Heterogeneity in fruit distribution and stem water potential variations within peach trees under different irrigation conditions

The hypothesis that different fruit loads between ‘scaffold branches’ of a tree can induce water status differences was evaluated. Two different experiments were carried out, one under full irrigation conditions and the other under conditions in which peach trees were deficit irrigated (50%). Thinning treatments were applied in both experiments at the begining of Stage III of fruit development: 1) EVEN-max with fruits distributed evenly with maximum crop, not thinned, 2) EVEN-min with fruits distributed evenly with minimal crop (< 90 fruit tree–1), and 3) UNEVEN with fruits distributed unevenly by totally defruiting half of the available main (scaffold) branches per tree and leaving the other half unthinned. Stem water potential (Psi(stem)) was measured with a pressure chamber at solar noon, and midday leaf conductance (gl) using a portable steady state porometer. UNEVEN trees always had intermediate Psi(stem) values between EVEN-max and EVEN-min, independent of irrigation treatments. Maximum fruit load differences between trees (EVEN-max compared to EVEN-min) produced Psi(stem) differences of 0.12 MPa and 0.25 MPa for full irrigation and deficit irrigation experiments, respectively. Although the magnitude of change in Psi(stem) was larger for deficit irrigated conditions, extreme differences in fruit load between main branches within an UNEVEN peach tree only induced differences in leaf conductance and had no effect on the water potential of scaffold branches independent of the irrigation experiment. Fruit load effects on branch water status were governed mainly by tree fruit load rather than scaffold branch fruit load. These results indicate that there is either little hydraulic isolation between the main stems, irrespective of tree water status, or an improved hydraulic efficiency associated with defruiting.