Thermographic measurement of canopy temperature is a useful tool for predicting water deficit effects on fruit weight in citrus trees

Alternative and more practical methods for plant water stress detection than stem water potential (ψs) and stomatal conductance (gs) are needed when regulated deficit irrigation (RDI) strategies are applied. The aim of this experiment was to compare sap flow and canopy temperature (Tc) measurements with more classical methods like ψs or gs to predict the effect of deficit irrigation on fresh fruit weight in citrus trees. 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 compensation heat pulse method in well-watered and RDI trees. Tc was measured continuously with infrared thermometers (IRTs) mounted over the canopies and also weekly with an infrared hand-operated thermographic camera taking frontal images of the sunlit side of tree crowns. Concurrently, ψs and gs were also measured on all trees. Results showed that the evolution of the relative transpiration obtained with the sap flow sensors was in agreement with the plant water stress experienced. The values of Tc obtained with the fixed IRTs, normalized by air temperature (Tc−Ta) were in general poorly related with ψs and gs. However, when Tc was obtained from thermal imaging, there was a good correlation with ψs in days of relatively high water stress (i.e. when ψs differences among treatments were >1.0MPa). The average fruit weight at harvest was significantly correlated with all the stress indicators, and the best correlation was that of thermographic Tc followed by ψs and gs. Overall, results showed that in citrus trees Tc measurement obtained from thermal imaging is a good tool to predict the effect of water deficit on fresh fruit weight.