Moderate Water Stress Increased the Photosynthesis Adaptability of Cucumber Seedlings Under Low Light

The effects of water stress(40% maximum moisture capacity)on the photosynthetic and fluorescent characteristics and the content of Rubisco of Cucumis sativus‘Deltastar'seedlings under low light(75-85 μmol · m-2 · s-1)were investigated in a phytotron.The results revealed that after 20 days of water stress under low light,the photosynthesis pigment contents per unit area of leaves of cucumber increased and the values of Chl.a/b decreased.The values of light saturated rate of the CO2 assimilation (Asat),apparent quantum yield(AQY),carboxylation efficiency(CE),maximum RuBP regeneration rate,light saturation point(LSP)increased significantly but CO2 compensation point(CCP)decreased remarkably.The chlorophyll fluorescence parameters including PSⅡ maximal photochemical efficiency (Fv/Fm),PSⅡ potential photochemical efficiency(Fv/Fo),PSⅡ actual photochemical efficiency(ΦPSⅡ), the photochemical reaction rate(Prate),the linear whole-chain electron transport rat(eJ)and the partitioning proportion for photochemical reaction of light energy absorbed by PSⅡ(P)showed increased trend,but the imbalance of partitioning of excitation energy between PSⅠand PSⅡ(β/α–1) decreased.The content of soluble protein and the content of Rubisco increased.These results indicated moderate water stress increased the photosynthesis adaptability of cucumber seedlings under low light.However,after 20 days of water stress under normal light(500–600 μmol · m-2 · s-1),the values of the items including Asat, maximum RuBP regeneration rate,CE,ΦPSⅡ,Prate and P decreased.But the values of(β/α–1) increased.So the photosynthesis of cucumber under normal light was inhibited by water stress.