Average daily light interception determines leaf water use efficiency among different canopy locations in grapevine

In Mediterranean areas grapevine water use efficiency (WUE) is becoming an important issue as grapevine production and quality are largely dependent on irrigation, and given the large area of this crop in this region. Under semi-arid conditions, grapevine water consumption rises up to 700mm/year (rainfall plus irrigation), thus great water amounts are needed for areas typically characterised by water scarcity during grapevine growing season. Therefore, improving WUE is a challenge to secure agriculture sustainability of viticulture in these areas. In the present work we evaluate the variation of leaf WUE over time (diurnal time) and space (at eight canopy positions) under irrigation, moderate and severe water stress in field-grown Tempranillo grapevines. Scaling up from the single leaf to the whole plant WUE values were tested comparing daily integrals of leaf water use efficiency (WUEDáµ¢) with midday leaf WUE ones showing a poor relationship, that become poured as water stress increased. To evaluate spatial variations of WUE inside the canopy daily integrals of WUEDáµ¢, were determined by leaf gas exchange measurements at eight canopy positions with marked differences in light exposition. Great variations in WUEDáµ¢ at different canopy locations were mostly determined by the daily integral of light interception with a high regression coefficient for irrigated (0.98) and moderately water stressed plants (0.92), and lower for plants under severe water stress (0.68). This indicates that increased daily PAR interception by the leaf increases WUEDáµ¢. Moreover, those data suggest that improvements in whole-plant WUE up to 27% could be achieved by selective thinning of basal (locations 1 and 7) and internal (location 8) leaves of the canopy and that these theoretical benefits would be minor but still significant under water stress.