| Photosynthesis is the fundamental secure of the crop yield and quality, environmental stresses were the important factors of limiting photosynthesis, in which high temperature and strong light is the main stress in the yangtze valley of China in summer. Moreover, heat and light stresses usually take place imultaneously under natural conditions, causing photoinhibition of photosynthesis, even make irreversible damage to photosynthetic apparatus. But much less is known about the combined effects of heat and light stresses on photosynthesis of plants. In this paper, three-year old Satsuma mandarin (Citrus unishiu Marc.) plants were selected to investigate the interactive impacts of different levels of light and temperature on photosynthetic apparatus operation and their mechanism by using gas exchange and chlorophyll fluorescence, Western-blotting, DAB-uptake method, exogenous SA and H2O2. Furthermore, the combined effects of strong light and high temperature on thylakoid membrane proteins and the protective role of SA were analyzed. The main results were as follows:1. The combined effects of high temperature and strong light on photosystems and their mechanism in Satsuma mandarin leaves were analyzed through the research of diurnal variations of photosynthetic apparatus operation in Satsuma mandarin leaves at midday in summer clear day. The results showed that the net photosynthetic rate (Pn) in Satsuma mandarin leaves decreased in agreement with internal CO2 concentration (Ci) increased, it was demonstrated that non stomatal factor was an important factor of limiting photosynthesis. At the same time, the maximal photochemical efficiency of PSII (Fv/Fm), photochemical quenching (qP), electron transport rate (ETR) and quantum yield of PSII (ΦPSII) declined, simultaneously, the relative content of D1, Degl, CP47, CP43, Cytf, Cytb6 protein reduced. Further reaseach suggested that photoinhibition is related to the decline of photochemical efficiency and the inactivation of photosynthetic apparatus. But the plant itself can be self-defense mechanism against photoinhibition. 2. The difference of photosynthetic apparatus operation in Satsuma mandarin leaves were separately studied under interactive impacts of four different levels of light and temperature:optimum temperature and moderate light (25℃and 800μmol·m-2·s-1), optimum temperature and strong light (25℃and 1600μmol·m-2·s-1), high temperature and moderate light (38℃and 800μmol·m-2·s-1), and high temperature and strong light(38℃and 1600μmol·m-2·s-1). The results showed that the initial fluorescence (Fo) was increased significantly in leaves when exposed to high temperature and strong light for 4 h. On the other hand, the maximal photochemical efficiency of PSII (Fv/Fm), electron transport rate (ETR) and quantum yield of PSII (ΦPSII) declined. The relative content of Degl, CP47, CP43, Cytf, Cytb6 protein reduced when the D1 protein degraded, simultaneously, H2O2 in the leaves also accumulated. High temperature and strong light broke the dynamic balance between generation and elimination of reactive oxygen that was responsible for the accumulation of H2O2, which has a crucial role in causing photo-oxidative damage and aggravating photoinhibition.3. The effects of H2O2 on photosynthetic apparatus and the protective effects of exogenously applied salicylic acid under high temperature and strong light of Satsuma mandarin detached leaves were explored. It was observed that the Fv/Fm, ETR,ΦPSII, the relative content of D1, Deg1 CP47, CP43, Cytf, Cytb6 protein were decreased by exogenous H2O2, whereas, these parameters were somewhat decreased by the exogenous SA under high temperature and strong light for 30min. These results suggested that the net content of D1 protein decreased due to accumulation of H2O2 induced by high temperature and strong light, which causeing damage to the reaction center of PSII in leaves. The exogenous SA could protect the photosynthetic apparatus against photo-damage in Satsuma mandarin leaves.
|
PDF Full Text Request