Abscisic acid acts essentially on stomata, not on xylem, to improve drought tolerance

Drought resistance is 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 less studied. We selected four tomato genotypes differing in their ability to accumulate ABA to investigate its role in drought-induced dysfunction. All genotypes displayed similar leaf and stem embolism resistance, along with comparable leaf hydraulic resistance. The only difference was observed between the two extreme genotypes: sitiens (sit; a strongly ABA-deficient mutant) and sp12 (a transgenic line that constitutively over-accumulates ABA). The water potential inducing 50% embolism was 0.25 MPa lower in sp12 than in sit. Maximum stomatal and minimum leaf conductances were considerably lower in plants with higher ABA biosynthesis (WT and sp12). Variations in gas exchange were associated with both ABA levels and differences in stomatal density and size. Plants with higher ABA biosynthesis exhibited lower water loss, delaying lethal water potentials associated with embolism during drought. In conclusion, ABA does not impact xylem embolism resistance; instead, its primary mechanism for enhancing drought tolerance involves reducing water loss, thus delaying dehydration and hydraulic dysfunction.