Maize Zmasft double mutant versus wildtype
The enrichment of water at the sites of evaporation above source water (Δ18Oe) was 4.1 times more responsive to changes in RH than to changes in light intensity, meaning thate a/e i is a more influential variable in determining Δ18Oe thang s-driven changes in εk. Δ18Oe did not differ between genotypes in either RH or light intensity, while maize Zmasft double mutants were less enriched in leaf water oxygen isotope enrichment (Δ18OLW) than wildtype in every treatment (14.24 ± 1.48 and 16.14 ± 1.41 ‰, respectively) (Table 1; Fig. 1a, b). In all treatments, Δ18OLW was consistently less enriched in the Zmasft double mutants than wildtype by 0.5 to 1 ‰ (Table 1; Fig. 1c, d). Additionally, Δ18Oe values trended lower in wildtype, contrary to what is expected if differences in Δ18Oe were driving the genotypic differences in Δ18OLW. Nonetheless, Δ18Oe has a drastic effect on Δ18OLW in that Δ18OLW was 11.65 ± 0.65 ‰ lower at 80 % RH than at 50 % RH, ranging from 9.44 to 22.80 ‰. The response to light intensity was much less in that Δ18OLW was 2.90 ± 0.50 ‰ lower at 300 than at 1200 μmol photons m-2 s-1, and there was no difference at 80 % RH (Table 1, S1; Fig. 1).
The fractional contribution of source water (unenriched xylem water;f sw) was slightly greater in Zmasftmutants (0.22 ± 0.03) than wildtype (0.16 ± 0.03). Alternatively, for both genotypes f sw was greater under 1200 compared to 300 µmol quanta m-2 s-1(0.27 ± 0.1 and 0.1 ± 0.02, respectively) (Table 1; Fig. 2a) and RH increased f sw under 300 but not 1200 µmol quanta m-2 s-1 (Table 1; Fig. 2a). Transpiration rate (E ; mol H2O m-2 s-1) was greater in theZmasft double mutants than in wildtype across treatments (P < 0.0001), and this difference was most apparent under high light intensity. At high light intensity, E was higher than at low light intensity, and RH only affected E at low light intensity (Table 1; Fig. 2b). Similarly, stomatal conductance (g s; mol H2O m-2 s-1) was 44 % greater inZmasft double mutant, and this genotypic difference was most apparent at high light intensity (Table 1).