A model-based diagnosis tool to evaluate the water stress experienced by grapevine in field sites

Abstract

Soil water deficit is a major factor influencing yield and grape berry composition, with an optimal situation corresponding to a moderate water deficit. A computer-based diagnosis tool requiring a limited number of measurements was developed and used to characterise the typology of soil water deficit experienced by grapevine in field sites over the cropping season with the aim of producing acceptable yields and achieving good quality red wine. The tool combined a simple soil water balance model parameterised for vine, to simulate the fraction of transpirable soil water (FTSW) at a daily time step, and an indicator of vine water status in terms of source/sink relationships and canopy microclimate. This indicator was developed from quantitative relationships between FTSW and maximal net assimilation (Anmax), under optimal conditions of light and temperature, the leaf emergence rate on first-order lateral branches (LER I), the leaf emergence rate on second-order lateral branches (LER II) and the final length of first-order lateral branches (BL I). These relationships were parameterised in a greenhouse on Syrah (syn. Shiraz) grafted onto Fercal rootstock. The computer-based diagnosis tool was used on 24 fields within 4 vineyards of Mediterranean southern France covering contrasting pedoclimatic conditions, cultivar/rootstock pairs and receiving different water regimes. The key soil parameter of the water balance model, the total vine's transpirable soil water (TTSW), was estimated in each field from measurements of predawn leaf water potential (Ψl) at different vine growth stages in conjunction with an optimisation algorithm of the simulated FTSW as a function of predawn Ψl obtained with an independent data set. The tool enabled classification of the fields on the basis of the changes of the indicator of vine water status across four phases of the plant cycle, from budburst to harvest, relative to an optimum change of this indicator defined for production of a good quality wine. Such a tool will be useful to farm advisers and researchers for the evaluation of management practices as for comparative analysis between vineyards.