Sensitivity and variability of soil and plant water stress indicators in response to withholding and resuming irrigation cycles in sweet cherry trees

Reduced water availability is the main limiting factor for crop production in semi-arid and arid regions. For this reason, irrigation water management needs to be based on reliable information and data that are rapidly and easily acquired. The aim of the present study was to assess the sensitivity and variability of several soil and plant water status indicators in response to two cycles of withholding and resuming irrigation in sweet cherry trees. The experiment was carried out during the summers of 2018 and 2019 in an experimental orchard of sweet cherry trees [Prunus avium (L.) ‘Lapins’] in SE Spain. Three irrigation treatments were studied: control, CTL, irrigated to ensure non-limiting soil water conditions (115% ETc) and two water stress treatments, medium water stress, MS, and severe water stress, SS. The threshold values of midday stem water potential (Ψ_stem) proposed to the first and second drought period for MS trees were -1.3 and -1.7 MPa and for SS trees were -1.6 and -2.5 MPa. After every irrigation withholding period, MS and SS trees were fully irrigated until reaching Y_stem values of CTL trees. The experimental design was a completely randomized block design with three blocks per treatment. Soil and plant water status were assessed by measuring the soil volumetric water content (θv), the Ψ_stem, the daily trunk growth rate (TGR), the maximum daily trunk shrinkage (MDS), the temperature of the canopy (Tc), the difference between Tc and air temperature (ΔT) and the crop water stress index (CWSI). The signal intensity (SI), the coefficient of variation (CV) and the sensitivity (S = SI/CV) of θv, Ψ_stem, MDS and Tc were determined.

θv at 25 cm dropped significantly during the drought periods. Ψ_stem of MS and SS trees reached minimum values close to those thresholds proposed both years of study. MDS and TGR had a rapid response to the irrigation regimen applied. Tc, ΔT and CWSI increased as an effect of the stomatal closure. Ψ_stem and Tc were the water stress indicators with the highest sensitivity. MDS showed SI values greater than that of Ψ_stem and Tc, although it also had greater variability (CV_MDS ≈ 29%). Ψ_stem showed high SI values and low CV both study years. When the linear relationships between Ψ_stem and the other plant water status indicators were calculated, it was observed that the Pearson correlation coefficients exceeded 0.75 in all cases, except for TGR. The relationship obtained between MDS and Ψ_stem was linear from −0.5 MPa to a threshold value of around −1.3 MPa, from that value onwards, Ψ_stem decreases were not related to MDS values. In contrast, ΔT and CWSI were always linearly related to Ψ_stem. These results suggest that: i) MDS could be used as a water stress indicator up to moderate water deficit; ii) Ψ_stem is a sensitive water stress indicator with low variability; and iii) the thermal indicators (Tc, ΔT and CWSI) can rapidly and easily assess sweet cherry tree water status.

This study was funded by the Spanish Economy and Competitiveness Ministry (AGL2013-49047-C2-1-R; AGL2016-77282-C33-R).