Effects of the interaction between vapor-pressure deficit and salinity on growth and photosynthesis of Cucumis sativus seedlings under different CO<sub>2</sub> concentrations

Abstract

We studied growth and photosynthesis of cucumber (Cucumis sativus) seedlings under two vapor-pressure deficit levels (VPD; 0.4 and 3.0 kPa), two salinity levels (0 mM and 34 mM NaCl), and two CO2 concentrations ([CO2]; 400 and 1,000 μmol mol–1). Relative growth rate (RGR) decreased with increasing VPD, but the causal factor differed between salinity levels and CO2 concentrations. Under ambient [CO2], RGR decreased with increasing VPD at low salinity mainly due to decreased leaf area ratio (LAR), and decreased net assimilation rate (NAR) at high salinity. The decrease in intercellular [CO2] (C i) with decreasing stomatal conductance caused by high VPD did not significantly limit net photosynthetic rate (P N) at low salinity, but P N was potentially limited by C i at high salinity. At high [CO2], high VPD reduced LAR, but did not affect NAR. This is because the decrease in C i occurred where slope of P N–C i curve was almost flat.