Partial rootzone drying (PRD) is a new irrigation technique which improves the water use efficiency (by up to 50%) of wine grape production without significant crop reduction. The technique was developed on the basis of knowledge of the mechanisms controlling transpiration and requires that approximately 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 concentration in leaves of grapevines under PRD only increased by 60% compared with a fully irrigated control. Stomatal conductance of vines under PRD irrigation was significantly reduced when compared with vines receiving water to the entire root system. Grapevines from which water was withheld from the entire root system, on the other hand, show a similar reduction in stomatal conductance, 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 stomatal conductance. In addition, there was a reduction in zeatin and zeatin-riboside concentrations in roots, shoot tips and buds of 60, 50 and 70%, respectively, and this may contribute to the reduction in shoot growth and intensified 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 assist 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 chemical signals in drying roots that are transported to the leaves where they bring about a reduction in stomatal conductance.

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Maintaining the most cost‐effective balance between vegetative and reproductive growth is one of the most testing problems in modern viticulture. Grapevines which exhibit excessive vegetative vigour are likely to produce less fruit of reduced quality, and vines with inadequate vigour may be compromised in terms of their yield potential. The requirement for techniques to better manage excess vigour has become more acute in recent years with the increased use of irrigation, adoption of vigour‐imparting rootstocks and the expansion of vineyards into cooler geographic regions. A number of strategies may be used to control vine vigour. Chemical growth regulators, although capable of reducing shoot vigour, have never received acceptance due to undesirable side effects and concerns over chemical residues. Devigorating rootstocks, likewise, may have the potential to control vigour but none are in wide commercial use. Restriction of the effective root volume, achieved through manipulation of planting densities, competition by cover crops, regulation of the soil volume wetted by drip irrigation or regulation of water availability can all achieve a degree of devigoration but often at the expense of fruit yield. Manipulation of vines through pruning and trellis design are probably the most commonly used methods for the control of shoot vigour. A high number of nodes retained at pruning combined with trellises which allow open canopies have proved successful. Advances in the understanding of the physiological factors influencing shoot growth and transpiration have allowed the development of novel irrigation methods for the control of vine vigour. These techniques exploit the fact that chemical signals originating in the roots are primarily responsible for the control of shoot growth and transpiration. Stimulation of the production of these signals through partial drying of the root system results in a significant reduction in shoot growth and water‐use while maintaining crop yield and improving fruit quality. These new techniques, in combination with appropriate pruning and trellising methods, are providing new viticultural tools for controlling vine vigour and water‐use efficiency.

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