CROP LOAD ALTERS WATER POTENTIAL AND DAILY VASCULAR FLOWS IN PEACH FRUIT

The daily patterns of peach fruit growth and of phloem, xylem and transpiration in/outflows were determined during cell expansion on 12 trees with different crop levels: unthinned (HCL), heavily thinned (LCL) and commercially thinned (CTRL). Vascular flows and transpiration were quantified by recording the diurnal patterns of diameter change of fruit using highly sensitive, custom-built fruit diameter gauges. The fruit were left intact at first and then fruit stems were girdled and the fruit were subsequently detached The daily patterns of stem and fruit water potential were also determined. Intact fruit always shrank in the morning due to transpiratory water loss following changes in VPD, and expanded in the afternoon and night. Xylem flow was quite low and did not balance transpiration water loss at sunrise, whereas in the afternoon it increased and caused the fruit to achieve positive growth. Daily phloem flow was always lower than xylem flow and contributed to fruit growth especially during mid-day hours. LCL fruit had higher daily relative growth rates than HCL fruit. Differences appeared mainly during the afternoon, because of the higher phloem flow and the lower transpiratory losses shown by LCL and CTRL fruit at this time of day. In all treatments fruit and stem water potentials decreased as VPD increased and an inverse relationship was found between specific fruit transpiration and fruit water potential. Due to their higher specific transpiration rates, HCL fruit reached lower water potential values than LCL during the day, possibly causing a higher mid-day phloem flow in HCL than in LCL and CTRL fruit. This was probably a response to a passive mechanism of phloem unloading. Later in the afternoon, the higher phloem fluxes to LCL fruit may have been driven by alternative active mechanisms of phloem unloading. Both fruit water relations and vascular flows were affected by crop load: the higher fruit growth rates recorded in LCL fruit could be attributed to the combined effect of a higher phloem flux and lower transpiratory losses.