Figure 3.

Working model for the role of ABA in chemical signaling of water stress. (A) Initial water stress induces root sulfate (SO₄) transport, which enhances the effect of ABA on stomata and may induce leaf ABA biosynthesis. Some ABA biosynthesized in leaves is transported to roots in phloem and from there cycled back via xylem to act on stomata. (B) As water stress continues, root β-glucosidases are expressed that cleave ABA-GE to release free ABA and initial pH increases facilitate redistribution of root ABA into the xylem. Additional SO₄ delivery and leaf water potential decreases enhance the increased ABA-induced stomatal depression. (C) Prolonged water stress leads to hydraulic changes in roots that induce biosynthesis of root ABA and large pH increases redistribute ABA from roots to xylem. The dramatically increased ABA delivery to leaves interacts with increasing SO₄ and decreasing leaf water potential to affect maximum stomatal closure. g_s, stomatal conductance to water; Ψ, water potential. Subscripts: X, xylem; L, leaf.