Abstract:
Partial rootzone drying (PRD) is a new irrigation strategy whereby water is withheld from part of the rootzone while another part is well watered. A successful PRD strategy should reduce tree water use through stomatal control of transpiration and reduce vegetative growth while maintaining fruit size and yield. A field experiment examined crop water relations and production performance of PRD in a commercial apple orchard on loam soil in the Goulburn Valley, Australia. The orchard consisted of high-density (1420�trees/ha) 8-year-old ?Pink Lady' apple trees trained as central leader and irrigated by microjets. The effects of PRD on leaf/stem water potential, vegetative growth, yield components and fruit quality were investigated during two seasons (2001?02, Year 1 and 2002?03, Year 2). The 2-year average growing season reference crop evapotranspiration and rainfall was 954 and 168�mm, respectively. Three irrigation treatments were established: (1) deficit irrigation (DI, supplied 50% of water to a fixed side of tree); (2) PRD supplied 50% of water to alternating sides of tree; (3) and conventional irrigation (CI, supplied 100% water to both sides of tree). Irrigation inputs under the CI treatment were 334 and 529�mm for Year 1 and Year 2, respectively. In Year 1, the volume of irrigation applied to CI treatment inputs equated to the replacement of predicted crop evapotranspiration (ETc) based on a mid-season FAO-56 crop coefficient with adjustment for tree size. Vegetative growth, fruit production and water status showed both PRD and DI treatments led to a classical ?deficit irrigation' water stress response. Leaf water potential, leaf conductance, fruit size, shoot growth and yield were reduced on PRD and DI trees compared to the fully watered (CI) trees. In Year 2, CI inputs exceeded estimated ETc by 2-fold. Consequently, minimal or no differences between irrigation regimes were measured in stem water potential, vegetative growth, yield components and fruit quality. Fruit disorders (sunburn, russet, misshape, markings, frost damage) were not affected by irrigation regime in either season. We contend that further effort is required to determine under what circumstances or environments there is a PRD response that saves water and maintains yield and quality for apple.