Studies On The Mechanism Water Deficiency Influences The Repair Of Photodamaged Photosynthetic Apparatus In Apple Leaves

Northwestern loess plateau is one of the most important regions in the production of apple in China. It belongs to arid and semi-arid area wherein water is the key environmental factor impacting physiological metabolism of apple trees. However, we still know little about the mechanism water deficiency influences the repair of photosynthetic apparatus after photodamage under strong light. In this study, we choose ’Fuji’ apple as the test material and investigated the effects of water deficiency on the the repair of photosynthetic apparatus after photodamage under strong light using changes of chlorophyll fluorescence, reactive oxygen species(ROS) contents, antioxidant enzymes activities and D1 content in photosystem II(PSII). The main results are as follows:1.The modulated chlorophyll fluorescence parameters showed, photochemistry capacity went on decreasing along with the aggravation and extension of water deficiency. On the other hand, nonphotochemistry capacity went up first and then went down, indicating the protecting mechanism in apple leaves still keep working in the initial period of water deficiency while the protecting mechanism maybe impaired when water deficiency exceeds the tolerance point of apple leaves.2.Noticeable difference can be observed on OJIP curves between water deficiency treatment and control. Further analysis showed, photosynthetic electron transport capacity down-regulates along with the aggravation and extension of water deficiency. Among all components in photosynthetic electron transport chain, the electron transport from QA to QB showed the most sensitive to the water deficiency. The decrease of D1 content also proved the action site of water deficiency is on the transport from QA to QB.3.Partitioning of absorbed light energy could reflect the performance of apparatus in more details. Our results showed, absorbed light energy and dissipated excess energy per PSII reaction center(RC) are both remarkably higher than the control for a smaller number of QA-reducing RCs of apple leaves with water deficiency treatment. Although absorbed light energy and dissipated excess energy per cross section in apple leaves with water deficiency treatment showed a up trend compared with control, they are not as sensitive as that per PSII RC.4.Antioxidant enzymes activities trials showed, the activities of all antioxidant enzymes in apple leaves with water deficiency showed an increasing trend but it is still not sufficient to clear the excess accumulation of ROS including H2O2 and O2?-, which could be responsible for the acceleration of photoinhibition.5.We found photoinhibition accelerated with the application of PMSF and EDTA, which inhibit Deg protease and FtsH protease respectively. Furthermore, the application of PMSF and EDTA together has more significant effect accelerating photoinhibition than the application of PMSF or EDTA individually. This indicates Deg protease and FtsH protease indeed participate in the repair cycle of photodamaged PSII in apple leaves.