Sap Flow and Vascular Functioning during Fruit Development

An expanding array of thermal sap flow techniques, combined with miniaturisation of sensors, is leading to the application of sap flow methodology to the understanding of vascular fluxes in small plants and distal organs that were previously difficult to study. Examples include the measurement of transpiration from individual leaves, and phloem and xylem flows to and from flowers and fruit. The nature of these organs means that high spatial and temporal variability, flow reversals and complex interactions between phloem and xylem flows can all be expected. In this paper we illustrate the application of miniature heat ratio sap flow gauges to the study of vascular functioning and growth of fleshy fruit. Relevant questions include the nature of pressure gradients driving vascular flows, the magnitude of xylem hydraulic resistance and the relative contributions of phloem and xylem flows to the fruit water balance. External heat ratio gauges were installed on the pedicels of developing kiwifruit two months after anthesis, a period when the fruit are transitioning from rapid initial growth to slower growth and accumulation of starch. Inward fluxes peaked during the day, lagging behind transpiration and in-phase with the diurnal minimum fruit diameter. Brief flow reversal, the movement of sap from fruit to vine, was observed consistently at dawn. The cumulative sum of sap flow and transpiration matched the dynamics of fruit growth measured using displacement transducers. Pedicel flow dynamics were explained by pressure gradients measured between fruit and vine, and the capacitance of the fleshy pericarp. Phloem girdling of the pedicel had no immediate effect on flow dynamics and fruit growth continued normally for 3-5 days after girdling, indicating that xylem flows were dominant, and that xylem water can substitute for phloem supplied water at this stage of development.