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The influence of partial rootzone drying on grape quality

by Brian Lovey^A, Jim Grant^A, Manfred Stoll^A, Peter Dry^B and Michael McCarthy^C
ACSIRO Division of Plant Industry, ^BDepartment of Horticulture, Viticulture & Oenology, University of Adelaide, ^CSouth Australian Research and Development Institute
SA Viticulture Technical Conference, 1997

Introduction

Over the past five years we have been developing a technique which we have called Partial Rootzone Drying (PRD). This technique has the potential to significantly reduce vine water use, reduce canopy vigour, maintain crop yields and fruit quality when compared with normal irrigation methods. Research work carried out over the last ten years or so by the CSIRO has shown that during the early stages of water stress a hormone called abscisic acid (ABA) is synthesised in drying roots and this is transported to the leaves where it reduces water loss by inducing the partial stomatal closure. The PRD technique maintains both wet and dry roots on the same vine. The drying roots produce ABA which induces many of the responses normally associated with water stress, for example reduced water use and reduced shoot growth, while the wet roots maintain an adequate water supply to the canopy. While one of the major benefits of the PRD technique is reduced water use, we have also shown that grape quality is changed and many of the desirable fruit characters normally associated with water stress are exhibited.

Materials and methods

Our experiments have been carried out in three main stages; in the first instance plants with two root systems were produced from cuttings which had been sawn longitudinally from the base up through the lower two internodes. Each root system was planted in a separate plot. This allowed us to investigate the effects on the physiology of the vine of drying one or both root systems. Secondly, these twin rooted vines were planted in the field with one root system on either side of a plastic sheet buried in the soil to a depth of about 1.4m. Irrigation was applied to each vine by drippers positioned on either side of the plastic sheet. This allowed us to dry either root system in a controlled way. Finally, large scale field trials have been carried out using pre-existing, conventionally grown vines. The irrigation systems of these vines were modified to allow water to be applied to either side of the vine. We have used a number of different configurations, all designed to provide water independently to either side of the vine; a) sub-surface drip lines positioned on either side of the vine row at a depth of 250-300mm, b) surface drippers supplied by two lines per vine row such that each vine has two drippers positioned on either side of the trunk, c) half-circle micro-sprinklers positioned along the centre line of the row, throwing to either side of the vines.

In all experiments we have monitored vine performance through measurements of shoot growth, stomatal conductance and production of the hormone ABA. Fruit quality measures included pH, titratable acidity, colour, total phenolics and glycosyl-glucose.

Results

We learned from our early pot experiments that by withholding water from one root system, stomatal conductance, photosynthesis and growth were reduced when compared with vines with both roots kept fully irrigated. A surprising finding was that the effect was transient, and despite the fact that one root system remained dry, conductance, photosynthesis and growth returned t pre-treatment levels within a few weeks. However, by alternately drying either side of the vine roots on a roughly two weekly cycle the effects could be sustained over an entire season. (Table 1).

Discussion

The PRD technique was originally conceived as a means of reducing vine vigour and the reduction in shoot growth and increase in light entering the bunch zone (bunch exposure index, Table 1) in treated vines shows that this has to some extent been achieved. However, the real benefit of the technique is likely to come from improvements in the water use efficiency of the vine and improved fruit quality.