Abstract
Physiological control of stomatal conductance (Gs) permits plants to balance CO2-uptake for photosynthesis (PN) 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 PN and reduction of Gs in many plants, enhancing carbon gain while reducing water-loss. It has also been hypothesized that the increase in WUE associated with lower Gs 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 PN and Gs sustained by the plants; indicative of close co-ordination of stomatal behavior and PN. 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 PN 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 Gs 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.
Highlights
Stomatal pores act as the interface between the plant and the atmosphere, regulating the uptake of CO2 for photosynthesis (PN) and the loss of water via transpiration
The maximum rate of stomatal conductance was significantly correlated to the speed of stomatal closure during the initial 50% reduction in Gs H2O (Figure 3C), but not to the maximum extent of stomatal closure (Figure 3D)
In plants grown at an ambient [CO2] of 400 ppm, no relationship was observed between stomatal pore length (SPL) and the speed of stomatal closure during the initial 50% reduction in Gs H2O (Figure 4A)
Summary
Stomatal pores act as the interface between the plant and the atmosphere, regulating the uptake of CO2 for photosynthesis (PN) and the loss of water via transpiration. The vast majority of crops currently cultivated possess active stomatal physiological behavior that permits the levels of Gs required to sustain high PN, and the capacity to respond rapidly to a change in environmental conditions (Kalaji and Nalborczyk, 1991; Haworth et al, 2015; Roche, 2015). It is unclear how active physiological stomatal behavior in crop plants can be affected by changes in the atmospheric concentration of [CO2], and whether growth at elevated [CO2] can induce a loss of stomatal control
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