SENSITIVITY OF YIELD AND FRUIT-QUALITY OF FRENCH PRUNE TO WATER-DEPRIVATION AT DIFFERENT FRUIT-GROWTH STAGES

The sensitivity of French prune (Prunus domestica L. syn. 'petite d' A gen') to water deprivation at various fruit growth stages was studied over 3 years in a drip-irrigated orchard. The soil was a poorly draine d Rocklin fine sandy loam with a hardpan that varied from approximate to 0.75 to 1 m from the surface at the northern end of the orchard (sh allow soil condition) to no hardpan apparent to 2 m below the surface at the southern end of the orchard (deep soil condition). Water depriv ation during a) the first exponential phase of fruit growth or stage I , b) lag phase of fruit growth or stage II, c) first half of stage II, d) second half of stage II, e) second exponential fruit growth phase or stage III, and f) postharvest was compared to a fully watered contr ol. Water deprivation caused the most severe reduction in tree water s tatus when it was imposed over longer periods of time and during perio ds of high evaporative demand and also had more severe effects under s hallow soil conditions. Compared to the control treatment, deprivation during all of stage II (the most severe deprivation treatment) was as sociated with increased flowering, reduced fruit hydration ratio, and smaller fruit size under all soil conditions. Under deep soil conditio ns, deprivation during all of stage II resulted in increased return bl oom, which was reflected in higher fruit loads and dry t.ha(-1) fruit yield. However, under shallow soil conditions, even though return bloo m was increased with this treatment, fruit loads and dry t.ha(-1) frui t yields were the lowest of all treatments. These differences in treat ment effects in shallow vs. deep soil conditions were most likely the result of increased fruit drop, which occurred under shallow soil cond itions as a result of rapid onset and increased severity of stress. Tr eatments that had parallel effects in shallow and deep soil conditions resulted in statistically significant overall treatment effects, whil e those that had opposing effects in shallow vs. deep soil conditions did not show significant overall treatment effects. Substantial altern ate bearing occurred, and, in general, dry fruit yields above approxim ate to 9 dry t.ha(-1) resulted in a decrease in fruit load the followi ng year, while loads below this value showed a subsequent increase. Ba sed on a separate estimate of the theoretically stable value for each treatment, all deprivation treatments resulted in a higher sustainable fruit load compared to the fully irrigated control. This suggests tha t, for the purpose of prune fruit production, there may be an optimal level of tree water stress.