Midday stem water potential as a useful tool for estimating irrigation requirements in olive trees

Abstract

Irrigation scheduling of fruit trees according to the water balance showed significant differences between locations. In recent years, water status measurements such as leaf water potential have been suggested as irrigation tools in different fruit trees. The aim of this study was to adjust water potential threshold values previously studied and water application approaches that permit the estimation of irrigation requirements of olive trees based on midday stem water potential. The experiments were performed during three seasons (from 2005 to 2007) in two different locations (Badajoz and Ciudad Real) with different weather and cultural conditions. In both locations, the olive orchards were seven years old at the beginning of the experiment but had significantly different canopy development. In Ciudad Real the canopy shaded area at the beginning of the experiment was 15% and the first crop was harvested in 2003. On the other hand, the canopy shaded area of the olive orchard in the Badajoz experiment was 40% and the first crop was harvested in 2001. We therefore considered the Ciudad Real orchard as young and Badajoz as mature. Three different irrigation treatments were compared in both locations: Control treatment with traditional water balance as irrigation scheduling and two treatments in which midday stem water potential (SWP) provided the information about water management. In the midday stem water potential irrigation (WI) treatment the threshold value of SWP was −1.2 MPa before the beginning of the massive pit hardening period and −1.4 MPa after this date. Finally, in the deficit irrigation (DI) treatment the threshold value of SWP was −2.0 MPa throughout the season. In the WI and DI treatments irrigation was applied when SWP reached the threshold value. No significant differences were found between Control and WI in any of the seasons or locations when SWP, leaf conductance, shoot and fruit growth and yield (fruit and oil) were considered. In both locations, the same SWP value in WI treatment resulted in similar water application as the Control treatment. In DI treatment, shoot growth was significantly reduced in both locations in all the seasons. The SWP in DI trees was clearly affected in both locations, while leaf conductance was only reduced in the Badajoz experiment. In the Ciudad Real experiment no significant differences between DI and the other treatments were found in fruit growth, whereas differences were found in Badajoz. However, in Ciudad Real yield in DI treatment was significantly reduced, but not in Badajoz. WI treatment was successful for non-water-stress conditions. On the other hand, DI treatment was a mild water stress treatment which reduced yield only in low covered orchards, but not in the orchards with almost maximum canopy shaded area.