Hormonal changes induced by partial rootzone drying of irrigated grapevine

Partial rootzone drying (PRD) is a new irrigation technique which improves the water use efficiency (by up to 50%) of wine grape production without si gnificant crop reduction. The technique was developed on the basis of knowl edge of the mechanisms controlling transpiration and requires that approxim ately half of the root system is always maintained in a dry or drying state while the remainder of the root system is irrigated. The wetted and dried sides of the root system are alternated on a 10-14 d cycle, Abscisic acid ( ABA) concentration in the drying roots increases 10-fold, but ABA concentra tion in leaves of grapevines under PRD only increased by 60% compared with a fully irrigated control. Stomatal conductance of Vines under PRD irrigati on was significantly reduced when compared with vines receiving water to th e entire root system. Grapevines from which water was withheld from the ent ire root system, on the other hand, show a similar reduction in stomatal co nductance, but leaf ABA increased 5-fold compared with the fully irrigated control, PRD results in increased xylem sap ABA concentration and increased xylem sap pH, both of which are likely to result in a reduction in stomata l conductance, In addition, there was a reduction in zeatin and zeatin-ribo side concentrations in roots, shoot tips and buds of 60, 50 and 70%, respec tively, and this may contribute to the reduction in shoot growth and intens ified apical dominance of vines under PRD irrigation. There is a nocturnal net flux of water from wetter roots to the roots in dry soil and this may a ssist in the distribution of chemical signals necessary to sustain the PRD effect. It was concluded that a major effect of PRD is the production of ch emical signals in drying roots that are transported to the leaves where the y bring about a reduction in stomatal conductance.