Scheduling peach orchard irrigation in water stress conditions: use of relative transpiration and predawn leaf water potential

Introduction. Plant water stress indicators have become valuable for moving towards deficit irrigation strategies and saving water. In this case evapotranspiration (ET) is below its maximum value for the crop and stage (ETc), and a stress coefficient (Ks) is applied to obtain actual ET (ETa). Predawn leaf water potential (Yp) can be related to relative transpiration (RT), the ratio between transpiration of a stressed plot (T) and transpiration of a well-irrigated plot (Tm). Estimating RT from Yp allows calculating ETa for determination of irrigation amounts, if deficit irrigation practices are used, as RT corresponds approximately to Ks. Materials and methods. RT and Yp were measured with the aim of establishing a relationship to estimate RT under moderate water stress for irrigation scheduling, in a peach orchard in south Portugal. RT was calculated using sap flow measurements (heat balance method) in two plots, one well-irrigated (daily drip irrigation amounts calculated for Tm) and another temporarily without irrigation. Results and discussion. A high correlation was found between RT and Yp, allowing the estimation of RT for the studied conditions. Significant differences regarding the relationship RT-Yp obtained for another peach orchard in the same region and similar soil conditions were found for Yp in the range between -0.11 and -0.45 MPa. The results suggest that the differences resulted from the different irrigation systems: drip and micro-sprinkling, as they determine different temporal and spatial water distribution and therefore different geometry of root systems. A formerly proposed equation to estimate RT from Yp with a general form for different fruit tree species was tested, proving to be adequate within some limits: when RT is lowered to 0.7, the error was below 9%. The threshold value RT = 0.7 was considered a minimum as it was successfully tested in deficit irrigation practices for peach orchards. [PUBLICATION ABSTRACT]