Apple tree and fruit response to drought stress

A range of late season drought stress was established across an orchard on a silt loam soil 1.1 m deep by terminating irrigation at different dates through harvest. Water potential, stomatal conductance, evapotranspiration and the rate of fruit growth declined slightly until 80% of available water in the soil was used. Below 20% available water, large changes in these parameters occurred with small changes in soil moisture. Soluble solids increased and starch, scald and water content of fruit at harvest decreased with increased drought stress. Firmness, adjusted for size, bitter pit, water core and titratable acidity were not consistently affected.;Improvement of tolerance to severe drought (below 20% available water) by growth habit (spur vs non-spur), thinning, nitrogen fertilization and pruning was evaluated by measuring wilting and fruit growth. There was no difference in drought tolerance between spur and non-spur trees. Nitrogen fertilization reduced mid-day wilt for moderately to severely stressed trees. Thinning improved fruit growth of lightly to moderately stressed but not severely stressed trees. Earlier and more severe pruning delayed development of drought symptoms. Dehorning plus nitrogen fertilization produced the largest fruit. There was no dieback of shoots or tree death.;Drought stress early in the cell elongation phase of fruit growth reduced vegetative growth and improved yield efficiency without reducing fruit size at harvest. Fruit growth rate was reduced to 50% of the controls by the deficits, but reduction in volume up to 15% and 20% was recovered after irrigation was restored by trickle and microsprinkle irrigation, respectively. Microsprinklers rewetted the entire soil volume and raised stem water potential to values similar to furrow. Trickle irrigation relieved stem water potential less than microsprinklers and rewetted about half the soil volume per tree. Fruit size at harvest was reduced by 5% for every increase in cropload of 1 fruit/cm...