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Water is critical for viticulture sustainability since grape production, quality and economic viability are largely dependent on water availability. The total water consumption of vineyards, 300 to 700 mm, is generally higher than the annual average precipitation in many viticultural areas, which induces a risk for sustainability of vineyards. Improving vineyard water use efficiency (WUE) is therefore crucial for a sustainable viticulture industry in semi-arid regions. Increased sustainability of water resources for vineyards can be achieved using both agronomical technology and cultivar selection. Here, we review advances in grapevine water use efficiency related to changes in agronomical practices and genetic improvements. Agronomical practices focus on increasing green water use by increasing soil water storage capacity, reducing direct soil water loss, or limiting early transpiration losses. Cover crops for semi-arid areas show a favorable effect, but careful management is needed to avoid excessive water consumption by the cover crop. Canopy management practices to reduce excessive water use are also analyzed. This is a genetic based review focused on identifying cultivars with higher WUE.

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Reduced stomatal conductance (gs ) during soil drought in angiosperms may result from effects of leaf turgor on stomata and/or factors that do not directly depend on leaf turgor, including root-derived abscisic acid (ABA) signals. To quantify the roles of leaf turgor-mediated and leaf turgor-independent mechanisms in gs decline during drought, we measured drought responses of gs and water relations in three woody species (almond, grapevine and olive) under a range of conditions designed to generate independent variation in leaf and root turgor, including diurnal variation in evaporative demand and changes in plant hydraulic conductance and leaf osmotic pressure. We then applied these data to a process-based gs model and used a novel method to partition observed declines in gs during drought into contributions from each parameter in the model. Soil drought reduced gs by 63-84% across species, and the model reproduced these changes well (r(2)  = 0.91, P < 0.0001, n = 44) despite having only a single fitted parameter. Our analysis concluded that responses mediated by leaf turgor could explain over 87% of the observed decline in gs across species, adding to a growing body of evidence that challenges the root ABA-centric model of stomatal responses to drought.

In woody plants, photosynthetic capacity is closely linked to rates at which the plant hydraulic system can supply water to the leaf surface. Drought-induced embolism can cause sharp declines in xylem hydraulic conductivity that coincide with stomatal closure and reduced photosynthesis. Recovery of photosynthetic capacity after drought is dependent on restored xylem function, although few data exist to elucidate this coordination. We examined the dynamics of leaf gas exchange and xylem function in Eucalyptus pauciflora seedlings exposed to a cycle of severe water stress and recovery after re-watering. Stomatal closure and leaf turgor loss occurred at water potentials that delayed the extensive spread of embolism through the stem xylem. Stem hydraulic conductance recovered to control levels within 6 h after re-watering despite a severe drought treatment, suggesting an active mechanism embolism repair. However, stomatal conductance did not recover after 10 d of re-watering, effecting tighter control of transpiration post drought. The dynamics of recovery suggest that a combination of hydraulic and non-hydraulic factors influenced stomatal behaviour post drought.

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Background and Aims: Improving water-use efficiency (WUE) is desirable for future grapevine growth and grape production, especially in Mediterranean areas where water is predicted to be limiting. Understanding the genetic variability in WUE is important to identify the most appropriate cultivars to be used in semi-arid areas. Most previous studies have focused at leaf-level WUE, while information on whole-plant level is scarce. This study explored the genetic variability of grapevine in whole-plant WUE (WUEWP) to determine whether several leaf WUE (WUEl) indicators are suitable as proxies of WUEWP. Methods and Results: Three similar experiments were performed to compare WUE in up to eight different grapevine cultivars under irrigation and water-stress treatments. Although WUEl and WUEWP varied with cultivar and treatment, WUEl was not a reliable parameter to predict WUEWP. Conclusions: Large variability in WUEWP between grapevine cultivars was observed, although this variability was not described by leaf-level indicators of WUE. Significance of the Study:This study showed that the large variability existing for WUEWP in different cultivars offers an potential method for selecting the more suitable cultivars to grow in water-scarce viticulture areas, although WUEl is not reliable for estimating WUEWP.Abbreviations d 13 C carbon isotope ratio; YMD midday leaf water potential; AN net CO2 assimilation rate; AN/E instantaneous water-use efficiency; AN/gs intrinsic water-use efficiency; E leaf transpiration rate; gs stomatal conductance; LA leaf area; SWC soil water content; WUEl leaf water-use efficiency; WUEWP whole-plant water-use efficiency Keywords: drought, grapevine, genetic variability, leaf water-use efficiency, whole-plant water-use efficiency 164Genotypic variability of water-use efficiency

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