Plant and soil water status indicators for irrigation scheduling in young lime trees

In Mediterranean regions water scarcity makes efficient irrigation management indispensable. The objective of this study was to evaluate the capability to detect water stress in lime trees using different physiological plant and soil water status indicators in order to increase precision of irrigation. A drip irrigation experiment was conducted on a plot of young lime trees (Citrus latifolia Tan., ‘Bearss’) cultivated in a clay-loam soil at the field station of CEBAS-CSIC in Murcia (southern Spain). The trees were submitted to two treatments: control (100% ET_c) and stress (irrigation withheld for 40 days). Volumetric soil water content (θ_v) was monitored with capacitance probes and the soil matric potential (Ψ_m) with resistive tensiometers probes. Plant water status was assessed by measuring stem water potential (Ψ_stem) with a pressure chamber, gas exchange by means of a portable photosynthesis system, and canopy temperature (T_c) using infrared temperature sensors located above the tree crown. From the beginning of the stress period a gradual decrease in both soil water status parameters was observed, while a delayed decrease was noted in Ψ_stem in stressed plants. Gas exchange decreased with high variability in response to water deficits. In stressed plants, canopy temperature showed values above air temperature during the greatest part of the day, with maximum values observed around solar noon and similar values to the control trees during the night and late afternoon. Real time soil water dynamics in the plant root zone provides water content data for irrigation management and/or automation. Ψ_m is a more standard indicator for triggering irrigation, while the delay in its response to water deficits suggest that θ_v, which is more reactive, should be used to stop irrigation. The combined use of soil water content and matric potential is proposed as threshold values for automating irrigation to improve irrigation management.