| The 10 leaf age seedling of Malus robusta Rehd. was used as a test material, and the pot experiments by controlling water content was applied in combination with spraying exogenous paclobutrazol (PBZ) to study the effects of different concentration exogenous paclobutrazol(0,50,100,150,200 mg·L-1) on Malus robusta Rehd. under water stress, and explore the changes of the photosynthesis, fluorescence and physiological characteristics. The results showed that:compared with the control, exogenous application of PBZ effectively slowed the increasing range of the intercellular CO2 concentration(Ci), non-photochemical quenching coefficient (NPQ), peroxidase activity (POD), electrolyte leakage(REC), Proline(Pro), and malondialdehyde (MDA) content of Malus robusta Rehd. under water stress, and alleviated the decreasing range of net photosynthetic rate(Pn), stomatal conductance(Gs), transpiration rate(Tr), the maximum quantum yield of PSII (Fv/Fm), actual photochemical efficiency(ΦPSⅡ), photochemical quenching coefficient(qP), the activity of catalase(CAT), the activity of superoxide dismutase(SOD). The NPQ in the control group and the treatment groups were increased by 400.00%, 348.28%, 212.90%, 153.13%, 292.86%, 266.67% respectively, the Pro respectively increased by 312.50%, 246.67%, 226.83%, 129.55%, 166.67%, 189.58%, and the Pn respectively decreased by 53.72%, 50.00%, 35.71%, 22.28%, 32.78%, 31.52%, with ΦPSⅡ decreased by 51.22%, 48.19%, 35.00%, 30.33%, 41.25%, 40.74%. Also the change degree of each index in PBZ treatment groups was lower than that in the control group, and the change degree of each index in 100 mg·L-1 PBZ treatment group was lower than that of the other PBZ treatment groups, which exhibited significant concentration-dependent effect. PBZ activate the defense system and protect the cell membrane system of seedlings under water stress, thus improving its drought tolerance The comprehensive evaluation of the indicators using the fuzzy membership analysis and principal component analysis showed that the 100 mg·L-1 PBZ treatment had the most significant effect on the water stress relief of Malus robusta Rehd. |