Effects Of High Temperature And Strong Light On Photosynthesis And Its Mechanisms In Satsuma Mandarin (Citrus Unshiu Marc.) Plants

Photosynthesis is the most important chemical reaction on earth. Light energy is the driving force of photosynthesis; however, too much light will reduce photosystem II (PSII) photochemical efficiency, which usually named as photoinhibition. Strong light and high temperature of the most common stresses simultaneously and severely restrict growth and photosynthesis of plant in many regions of the world, but much less is known on how their combination impacts plants. So, it is important to investigate the combined effects of strong light and high temperature on the operation of photosynthetic apparatus and look for some measures to alleviate the depression of photosynthesis. In this dissertation, Satsuma mandarin (Citrus unshiu Marc), broadly cultivated citrus plants, were selected to investigate the diurnal variations of photosynthetic capacity, the factors of photosynthesis mid-depression in different seasons and the functions of mist-spraying to alleviate the mid-depression in summer. Furthermore, the combined effects of strong light and high temperature on photosystems and their mechanism in Satsuma mandarin leaves were analyzed; the contribution of D1 turnover in protecting of PSII against photodamage in Satsuma mandarin leaves was also explored. The main results were as follows:1. Diurnal variations of photosynthetic apparatus operation in Satsuma mandarin leaves were separately investigated under strong light with moderate temperature (spring) and strong light with high temperature (summer). The results showed that the net photosynthetic rate (Pn) in Satsuma mandarin leaves decreased at midday in spring and it decreased seriously in summer. The decrease of Pn was in agreement with Stomatal closure at midday in spring; whereas major changes in the maximal photochemical efficiency (Fv/Fm) were not observed. It was demonstrated that the Stomatal closure played an important role in the midday depression of photosynthesis in spring. Stomatal closure and Fv/Fm decreased with the decrease of Pn in summer, which suggested that photoinhibition was another factor in the mid-depression of photosynthesis in Satsuma mandarin leaves in summer. The decrease in electron transport and the increase in ratio of photoinactivation of photosystem II were found at midday in summer, not in spring. All these results suggested that decline of photochemical efficiency resulting from inactivation of PSII reaction was an important factor in mid-depression of photosynthesis under summer conditions, not under spring conditions.2 Mist-spray was adopted as an alleviative strategy to investigate the effects of Stomatal conductance (gs) on mid-depression of photosynthesis in Satsuma mandarin leaves. Depressions in gs, Pn and Fv/Fm were alleviated by mist-spraying in Satsuma mandarin leaves at midday. Photoinactivation of PSII at midday, accumulations of peroxide and degradation of D1 protein were also ameliorated in mist-sprayed leaves. The proton motive force (pmf) across the thylakoid membrane in mist-sprayed leaves was maintained, which was benefit for the operation of photosynthetic apparatus. It was concluded that limitation of CO₂ assimilation resulting from leaf Stomatal closure induced the accumulation of H₂O₂ and damaged the PSII reaction centers, which resulted in decrease of PSII photochemical efficiency and depression of photosynthesis in Satsuma mandarin leaves at midday in summer. Moreover, mist-spraying was also benefit for normal operation of photosynthetic apparatus.3. The combined effects of strong light and high temperature on photosystems in Satsuma mandarin leaves were analyzed. Fv/Fm and electron transport rate (ETR) decreased in the leaves exposed to combined stress with strong light and high temperature. Significant degradation D1 protein and accumulation of H₂O₂ were also found accompanying with the decreases of pmf and ATP content. The combined effects caused photodamage of PSII but not PSI, declined of the ratio of PSII to PSI and inhibition of the cyclic electron flow around PSI (CEF-PSI). In conclusions, the decrease of photochemical efficiency in Satsuma mandarin leaves under combined stresses was caused by inhibition of electron transport and degradation of D1 protein which result from accumulation of H2O2. In addition, synthesis of ATP was limited by the decrease of pmf resulting from the inhibition of electron transport rate, which could exacerbate the degradation of PSII reaction center.4 The contributions of D1 protein turnover in PSII protection against photodamage in Satsuma mandarin leaves under strong light illumination were explored. Illuminations decrease Fv/Fm, ETR and stimulated qP compared with darkness. Illuminations also significantly increased the (Fi-Fo)/(Fp-Fo) and decreased the slope of I to P , which indicated the increase of PSII photoinactivation ratio. In the presence of lincomycin, Fv/Fm and ETR significantly decreased compared with the leaves fed with water instead. Compared with darkness, illumination induced the decrease of D1 protein and this depression was more significant in lincomycin treated leaves than that in dark-treated leaves. All these facts supported a conclusion that photoinhibition of photosynthesis in Satsuma mandarin leaves was caused by inactivation of PSII reaction center with increase of net degradation of D1 protein and the rapid turnover of D1 protein under light played an important role in the photoprotection of the PSII reaction center.