| Photosynthesis is the foundation of yield and quality formation of apple. Drought is themain limiting factor in the production of apple in the northwest region. Three years old ‘Na-gafu No.2’apple (Malus domestica Borkn. cv. Red Fuji) trees on Malus Robusta rootstockswith M26inter-stock grown in plastic pot were used in this study. The apple trees grown in aplastic greenhouse for shelter from the rain under severe stress (SS), moderate stress (MS),slight stress (LS) and control conditions. The study aim is to explore the responses of photo-synthetic apparatus operation of apple trees to different drought stress, to provide theoreticreferences for cultivation of apple trees in drought region. The main results of this paper areas follows:1. Both leaf water potential (ψw) and photosynthesis were related to the soil water con-tent. Through the treatments of different levels of water, the present study found that ψwde-crease significantly with increasing intensity of drought stress. The net photosynthetic rate (Pn)and transpiration rate (Tr) also decreased significantly, presenting approximate linear rela-tionship with ψw. The correlations of stomatal conductance (Gs) and intercellular CO2con-centration (Ci) with ψwwere polynomial regressions. As increasing intensity of drought stress,Gs dropped at first and maintained stability in the end, while Ci decreased firstly before in-creasing. Therefore, both stomatal limitation and non-stomatal limitation inhibit the activity ofphotosynthesis; in LS and MS were mainly because of stomatal limitation play dominant rolein LS and MS，while non-stomatal limitation was primary factor in SS.2. Using the analysis of chlorophyll fluorescence kinetics, the results suggested that: themaximal photochemistry efficiency (FV/FM), effective photochemical quantum yield of PS II(YII), relative electron transport rate (ETR) and the capacity of electron transport (PIABStotal)in apple leaves decreased significantly. The transmission efficiency (ETO/TRO) and capacity(PIABS) of electron transport between plastoquinone QAand QBdecreased with the most signi-ficance. In LS and MS, non-photochemical fluorescence quenching (qN, NPQ and YNPQ)could effectively dissipate the excess excitation energy, which leaded to irreversible photo-damage in SS (YNOincreased). In contrast, transmission efficiency of electron transport fromQBto the end of PSI accepters (REO/ETO) and relative content of PSI increased in LS and MS.These results suggest that mainly the reduced transmission efficiency of photosynthetic elec- tron transport chain lowered photochemistry efficiency and photosynthesis under water stressconditions, especially the the inactivation of QBnon-reducing centers.3. The content of chlorophyll increased in apple leaves in a certain degree of droughtstress, while it decreased significantly under long-term stress, especially the chlorophyll a.The content of reactive oxygen species (H2O2and O2-, AOS) increased significantly underdrought stress. In SS, the active oxygen scavenger decreased significantly the content of AOS,and increased the content of chlorophyll; FV/FM, ETO/TROand PIABSincreased, also innon-photochemical quenching. While there no difference in Y(II), ETR, PIABStotaland relativecontent of PSI. This suggests that the main reason for the inhibition of photosynthesis is thereduced transmission efficiency of photosynthetic electron transport chain, not the accumula-tion of reactive oxygen species.4. Through treating the inhibitors of FtsH proteases and Deg proteases，the resultsshowed that both photochemistry efficiency and transmission capacity of photosynthetic elec-tron transport chain decreased, with ETO/TROdecreasing and REO/ETOincreasing. Comparedcomprehensive, the prominent effect of inhibition was the inactivity of FtsH proteases. It issuggested that both FtsH proteases and Deg proteases play important role in the repair anddegradation of D1protein, while the former may play an important role. |
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