Night-time responses to water supply in grapevines (Vitis vinifera L.) under deficit irrigation and partial root-zone drying

Night-time water uptake (Sn) mainly corresponds to stem and organ rehydration and transpiration, the latter through stomata and cuticle. Nocturnal transpiration is uncoupled from photosynthesis, therefore it contributes to reduce water use efficiency (WUE). Night-time grapevine physiology was measured on field grown grapevines (cv. Shiraz) under partial root-zone drying (PRD) and deficit irrigation (Exp. 1), on potted vines (cv. Tempranillo) (Exp. 2) and on potted vines (cv. Cabernet Sauvignon) on a progressive drought treatment in the glasshouse (Exp. 3). Sap flow probes using the compensated heat pulse method (cHP) were installed in vines (Exp. 1 and 3). Night-time gas exchange measurements were performed for Exp. 3. Other vine water status monitoring methods used were: midday stem water potential (Ψs) for all experiments, and abscisic acid (ABA) concentration monitored from leaf sap for Exp. 3. Results showed that Sn was parabolically correlated to Ψs measured on the previous day for all treatments and cultivars. Two distinct zones where vines exhibit different night-time behaviour within the Ψs vs Sn parabolic relationships were identified for all experiments. The differences between the two identified areas were related to the water status conditions of the vines: (i) non-water stress conditions (0 < Ψs < −1.0 MPa); (ii) water stress conditions (−1.0 MPa < Ψs < −2.0 MPa). Furthermore, levels of water stress were negatively correlated to concentrations of leaf sap ABA, which helped to explain the parabolic curve found for cv. Cabernet Sauvignon.