Water use efficiency in Sangiovese grapes (Vitis vinifera L.) subjected to water stress before veraison: different levels of assessment lead to different conclusions.

Several recent papers have shown that in grapevine (Vitis vinifera L.), interpretation of responses to drought can differ depending upon the parameter chosen to express water use efficiency (WUE). In the present paper, a series of WUE expressions, including physiological and agronomical, were compared in potted grapevines (Vitis vinifera L. cv. Sangiovese) that were either well-watered (WW) or subjected to progressive drought before veraison (WS) by supplying decreasing fractions (i.e. 70%, 50% and 30% of daily vine transpiration (Tr_d) determined gravimetrically before vines were fully rewatered. Although single-leaf intrinsic and instantaneous WUE increased with water stress severity, seasonal and whole-canopy WUE were similar to that before stress, at 70% Tr_d and upon rewatering, but dropped during severe water stress. WUE calculated as mass of DW stored in annual biomass (leaves, canes and bunches) per litre of water used did not differ on a seasonal basis, whereas WS plants showed lower must soluble solids at harvest, and unchanged colour and phenolic concentration in spite of smaller berries with higher relative skin growth. Results confirm that whole-canopy WUE is a much better index than any single-leaf based WUE parameter for extrapolation to agronomic WUE and actual grape composition. In our specific case study, it can be recommended that water supply to drought-stressed Sangiovese grapevines before veraison should not be lower than 70% of daily vine water use. How grapevines (Vitis vinifera L.) use irrigation under drought needs to be determined. Previous work shows contradictory results regarding the method used. Single-leaf and whole-canopy methods for assessing water use efficiency (WUE) were compared in V. vinifera cv. Sangiovese subjected to progressive water deficit before veraison. Single-leaf WUE assessment was poorly correlated with yield and grape composition, whereas whole-canopy assessment was more reliable. Guidelines were that not less than 70% of daily water use should be supplied to Sangiovese vines before veraison. [ABSTRACT FROM AUTHOR]