Phloem and xylem flow contributions to nectarine fruit development

Abstract

This study aimed at determining how source-sink balance and phloem/xylem flows affect nectarine fruit growth during development. Different levels of water and assimilate availability to growing fruits were induced in vivo by varying leaf:fruit ratio (L:F) of fruiting shoots and by interrupting the phloem stream (girdling) at the base of entire fruiting shoots. Two fruiting shoots in each of six ‘Big Top’ adult nectarine trees were selected, labeled and their L:F was adjusted by thinning fruits or removing leaves to two levels: high L:F and low L:F. Stem water potential, stomatal conductance, continuous fruit diameter and leaf turgor pressure were measured before and after girdling at stage I and stage III of fruit development. At stage I, fruits from high L:F shoots grew more than those from low L:F shoots both before and after stem girdling. At this stage, xylem flow from the whole tree and phloem flow from leaves of the same stem seem to be the major factors contributing to fruit growth. At stage III, fruits from high and low L:F shoots exhibited similar growth before girdling, whereas girdling induced a generally sharp decline of fruit growth and a marked shrinking of low L:F fruits. At this stage, regular fruit growth mostly relied on both water and carbon phloem imports from neighboring branches. Regardless of girdling or stage, the diel leaf turgor pressure (pc) range (maximum pc – minimum pc over 24 h) was always smaller in leaves from low L:F shoots compared to those from high L:F shoots, indicating that the source-sink balance is a major factor driving water status changes in leaves. Overall, at stage I, nectarine fruit growth was mainly driven by water movements through the xylem, whereas at stage III, it mostly relied on phloem contribution (water + assimilates). Leaf pc variations were mostly influenced by L:F possibly via xylem backflow from fruits in both stages I and III.