR), quantum yield of PSII (φPSII) declined when’Cuiguan’and’Wonhwang’ plants were exposed to combined light and heat stresses, and (Fi-Fo)/(Fp-Fo) increased while the slope of I to P decreased, which indicated the increase of PSII photoinactivation ratio. At the same time, the relative content of D1, CP43, CP47, Cyt f, Cyt b6and Degl proteins in thylakoid membrane reduced. These results showed that high temperature and strong light caused a serious damage to leaf photosynthetic apparatus, and Degl protease was not important in the D1degradation. Moreover, the recovery of chlorophyll fluorescence parameters and thylakoid membrane proteins at18:00indicated that low light adapted’Cuiguan’and’Wonhwang’leaves had low repair efficiency.2. The combined effects of strong light and high temperature on Dl protein turnover in’Cuiguan’and’Wonhwang’leaves were analyzed. The results showed that net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and apparent quantum yield(AQY) decreased, D1protein and Degl protease in leaves degraded, while the expression of psbA mRNA enhanced when’Cuiguan’and ’Wonhwang’leaves were exposed to high temperature and strong light. Moreover, Pn increased slightly and the relative contents of D1protein and Degl protease recovered to more than90%of the original level at17:00in’Cuiguan’leaves, whereas no obvious sign of recovery in’Wonhwang’leaves. These results indicated that high temperature and strong light could induce photoinhibition in leaves of both’Cuiguan’ and’Wonhwang’pear cultivars. Accumulation of PsbA mRNA and degradation of Degl protease suggested that the decrease of net content of D1protein was not attributed to degradation of D1protein by Degl protease, but directly related to inhibition of the synthesis of D1protein at the translation step under heat and light stresses. Moreover, the recovery of Dl protein and Pnat17:00in’Cuiguan’while not in’Wonhwang’leaves indicated that the PSII in’Cuiguan’was more tolerant to heat than in’Wonhwang’. |
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