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Xylem, phloem and transpiration flows in developing strawberries

Abstract:

The fruit of many species suffer a loss of xylem functionality during development. This reduces Ca import, so lowers fruit Ca concentration and impairs postharvest performance. Even though not a true fruit, this pattern may also apply to strawberry. Our aim was to measure xylem and phloem inflows to a developing strawberry and transpiration outflows. To do this, we used linear variable displacement transducers mounted on still-attached fruit and potometers on detached fruit. In attached fruit, the stalk inflows were partitioned into xylem and phloem components by killing the phloem with steam. Fruit mass increased sigmoidally till the fully-mature. The balance between xylem, phloem and transpiration flows followed a characteristic pattern of change during development. Early on, xylem inflow was 70–80% of total water inflow, towards maturity this decreased to ~36%. In contrast, phloem inflow increased from 20 to 30% of total early on, rising to ~64% at maturity. Aqueous acid fuchsin was used as a marker of xylem sap flow. When fed through the cut surface of the stalk of a detached fruit, staining of the fruit vascular system was uniform in young fruit, but progressively less uniform as the fruit developed. By full maturity no dye penetrated beyond the base of the fruit. Potometric flow rates into the fruit increased from 8 to 27 days after full bloom, then decreased until maturity. The inflow rate of a fruit held at ~0% RH always exceeded that in one held at ~100% RH. Removing the calyx decreased the inflow rate. The calyx accounted for ~50% of total fruit transpiration (i.e. of fruit + calyx). As indexed both by dye penetration and by xylem flow measurements, xylem functionality decreased during fruit development.