Correlation Between Maximum Trunk Daily Shrinkage, Soil Matric Potential and Weather Variables for Mature Plum Trees in Medium-textured Soils and Temperate Climate

The objective of this work is to test the strength of relationships between maximum trunk daily shrinkage (MDS), soil matric potential (SMP) for various soil depths, and some weather variables in grown plum trees and medium-textured soils under continental-temperate climate conditions. The obtained relationships might be used in identification of water stress for irrigation scheduling. The study was carried out in a 7–9-year-old plum tree orchard plot with the ‘Stanley’ cultivar grafted on Saint Julien A semi-dwarf rootstock. Micrometric trunk diameter fluctuations of three trees were automatically measured by phytomonitors, and SMP was monitored continuously with resistance blocks installed in three profiles at four depths: 0.2, 0.4, 0.6 and 0.8 m during many wetting-drying cycles during two growing seasons. Strong and highly significant regression equations were obtained between MDS and air temperature (T) and daily vapor pressure deficit (VPD) for three growing seasons of observations. There were also weaker but significant relationships between MDS and SMP for all the four depths studied. Multiple linear regression equations between MDS, SMP for all the four depths, and either maximum T or VPD generally improved R2 from the simple regression equations obtained with each of the same variables. The MDS threshold of 0.08–0.12 mm approximately corresponds to a SMP range of about −60 to −40 kPa; however, due to the high MDS variability in time and the weight of the weather variables in the relationships between MDS, SMP and weather variables like T or VPD, the usefulness of these relationships is limited.