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

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