Impaired Stomatal Control Is Associated with Reduced Photosynthetic Physiology in Crop Species Grown at Elevated [CO(2)]

Physiological control of stomatal conductance (G(s)) permits plants to balance CO(2)-uptake for photosynthesis (P(N)) against water-loss, so optimizing water use efficiency (WUE). An increase in the atmospheric concentration of carbon dioxide ([CO(2)]) will result in a stimulation of P(N) and reduction of G(s) in many plants, enhancing carbon gain while reducing water-loss. It has also been hypothesized that the increase in WUE associated with lower G(s) at elevated [CO(2)] would reduce the negative impacts of drought on many crops. Despite the large number of CO(2)-enrichment studies to date, there is relatively little information regarding the effect of elevated [CO(2)] on stomatal control. Five crop species with active physiological stomatal behavior were grown at ambient (400 ppm) and elevated (2000 ppm) [CO(2)]. We investigated the relationship between stomatal function, stomatal size, and photosynthetic capacity in the five species, and then assessed the mechanistic effect of elevated [CO(2)] on photosynthetic physiology, stomatal sensitivity to [CO(2)] and the effectiveness of stomatal closure to darkness. We observed positive relationships between the speed of stomatal response and the maximum rates of P(N) and G(s) sustained by the plants; indicative of close co-ordination of stomatal behavior and P(N). In contrast to previous studies we did not observe a negative relationship between speed of stomatal response and stomatal size. The sensitivity of stomata to [CO(2)] declined with the ribulose-1,5-bisphosphate limited rate of P(N) at elevated [CO(2)]. The effectiveness of stomatal closure was also impaired at high [CO(2)]. Growth at elevated [CO(2)] did not affect the performance of photosystem II indicating that high [CO(2)] had not induced damage to the photosynthetic physiology, and suggesting that photosynthetic control of G(s) is either directly impaired at high [CO(2)], sensing/signaling of environmental change is disrupted or elevated [CO(2)] causes some physical effect that constrains stomatal opening/closing. This study indicates that while elevated [CO(2)] may improve the WUE of crops under normal growth conditions, impaired stomatal control may increase the vulnerability of plants to water deficit and high temperatures.