Hormonal Balance, Photosynthesis, and Redox Reactions in the Leaves of <i>Caragana korshinskii</i> Kom. under Water Deficit

To evaluate the physiological responses of Korshinsk peashrub (<i>Caragana korshinskii</i> Kom.) to water deficit, photosynthetic gas exchange, chlorophyll fluorescence, and the levels of superoxide anion (O₂^•−), hydrogen peroxide (H₂O₂), malondialdehyde (MDA), antioxidant enzymes, and endogenous hormones in its leaves were investigated under different irrigation strategies during the entire growth period. The results showed that leaf growth-promoting hormones were maintained at a higher level during the stages of leaf expansion and vigorous growth, and zeatin riboside (ZR) and gibberellic acid (GA) gradually decreased with an increase in water deficit. At the leaf-shedding stage, the concentration of abscisic acid (ABA) dramatically increased, and the ratio of ABA to growth-promoting hormones increased to a high level, which indicated that the rate of leaf senescence and shedding was accelerated. At the stages of leaf expansion and vigorous growth, the actual efficiency of photosystem II (PSII) (Φ_{PS<span style="font-variant: small-caps;">ii</span>}) was downregulated with an increment in non-photochemical quenching (<i>NPQ</i>) under moderate water deficit. Excess excitation energy was dissipated, and the maximal efficiency of PSII (<i>F</i>_v/<i>F</i>_m) was maintained. However, with progressive water stress, the photo-protective mechanism was inadequate to avoid photo-damage; <i>F</i>_v/<i>F</i>_m was decreased and photosynthesis was subject to non-stomatal inhibition under severe water deficit. At the leaf-shedding stage, non-stomatal factors became the major factors in limiting photosynthesis under moderate and severe water deficits. In addition, the generation of O₂^•− and H₂O₂ in the leaves of <i>Caragana</i> was accelerated under moderate and severe water deficits, which caused an enhancement of antioxidant enzyme activities to maintain the oxidation–reduction balance. However, when the protective enzymes were insufficient in eliminating excessive reactive oxygen species (ROS), the activity of catalase (CAT) was reduced at the leaf-shedding stage. Taken all together, <i>Caragana</i> has strong drought resistance at the leaf expansion and vigorous growth stages, but weak drought resistance at the leaf-shedding stage.