Xylem conductance of sweet cherry pedicels

Abstract

Flow rates were quantified on detached pedicels and conductances calculated. Flow rates and conductances were independent of time and temperature, increased with the pressure applied and slightly decreased during development. Water uptake is thought to contribute to cracking of sweet cherry fruit. Water uptake/loss occurs not only through the fruit surface but also through the xylem and phloem of the pedicel. The objectives here were: (1) to quantify the hydraulic conductance of the pedicel xylem and (2) to identify the factors affecting pedicel xylem conductance. Using a modified pressure probe, water was forced through excised pedicels. The rates of axial flow were quantified gravimetrically by collecting water at the pedicel’s distal end in a pre-weighed Eppendorf tube. Applying pressure to the proximal end of a pedicel induced a constant axial flow of water through it. The introduction of air embolisms to the xylem markedly decreased axial conductance from 1.3 ± 0.1 × 10−10 m3 MPa−1 s−1 to near zero. Steam girdling had no effect on axial conductance indicating the water flows to be through the xylem, not significantly through the phloem in our ex situ arrangement. Compared with a petiole, the pedicel conductance was 50-fold lower. A cross-section of a pedicel near its distal end revealed about 10 vascular bundles that diverged from a complete ring closer to the proximal end. Conductance estimates from the numbers and sizes of xylem vessels using Hagen–Poiseuille’s law exceed those determined experimentally by about three to fourfold. Conductance of pedicels decreased slightly in developing fruit and then remained about constant until maturity. Surprisingly, there was no significant effect of temperature on conductance. The conductance estimates may be used to model xylem water flows through the pedicel.