Reverse Water Flow from Fruit to Leaves in Grapefruit Under Severe Drought Stress

Abstract

Detached fruit-bearing branches(1 fruit and ca 5 leaves each)were harvested from 10-year-old grapefruit trees to quantify the extent to which xylem back-flow can remove water from fruit to leaves, and the amount of water arising from different sources in fruit. Tritiated water was used to trace the movement from fruit to leaves and, together with total water, to quantify th efflux from fruit. The results showed that both fruit transpiration and water exit via xylem back-flow were found to occur within a given 24-hour period. Both leaves and fruit transpired more water when on fruited branches, than when fruit were severed. Total water losses from fruit via xylem back-flow and fruit transpiration were similar (2.1% vs 2.2% of original tissue water). A comparison between exit of total and tritiated water indicated that newly arrived water in juice sacs was the first to leave. Albedo and possibly juice sacs both contributed to the water efflux from fruit via xylem back-flow, although peel lost a mean of 5.4% of its tissue water in 4 days. vs 1.0% from juice sacs during the same period. Concomitant decreases in water potential were ca 0.6 MPa in albedo and ca 0.1 MPa in juice sacs due to effects of xylem back-flow. Transpirational water loss from fruit appeared to arise from albedo rather than from juice sacs during this experimental period.