Apple tree and fruit responses to early termination of irrigation in a semi-arid environment

Drought stress was imposed in two 'Delicious' apple (Malus xdomestica Borkh.) orchards on a sandy loam soil of different soil depths (0.8 and 1.2 m) in the semi-arid environment of central Washington by withholding irrigation all season or from 3,5,7,9, 11, 13, 15, or 17 weeks before harvest. Total pan evaporation was 1005 mm and precipitation was negligible from May through Sept. Soil of the control trees was near field capacity all season, and stem water potential (Psi(stem)) averaged -1.29 MPa. Total available soil water (TAW) declined after irrigation was terminated for each treatment. As TAW declined to 35%, the TAW that commercial growers are recommended to allow soil to dry to before irrigating, Psi(stem) was 93% of the controls, fruit growth rate was 97% of the controls, and leaf senescence did not exceed the control trees. As TAW decreased below 30%, leaves senesced acropetally starting with transition leaves near the bud-scale sear. Soil moisture of nonirrigated trees was depleted in July in the orchard on shallow soil and In late August in the orchard on deep soil. Normal June drop was reduced in the driest treatments, but crop load was not affected in the other treatments. There was no difference In drought response between the two rootstocks studied (M.7 and MM.111), but nonspur-type trees exhibited slightly greater symptoms of drought stress than the smaller spur-type trees. A Crop Water Deficit Index (CWDI) based on Psi(stem) measurements was linearly related to fruit weight at harvest (r(2) = 0.87). All trees were well-watered the following year and yield was reduced only for trees that were severely stressed the previous year.