Are sap flow and canopy temperature measurements useful alternatives to stem water potential for detecting plant water stress in citrus trees?

Alternative and more practical methods than stem water potential (ψs) and stomatal conductance (gs) are needed for detecting plant water stress when regulated deficit irrigation (RDI) strategies are to be applied. The aim of this experiment was to assess the usefulness of sap flow and canopy temperature (Tc) measurements, as plant water stress indicators in citrus trees compared with more classical methods like ψsor gs. The experiment was performed during the summer of 2011 in a "Clementina de Nules" orchard undergoing RDI. Sap flow was determined by means of the compensated heat pulse method in well-watered and RDI trees. Tcwas measured continuously with infrared thermometers (IRTs) mounted over the canopies pointing vertically downward but also weekly with an infrared hand-operated thermographic camera taking frontal images of the sunlit side of the crowns. Concurrently, ψsand gswere also measured on all the trees. Results showed that the evolution of relative transpiration, obtained with the sap flow gauges, was in agreement with the water deficit applied. The values of Tcobtained with the IRTs normalized by air temperature (e.g., Tc-Ta) were in general poorly related with ψsand gs. However, when Tcwas obtained from thermal imaging there was a good correlation with ψsin days of relatively high stress (i.e., when ψsdifferences among treatments were > 1.0 MPa). ψs, gsand Tcobtained from thermal images were significantly correlated with the average fruit weight at harvest. The best correlation was that of thermographic Tcfollowed by ψsand gs. Overall, results showed that both sap flow and Tcmeasurements can detect plant water stress in cases of severely stressed citrus trees. The determination of Tcfrom thermal imaging, which integrates a large number of leaves in the measurement, was better than the IRTs method for detecting plant water stress.