WATER RELATIONS OF FRUIT END CRACKING IN FRENCH PRUNE (PRUNUS-DOMESTICA L CV FRENCH)

Irrigation of previously water-stressed French prune trees is known to induce fruit end cracking. The relationships between end cracking, water relations, and mechanical properties of the skin of French prune were studied as a function of irrigation regimes under field conditions. Water stress resulted in the accumulation of solutes in the fruit of nonirrigated trees. A gradient in osmotic potential (PSI(s)) existed along the vertical axis of fruit from all treatments; PSI(s) was always lower at the stylar than stem end. Irrigation of previously water-stressed trees (irrigated-dry treatment) resulted in PSI(s) gradients exceeding those of all other treatments. Moreover, estimated turgor (PSI(p)) at the stylar end of the fruit increased 2-fold within 24 hours after irrigation. These changes were accompanied by the onset of fruit end cracking, and neither the well-watered controls nor the continuously droughted fruit exhibited such changes. During the 24 hours following irrigation, the overall PSI(s) of irrigated-dry treatment fruit was diluted by the same amount as the calculated increase in fruit volume. However, during the same period, PSI(s) at the stem end of the fruit showed more dilution than expected, and PSI(s) at the stylar end of the fruit concentrated, indicating a redistribution of solutes. There were no differences in skin mechanical properties along the fruit vertical axis and, hence, this could not have accounted for the.observed changes in PSI(s) and PSI(p). Thus, when previously stressed French prune trees were irrigated, the overall recovery in water potential (PSI) and the subsequent movement of solutes to the stylar end of the fruit resulted in apparently excessive turgors in this region and hence the observed pattern of end cracking.