Effect of Rapid Dehydration on Photosynthetic and Fluorescent Properties of Cucumber Leaves Detached from Low Light Treated Seedlings

Two cucumber (Cucumis sativus L.) cultivars, including the shade-susceptible cv. 'Jingyan 2' and shade-tolerant cv. 'Deltastar', were compared for the changes in gas exchange and chlorophyll a fluorescent parameters during rapid dehydration of mature leaves detached from seedlings grown for 20 days under low light (PFD: 75–85 μmol m-2 s-1). During the continuous water loss process after separation from plants, leaf net photosynthesis rate (Pn), transpiration rate (Tr), efficiency of excitation energy captured by open photosystem II (PSII) reaction centre (Fv′/Fm′), quantum efficiency of PSII photochemistry (ΦPSII) and partitioning proportion for photochemical reaction of light energy absorbed by PSII (P) gradually decreased, the antennae heat dissipation (D) and reaction centres dissipation of light energy absorbed by PSII (Ex) gradually increased, and the maximal photochemical efficiency of PSII (Fv/Fm) remained largely stable. The declining of the photosynthetic parameters was faster and lasted shorter period than the fluorescent parameters. The impact of dehydration was much more severe in the control plants grown under normal light intensity, which indicates that plants grown under low light had improved tolerance to subsequent water deficient stress. Between the two cultivars both exposed to low light, cv. 'Deltastar' was able to maintain more stable chl fluorescence, photosynthetic parameters and the critical water content during rapid dehydration process. These characteristics may account for the better tolerance to shade and drought condition of 'Deltastar' compared to 'Jingyan 2'.