Abstract
A precise knowledge of the grapevine responses to increasing level of water stress is of the utmost relevance for an effective application of deficit irrigation strategies in viticulture. Against this background, a study was conducted on potted grapevines subjected to two ten-day drought cycles to assess their water status by integrating the information derived from different physiological indexes including whole-plant transpiration (measured gravimetrically and with sap flow sensors), leaf gas exchanges and chlorophyll fluorescence. When soil water availability was not limited, vine transpiration rate was determined mainly by environmental factors and ranged between 0.5 and 2 L day−1 m−2 of leaf surface. Transpiration was affected by the soil water availability only when water stress became evident (midday stem water potential < −1 MPa), with vines showing a strong limitation of the stomata functioning (stomatal conductance < 0.05 mol m−2 s −1 ) and, consequently, low transpiration rates (<0.5 L m−2 d −1 ). Transpiration rates measured with the sap flow sensors were correlated with those measured gravimetrically, showing daily patterns that were highly affected by the intensity of the water stress. Nevertheless, these sensors highly underestimated actual transpiration rates, limiting their reliability for vineyard irrigation management. At the end of the second drought cycle, vines showed very limited responses to daily changes in environmental conditions (same photosynthetic rate and stomatal conductance at morning, midday and afternoon), likely reflecting a carryover stress effect from the first drought cycle and an incomplete physiological recovery before the beginning of the second. Evidence of the severe water stress reached by vines was also given by the high value of the quantum yield of nonregulated energy dissipation (Y(NO) higher than 0.4) found at the end of the first drought cycle. Taken together, the obtained results integrate the current knowledge on water stress development in grapevine, also highlighting the relevance of specific physiological indexes that could be used effectively for the correct management of deficit strategies in viticulture.