Relationship of stem water potential and leaf conductance to vegetative growth of young olive trees in a hedgerow orchard

In 2005, four irrigation treatments were applied to a 3-year-old cv. Cornicabra orchard. In T1, wetted soil volume was maintained close to field capacity by irrigating when soil sensors indicated that soil water potential in the root zone had fallen to -0.03 MPa and 0.06 MPa from spring until 15 August and from 15 August until September, respectively. On those days, 8, 6, 4, and 2 h of irrigation was applied to T1, T2, T3, and T4, so that over the season they received 106, 81, 76 and 31 mm of irrigation, respectively. The high value for T3 was the result of a valve failure on 13 June. Measurements were maintained throughout the experimental period of relative extractable water (REW) to 1 m depth at the wetted volume (0.30 m from a drip emitter), shoot length, trunk diameter, stem water potential (Psi(stem)) and leaf conductance (g(1)). The irrigation treatment significantly affected REW (P < 0.10), Psi(stem), g(1) and vegetative growth (P < 0.05). Psi(stem), and trunk diameter were the least variable parameters and Psi(stem) and shoot growth were the most sensitive to water stress. Although T1 received 24% more water than T2, no significant differences were detected in vegetative growth. T2 should be considered the optimum irrigation value. The mean monthly Kc for T2 was 0.086. The failure of the valve in T3 simulated a wet spring followed by limited irrigation. Irrigation applied was similar to T2 but shoot growth stopped one month earlier and lower values of Psi(stem) and g(1) were observed after mid August. REW was highly related to vegetative growth, 66% of maximum being achieved at REW 0.53 and 50% at 0.45. g(1) was independant of plant or soil water status and did not determine vegetative growth. A strong relationship established Psi(stem) as a good indicator of vegetative growth and hence of water stress. Shoot growth was 66% of maximum at Psi(stem) -1.5 MPa and 50% at -1.8 MPa.