Impaired Stomatal Control Is Associated with Reduced Photosynthetic Physiology in Crop Species Grown at Elevated [CO2]

Abstract

Physiological control of stomatal conductance (<i>G</i>_s) permits plants to balance CO₂-uptake for photosynthesis (<i>P</i>_N) against water-loss, so optimizing water use efficiency (WUE). An increase in the atmospheric concentration of carbon dioxide ([CO₂]) will result in a stimulation of <i>P</i>_N and reduction of <i>G</i>_s in many plants, enhancing carbon gain while reducing water-loss. It has also been hypothesized that the increase in WUE associated with lower <i>G</i>_s at elevated [CO₂] would reduce the negative impacts of drought on many crops. Despite the large number of CO₂-enrichment studies to date, there is relatively little information regarding the effect of elevated [CO₂] on stomatal control. Five crop species with active physiological stomatal behavior were grown at ambient (400 ppm) and elevated (2000 ppm) [CO₂]. We investigated the relationship between stomatal function, stomatal size, and photosynthetic capacity in the five species, and then assessed the mechanistic effect of elevated [CO₂] on photosynthetic physiology, stomatal sensitivity to [CO₂] and the effectiveness of stomatal closure to darkness. We observed positive relationships between the speed of stomatal response and the maximum rates of <i>P</i>_N and <i>G</i>_s sustained by the plants; indicative of close co-ordination of stomatal behavior and <i>P</i>_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 [CO₂] declined with the ribulose-1,5-bisphosphate limited rate of <i>P</i>_N at elevated [CO₂]. The effectiveness of stomatal closure was also impaired at high [CO₂]. Growth at elevated [CO₂] did not affect the performance of photosystem II indicating that high [CO₂] had not induced damage to the photosynthetic physiology, and suggesting that photosynthetic control of <i>G</i>_s is either directly impaired at high [CO₂], sensing/signaling of environmental change is disrupted or elevated [CO₂] causes some physical effect that constrains stomatal opening/closing. This study indicates that while elevated [CO₂] 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.