Effects of doubled CO2 concentration on physiological characteristics of cucumber seedlings under drought stresses

The purpose of this study is to explore the physiological response mechanism of cucumber seedlings to doubled CO2 concentration ([CO2]) and drought stresses. Using split plot design, effects of doubled [CO2] on photosynthetic characteristics and the mitigation mechanism of doubled [CO2] to oxidative damage of cucumber (Cucumis sativus L. ‘Jinyou No.1’) seedlings under drought stresses were investigated in two Open-Top-Greenhouse (OTG). The main plot was two [CO2] (ambient [CO2], ≈380 µmol/mol, and doubled [CO2], 760±20 μmol/mol), and the split-plot was three water treatments (control, mild and severe drought stress) simulated by PEG 6000. The results showed that the fall of net photosynthetic under mild drought stress was due to stoma restriction, while it was duo to stoma and non-stoma combined restriction under severe drought stress. Under control and mild drought stress conditions, doubled [CO2] improved apparent quantum efficiency (AQY) and CO2 assimilation maximum rate (A(maxl)), improved maximum rate of carboxylation by Rubisco (V(cmax)), PAR-saturated rate of electron transport (J(max)) and rate of triose phosphate utilization (TPU). With the aggravation of drought stress, the accumulation of reactive oxygen species (ROS) induced, while the antioxidative enzyme (SOD, POD, CAT) activity and antioxidants (AsA and GSH) content increased significantly. Doubled [CO2] enhanced the activity of antioxidative enzymes rather than increased the content of antioxidants of cucumber seedlings to scavenge the accumulation of ROS resulted from drought stresses. In conclusion, under drought stress conditions, doubled [CO2] can promote photosynthetic electron transport and photophosphorylation capacity of cucumber seedlings, meliorate plant water status and improve drought resistance ability, subsequently alleviate the negative effect of drought stress.