Maximum daily trunk shrinkage and stem water potential reference equations for irrigation scheduling in table grapes

A two-year experiment was conducted to investigate the suitability of reference lines for irrigation scheduling based on maximum daily trunk shrinkage (MDS) and midday stem water potential (Ψs) in a commercial orchard of table grape cv. Crimson Seedless grafted onto Paulsen 1103 (V. berlandieri R. × V. rupestres du Lot). Vines were irrigated (from April to October) above their full crop water requirements (110% of crop evapotranspiration, ETc) in order to obtain non-limiting soil water conditions. The reference equations obtained for MDS and Ψs with meteorological factors differed between the pre and post-veraison periods. Before veraison, MDS was the most reliable indicator for assessing the water status of vines, whereas Ψs correlated better with meteorological variables after veraison. The sensitivity of MDS to ascertain the plant water status decreased during post-veraison due to its dependence on growth and to daily fluctuation of stem diameter. Moreover, it can also be induced by changes in the transpiration and also on the accumulation of xylem abscisic acid ([ABA]xylem). Mean temperature (Tm) was the environmental variable that best correlated with MDS and Ψs at pre-veraison. However, post-veraison reference lines can be obtained for MDS and Ψs using reference evapotranspiration (ET0) and mean daily vapour pressure deficit (VPDm). The use of MDS signal intensity (SIMDS) around the unity and Ψs around −0.65 MPa were the best criteria for irrigation scheduling in well-irrigated 'Crimson Seedless' table grapes growing in a semiarid climate of south-eastern Spain, during pre and post-veraison periods, respectively.