3.4 | Stomatal conductance and stomatal limitation
Stomatal conductance did not exhibit strong temperature responses (Fig. 5). While conductance tended to be higher in control plants at low temperatures, this pattern reversed at higher temperatures, as illustrated by diverging temperature trends in Fig. 5. Stomatal conductance decreased in response to increasing VPD, with conductance at low VPD being significantly higher in controls (as indicated by non-overlapping confidence intervals in Fig. 4). Transfer to treatment conditions reduced stomatal conductance by ~43% (ANCOVA with Tukey post hoc test, t = −3.58, P = 0.003), whereas transfer from treatment to control conditions did not change conductance (Fig. 5). Consistent with the weak temperature response of stomatal conductance, stomatal limitation did not increase with increasing measurement temperature (Fig. 6). Below 35°C stomatal limitation was higher in control plants transferred to treatment conditions than in control conditions (two-tailed t-test, df = 26, t = 3.3, P = 0.003), consistent with their reduced stomatal conductance (Fig. 5). Temperature responses of photosynthesis at fixed Ci of 270 ppm were comparable to those of P400, and curves with Ci of 505 were similar to curves of P800. TOpt of P270 was moderately higher than that of P400, particularly for control plants and treatment plants moved to control conditions, but the 90% credible intervals overlapped; TOpt differences between P505 and P800 were much smaller (Fig. S2). These results suggest that stomatal conductance had some influence on TOpt at ambient but not elevated measurement CO2.
The Ci/ Ca ratio was not affected by treatment conditions, and there were no short-term temperature response patterns in Ci/ Ca (Fig. S3).