Europ.J.Hort.Sci., 74 (5). S. 210–217, 2009, ISSN 1611-4426. © Verlag Eugen Ulmer KG, Stuttgart Europ.J.Hort.Sci. 5/2009 Effect of Rapid Dehydration on Photosynthetic and Fluorescent Properties of Cucumber Leaves Detached from Low Light Treated Seedlings W. Li 1,2) , X.L. Sui 1) , L.H. Gao 1) , H.Z. Ren 1) and Z.X. Zhang 1) ( 1) College of Agronomy and Biotechnology, China Agricultural University, Beijing, China and 2) College of Agronomy, Guizhou University, Guiyang, China) Summary Two cucumber (Cucumis sativus L.) cultivars, includ- ing the shade-susceptible cv. ‘Jingyan 2’ and shade-tol- erant cv. ‘Deltastar’, were compared for the changes in gas exchange and chlorophyll a fluorescent parame- ters during rapid dehydration of mature leaves de- tached from seedlings grown for 20 days under low light (PFD: 75–85 µmol m –2 s –1 ). During the continu- ous 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′), quan- tum 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) grad- ually increased, and the maximal photochemical effi- ciency of PSII (Fv/Fm) remained largely stable. The declining of the photosynthetic parameters was faster and lasted shorter period than the fluorescent param- eters. The impact of dehydration was much more se- vere in the control plants grown under normal light in- tensity, which indicates that plants grown under low light had improved tolerance to subsequent water de- ficient 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 dehydra- tion process. These characteristics may account for the better tolerance to shade and drought condition of ‘Deltastar’ compared to ‘Jingyan 2’. Key words. Cucumis sativus – 1ow light – relative leaf water content – gas exchange parameters – chlorophyll a fluorescence – photosynthetic critical water content Introduction Low light is a major limiting factor for greenhouse cu- cumber production in the spring season. Insufficient light intensity reduces photosynthetic activities, and affects plant growth and development. Crop production also de- pends on appropriate water supply; drought stress causes reduction in net photosynthesis (Pn), apparent quantum yield (AQY), transpiration (Tr), photosynthetic satura- tion light intensity, and CO 2 saturation point. Concur- rently, light and CO 2 compensation points become higher (ZHU et al. 2005; KE and Y ANG 2007). The reduction of the fluorescent light parameters (such as Fo, Fm, Fv, and Fv/Fm) suggests that photo- chemical conversion of harvested light energy in the PSII system is inhibited by drought stress. The lower T 1/2 and PQ value represents a decline of photosynthetic potential activity in PSII (Fv/Fo). In plant leaves, inhibition of pho- tosynthetic electron transmission and photosynthetic phosphorylation can directly cause damages to the light emission complexes, such as the PSII oxygen-evolving complex, PSII light-harvesting pigment-protein complex, which eventually affects photosynthetic activities (LU et al. 1993, 1999). Desiccation stress activates production of reactive ox- ygen species (ROS). The subsequent physiological re- sponses include marked concentration of malondialche- hyche (MDA) due to membrane lipid peroxidation, re- duced membrane lipid fluidity with lower unsaturated fatty acid content, and structural deformation of chloro- plast and mitochondria in the mesophyll cells (LI et al. 2005). Leaves experience natural water loss after being separated from plants, meanwhile, their chlorophyll flu- orescent parameter qN increases, and the xanthophyll cy- cle is activated to dissipate excessive light energy in order to alleviate or avoid damage to the photosynthetic appa- ratus (FENG et al. 2002). Those plant responses are incurred when they are ex- posed to either water or light stress. No relevant studies have been conducted to understand the combined impact of water and low light stresses on photosynthesis, such as the physiological response of leaves grown under low light conditions to the ensuing dehydration condition. In this research, cucumber seedlings were treated with low