Influence of water stress on photosynthetic characteristics in barley plants under ambient and elevated CO2 concentrations

We evaluated the combined effects of elevated CO2 and water availability on photosynthesis in barley. Soil and plant water content decreased with water stress, but less under elevated CO2 concentration (EC) compared with ambient CO2 concentration (AC). During water stress, stomatal conductance, carboxylation rate, RuBP regeneration, and the rate of triose phosphate utilisation (TPU) were decreased but less when plants grew under EC. Drought treatments caused only a slight effect on maximum photochemical efficiency (variable to maximum fluorescence ratio, Fv/Fm), whereas the actual quantum yield (ΦPS2), maximum electron transport rate (Jmax) and photochemical quenching (qP) were decreased and the non photochemical quenching (NPQ) was enhanced. Under water deficit, the allocation of electrons to CO2 assimilation was diminished by 49 % at AC and by 26 % at EC while the allocation to O2 reduction was increased by 15 % at AC and by 12 % at EC.