Elevated CO2 reduces stomatal and metabolic limitations on photosynthesis caused by salinity in Hordeum vulgare.

The future environment may be altered by high concentrations of salt in the soil and elevated [CO(2)] in the atmosphere. These have opposite effects on photosynthesis. Generally, salt stress inhibits photosynthesis by stomatal and non-stomatal mechanisms; in contrast, elevated [CO(2)] stimulates photosynthesis by increasing CO(2) availability in the Rubisco carboxylating site and by reducing photorespiration. However, few studies have focused on the interactive effects of these factors on photosynthesis. To elucidate this knowledge gap, we grew the barley plant, Hordeum vulgare (cv. Iranis), with and without salt stress at either ambient or elevated atmospheric [CO(2)] (350 or 700 μmol mol(-1) CO(2), respectively). We measured growth, several photosynthetic and fluorescence parameters, and carbohydrate content. Under saline conditions, the photosynthetic rate decreased, mostly because of stomatal limitations. Increasing salinity progressively increased metabolic (photochemical and biochemical) limitation; this included an increase in non-photochemical quenching and a reduction in the PSII quantum yield. When salinity was combined with elevated CO(2), the rate of CO(2) diffusion to the carboxylating site increased, despite lower stomatal and internal conductance. The greater CO(2) availability increased the electron sink capacity, which alleviated the salt-induced metabolic limitations on the photosynthetic rate. Consequently, elevated CO(2) partially mitigated the saline effects on photosynthesis by maintaining favorable biochemistry and photochemistry in barley leaves.