Deficit irrigation strategies using midday stem water potential in prune

The effects of two levels of water stress on fruit yield, quality and relative economic return were investigated in French prune over a 3-year period. Water stress was achieved by allowing a progressive decrease in midday stem water potential (midday Psi (stem)) over the season. The control treatment (T1) received approximately 100% of the estimated seasonal crop water requirements, but did exhibit a decline in midday Psi (stem) during the season to below that expected for fully irrigated prune trees (-1.0 MPa). For the moderate and severe stress treatments (T2 and T3 respectively), irrigation was managed to reach a target midday Psi (stem) of approximately -1.5 and -2.0 MPa, respectively, by harvest. Treatments T2 and T3 received an average of 57% and 38% of estimated seasonal crop water requirements. respectively, over the 3 years of the study but this did not negatively impact fruit dry mass accumulation or fruit quality. Over a 3-year period, the stress treatments had less dry mass of fruit dropped, fewer fruit dropped, improved (lower) fruit hydration ratios, less fruit side-cracking and decreased vegetative growth compared with the control. There were no significant effects of either of the stress treatments on flowering, dry fruit yields or relative economic return for the 3-year average data. However, a significantly lower relative economic return for T3 during the third year of the study suggests that the cumulative effect of this level of stress may be detrimental to long-term prune productivity. Allowing a progressive decline in midday Psi (stem) to approximately -1.5 MPa by harvest appears to be an effective way to reduce irrigation and maintain economic returns in French prune.