Source and sink limitations in vascular flows in peach fruit

The daily patterns of vascular and transpiration flows to and from peach fruit were compared between heavily thinned (LCL) and unthinned (HCL) trees, in order to assess when these flows were limited by resource availability (source-limited) or by the genetic potential of the fruit (sink-limited) during the day. Minute variations in fruit growth and in phloem, xylem, and transpiration flows were determined at cell division (stage I) and at cell expansion (stage III) during fruit development on several fruit per treatment, using highly sensitive fruit gauges. During cell expansion, thinning treatments were also compared for their effects on fruit water potential as well. At stage I, no difference between treatments was found in any of the flows, suggesting that fresh matter (FM) import from phloem and xylem is sink-limited during early fruit development. At stage III, HCL fruit were smaller and had higher specific transpiration rates during the day. Xylem flow did not show any source limitations due to high crop load. However, it was “sink-strengthened” in the afternoon, in HCL, as these fruit reached lower water potentials during the day. Phloem flow to HCL fruit was source-limited during the afternoon and at night due to fruit to fruit competition. However, HCL fruit appeared to take advantage of their lower water potential at midday, when they showed higher rates of phloem flow. Although daily growth in HCL fruit undergoes periods of source limitation, this study showed how, at certain times of day, fruit may be more active sinks in attracting resources in high cropping conditions than at low crop loads.