Whole-plant hydraulic conductance and root-to-shoot flow of abscisic acid are independently affected by water stress in grapevines

In order to investigate whether plant hydraulic conductance (g(plant)) is reduced under drought conditions via an ABA-related mechanism, a water-stress experiment was carried out using split-rooted grapevines. In addition, inversion of shoot growth orientation was imposed to reduce g(plant) independently of soil water availability, and thus of the putative ABA root-generated stress message. As expected, water stress imposed on split-roots affected ABA accumulation. ABA drought-stress message negatively affected stomatal conductance (g(s)) and transpiration (E), but modified neither leaf or stem water potentials (Psi(leaf) and Psi(stem), respectively), nor g(plant). When g(plant) was reduced in split-rooted, shoot-inverted (s-r/s-i) grapevines, Psi(leaf) and Psi(stem) decreased, without changes in ABA accumulation, g(s) and E. The ABA drought-stress message did not modify g(plant), nor did g(plant) (impaired by shoot-growth inversion) decrease ABA delivery to the leaves. However, leaf growth was depressed in s-r/s-i grapevines. The fact that no interaction between ABA stress messages (caused by split-root technique) and hydraulic constraints to sap flow (caused by shoot inversion) was necessary to impair leaf growth suggests that the targets of ABA and hydraulic-limitation effects on leaf expansion are not the same.