Intra-block variations of vine water status in time and space

Vine water status was measured on 96 plots of three vines inside a vineyard block of 0.28 ha during three years: 2003, 2004 and 2005. Three physiological indicators were implemented: stem water potential, carbon isotope discrimination measured on grape sugars at ripeness (δ13C) and canopy temperature measured by high resolution remote sensing. For stem water potential, measurements were taken on every single vine of each plot. The objectives of this study were to assess (i) the spatial distribution of vine water status inside a vineyard block, (ii) the temporal stability of this distribution from one date to another in the same year and (iii) the temporal stability of this distribution from one year to another. The three physiological indicators provided accurate data of vine water status, as was shown by high correlation coefficients between stem water potential values and canopy temperature, as well as between stem water potential and δ13C. Vine water status maps obtained with either stem water potential data or δ13C data showed similar patterns of spots that were more or less affected by water deficit stress, in relation to local soil water holding capacity. Stem water potential was measured in September 2004 on two days in a row, one cloudy day and the next day with higher temperatures and clear conditions. Stem water potential values were highly correlated between these two days, which confirms the fact that stem water potential is mainly influenced by soil water status. However, stem water potential values were in average 0.08 MPa higher on the cloudy day, which shows a measurable but limited influence of evaporative demand on absolute stem water potential values. Both stem water potential values and δ13C data were well correlated from one year to another, which shows a stability of spatial distribution of vine water status inside the block. This can be explained by the fact that soil water holding capacity is invariable from one year to another. Surprisingly, stem water potential values measured at the same time between vine 1, vine 2 and vine 3 of each plot were not very well correlated, although the soil can be considered homogeneous inside a plot (3 m2). This observation shows high variability in vine to vine water status, in relation to individual vine rooting depth and canopy size. Consequently, replicates on several adjacent vines have to be averaged out to obtain accurate vine water status data for each plot.