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).