In our study, the effects of water stress on photosynthesis and photosynthetic electron transport chain (PETC) were studied in several ways, including monitoring the change of gas exchange parameters, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, reactive oxygen species (ROS), antioxidant enzyme activities and D1 protein levels in apple leaves. Our results show that when leaf water potential (ψ _w) is above -1.5 MPa, the stomatal limitation should be the main reason for a drop of photosynthesis. In this period, photosynthetic rate (P _N), stomatal conductance (G _s), transpiration rate (E) and intercellular CO₂ concentration (C _i) all showed a strong positive correlation with ψ _w Modulated chlorophyll fluorescence parameters related to photosynthetic biochemistry activity including maximum photochemical efficiency (F_v/F_m), actual photochemical efficiency of PSII (Φ_PSII), photochemical quenching coefficient (q _P) and coefficient of photochemical fluorescence quenching assuming interconnected PSII antennae (q _L) also showed a strong positive correlation as ψ _w gradually decreased. On the other hand, in this period, Stern-Volmer type non-photochemical quenching coefficient (NPQ) and quantum yield of light-induced non-photochemical fluorescence quenching [Y _(NPQ)] kept going up, which shows an attempt to dissipate excess energy to avoid damage to plants. When ψ _w was below -1.5 MPa, P _N continued to decrease linearly, while C _i increased and a 'V' model presents the correlation between C _i and ψ _w by polynomial regression. This implies that, in this period, the drop in photosynthesis activity might be caused by non-stomatal limitation. F_v/F_m, Φ_PSII, q _P and q _L in apple leaves treated with water stress were much lower than in control, while NPQ and Y _(NPQ) started to go down. This demonstrates that excess energy might exceed the tolerance ability of apple leaves. Consistent with changes of these parameters, excess energy led to an increase in the production of ROS including H₂O₂ and O₂ ^{•^-} Although the activities of antioxidant enzymes like catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD) increased dramatically and ascorbate peroxidase (APX) decreased in apple leaves with drought stress, it was still not sufficient to scavenge ROS. Consequently, the accumulation of ROS triggered a reduction of net D1 protein content, a core protein in the PSII reaction center. As D1 is responsible for the photosynthetic electron transport from plastoquinone A (Q_A) to plastoquinone B (Q_B), the capacity of PETC between Q_A and Q_B was considerably downregulated. The decline of photosynthesis and activity of PETC may result in the shortage of adenosine triphosphate (ATP) and limitation the regeneration of RuBP (J _max), a key enzyme in CO₂ assimilation. These are all non-stomatal factors and together contributed to decreased CO₂ assimilation under severe water stress.

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