PEAR WATER RELATIONS UNDER PARTIAL ROOTZONE DRYING

Abstract

The effects of partial rootzone drying (PRD) on microjet-irrigated pear trees on a loam soil in the Victorian Goulburn Valley, Australia were investigated during the 2002/03 season. PRD effects on soil water content, midday stem water potential (ψ ψ ψ ψ stem), leaf conductance (gl) and sap velocity were examined. Treatments included 50% (PRD50) and 100% (PRD100) of estimated crop water requirement (ETc) applied daily to one side of the tree on a 14-day alternating cycle while the Control treatment received 100% of ETc applied to both sides of the tree. ETc was calculated using reference crop evapotranspiration (ETo) and FAO crop coefficient for pear adjusted for effective canopy cover (ECC). ECC was measured by canopy PAR interception over the planting square at solar noon and remained at low levels (≤32 %) during the season. ECC did not differ between treatments. Irrigation inputs to harvest were 1.9 Ml ha -1 for PRD50, and 3.5 Ml ha -1 for the fully watered PRD100 and Control trees, compared to the calculated ETc of 3.1 Ml ha -1 over the same period. Crop water relations of the PRD100 trees during a 14-day PRD cycle, as measured by ψ ψ ψ ψ stem and gl, was similar to the Control. Decreased ψ ψ ψ ψ stem under PRD50 indicated water stress. Sap velocity, which was assumed to equate to transpiration, increased in a curvilinear fashion to instantaneous ETo in all trees. This sap velocity-ETo relationship was maintained throughout the 14-day PRD cycle. No evidence of transient changes in stomatal control of transpiration over a drying cycle occurred in the PRD trees. These results indicate that PRD50 led to water stress. Furthermore there was no evidence of the PRD stomatal control and maintenance of plant water status previously reported for grapevines (Dry and Loveys, 1999; Stoll, 2000). Thus, further work on crop species and soil type influence with respect to PRD is required. Also, research into the establishment of ψ stem baselines for fully watered crops would aid irrigation management of orchards.