The Mechanism of Regulation of ‘Bartlett’ Pear Fruit and Vegetative Growth by Irrigation Withholding and Regulated Deficit Irrigation

After initially withholding irrigation (WI) to dry out the root zone of pear trees, regulated deficit irrigation (RDI) applied to replace 23% and 46% of evaporation over the planting square (Eps) was compared with 69% and 92% Eps applied during the WI and RDI periods, respectively (full irrigation). Irrigation was increased to 120% Eps on all treatments after rapid fruit growth commenced. Leaf water potential (ψ 1 ) measured at dawn and midday became less negative during RDI than during WI but in both periods was more negative than the control (69%/92% Eps). On the other hand, ψ 1 of treatments receiving WI and RDI became less negative than the control when all irrigation treatments were increased to 120% Eps. Withholding irrigation followed by RDI reduced vegetative growth by 52%. In contrast, however, WI did not inhibit fruit growth, while, during RDI following WI fruit, growth was stimulated. A similar but greater stimulation of fruit growth (consistent with relatively less negative ψ 1 ) was measured on WI/RDI plants when all treatments received 120% Eps. This stimulation of fruit growth increased yields by about 20%. The results indicate fruit osmoregulate to maintain and/or increase growth at the expense of inhibited vegetative growth when WI and or RDI reduce ψ 1 in spring to values approaching −0.5 MPa at dawn.