Establishing reference values of trunk diameter fluctuations and stem water potential for irrigation scheduling of olive trees

diameter fluctuations (TDF) and stem water potential (SWP) were recorded in well-irrigated olive trees with the aim of establishing reference values of both indicators for irrigation scheduling. The measurements were performed in a mature (18 years) and a young olive orchard (3 years) during 4 years. Two different indicators derived from TDF: maximum daily shrinkage (MDS) and trunk growth rate (TGR) were evaluated. The annual relationship between MDS and vapour pressure deficit (VPD) was hysteretic, probably caused by the presence of fruit as no hysteresis was detected in 'off' trees. All data pooled gave an equation of MDS = 0.09 + 0.19VPD (R-2 = 0.60). When only data until mid-August in mature "on" trees was used, the fit was improved (R-2 = 0.71; MDS = 0.05 + 0.22VPD). Young trees had a slightly different relationship with a steeper slope; MDS = -0.18 + 0.27VPD (R-2 = 0.80). The relationships between the SWP values and VPD were not as good as with MDS (R-2 around 0.3), and the regression slope was steeper in 'on' than in 'off' trees. TGR in the young orchard had two maxima in the year; the first at mid-Spring (0.2 mm day(-1)) and the second at the end of the summer (0.15 mm day(-1)). In the mature orchard, TGR had a single maximum value, at the end of summer for 'off' trees, and in spring for 'on' trees.