Research Plant Physiologist USDA ARS Raleigh, North Carolina
Drought tolerance mechanisms are essential for plant production under water limiting environments. Abscisic acid (ABA) plays a critical role in stomatal function during drought but its impact on hydraulic function beyond the stomata is far less studied. In this study, we selected four tomato genotypes differing in their ability to accumulate ABA to investigate the role of this hormone on drought-induced hydraulic dysfunction. All four genotypes exhibited similar leaf and stem embolism resistance as well as leaf hydraulic resistance. The only difference was observed between plants of the two most extreme genotypes: sitiens (sit; an ABA-deficient mutant) and sp12 (an ABA overaccumulating genotype), in which sp12 plants exhibited xylem that was 0.25 MPa more resistant to embolism than the leaves of sit plants. Maximum and minimum stomatal conductances were considerably higher in the two ABA-deficient mutant lines than in WT and sp12 plants. Variation in gas exchange across genotypes were associated with both ABA levels and differences in stomatal density and size. Higher stomatal conductance in the ABA-deficient mutant plants meant that lethal water potentials associated with embolism occurred earlier than in WT and sp12 plants in dry-down experiments. The primary pathway by which ABA enhances drought tolerance is via stomatal regulation and only to minor extent embolism resistance.