Abstract:

【Objective】 The purpose of this study is to explore the photosynthetic response and mechanism of cucumber seedlings to double CO2 concentration combined with drought stresses.【Method】 Using split plot design, the effects of doubled CO2 concentration on photosynthesis of cucumber (Cucumis sativus L. ‘Jinyou No.1’) seedlings under drought stresses were investigated. The main plot was two CO2 concentrations (ambient CO2 concentration, ≈380 μmol?mol-1, and doubled CO2 concentration, 760±20 μmol?mol-1), and the split-plot was three water treatments (control, moderate drought stress and severe drought stress) simulated by PEG 6000.【Result】 Drought stresses remarkably reduced net photosynthetic rate, transpiration rate and stomatal conductance of leaves of cucumber seedlings. Under the condition of doubled CO2 concentration, net photosynthetic rate dramatically increased while transpiration rate and stomatal conductance decreased, thereby water use efficiency dramatically increased. The interact effects of doubled CO2 concentration and drought stress on net photosynthetic rate, transpiration rate, water use efficiency as well as stomatal conductance were all significant (P<0.01). Drought stresses significantly reduced apparent quantum efficiency, maximum assimilation rate, maximum rate of carboxylation by Rubisco, PAR-saturated rate of electron transport, rate of triose phosphate utilization and light saturated points of leaves of cucumber seedlings, but enhanced light compensatory points. Under control and mild drought stress conditions, doubled CO2 concentration dramatically improved apparent quantum efficiency and maximum assimilation rate, but there were no significant differences under severe drought stress. Under control condition, doubled CO2 concentration dramatically enhanced maximum rate of carboxylation by Rubisco, PAR-saturated rate of electron transport and rate of triose phosphate utilization, but reduced CO2 compensatory points and respiration rate, there were no significant differences under drought stresses except CO2 compensatory points reduced by 4.9% under moderate drought stress. 【Conclusion】 The decrease of net photosynthesis under moderate drought stress was due to stoma restriction, while it was due to stoma and non-stoma combined restriction under severe drought stress. Drought stresses resulted in dramatic decline of photosynthetic electron transport and photophosphorylation capacity of cucumber seedlings. Doubled CO2 concentration improved water use efficiency of cucumber seedlings by improving net photosynthetic rate and reducing transpiration rate, and improved apparent quantum efficiency and CO2 assimilation maximum of cucumber seedlings under moderate drought stress condition to meliorate photosynthetic capacity. Therefore, it is speculated that in arid or semi-arid areas, CO2 fertilization in protected agriculture or rising atmospheric CO2 concentration in future could meliorate plant water status, enhance drought resistance ability and improve water use efficiency to some extent, subsequently alleviate the negative effect of drought stresses.

Key words: doubled CO2 concentration, drought stress, cucumber, photosynthetic characteristics