Fruit apoplast tension draws xylem water into mature sweet cherries

Abstract

Rain cracking of sweet cherry fruit (Prunus avium L.) is likely related to fruit water balance. Most research on fruit water balance has focused on water transfers through the fruit skin (e.g. osmotic inflows, transpiration outflows) with little work on the water flows through the xylem and phloem vascular systems of the pedicel. The objectives here were to use a potometer and a pressure probe to study xylem sap flows in the pedicels of fruit isolated from the tree by a cut at the proximal end of the fruit+pedicel.Xylem water inflow rates were constant for up to 8h but then slowed. The pressure probe established the presence of a tension (a negative pressure) in the pedicel xylem. Xylem sap flow rates and xylem tensions were instantly, totally and permanently eliminated if the fruit was detached from the pedicel. Progressive removal of slices of the fruit flesh beginning at the distal end (stylar scar) and work towards the proximal end (pedicel) gradually decreased sap flow rates and tensions. The length of the pedicel had only a marginal effect on flows. Fruit mass and osmotic pressure of the flesh increased sigmoidally during stage II (pit development) and stage III (final swell) of fruit development. Flow rates and tensions both increased to maxima at early stage III, and both decreased thereafter. Flow rates and tensions were higher at ∼0% relative humidity (RH) than at ∼100% RH. The flow rate difference at ∼0% and at ∼100% RH decreased during development. There was no difference in flow rate of fruit at ∼100% RH and fruit submerged in water. During early development, xylem flow in the pedicel was inversely related to RH. As development proceeded, xylem flow became progressively less dependent on RH. Abrading the cuticle or slashing the skin had no effect on xylem sap inflow. There was no relationship between the tension in the xylem and the osmotic pressure of the expressed juice of the flesh (r2=0.13). Feeding the cut end of the pedicel xylem with sucrose solutions of increasing osmotic pressure decreased xylem inflow and tension up to osmotic pressures of about 2.5MPa. Beyond 2.5MPa, some inflow and tension remained detectable. The results establish that xylem flow is likely drawn into the fruit from the tree by apoplastic tension in the fruit, resulting from osmotic water uptake from apoplast to symplast and from transpiration. Minor contributions may result from cell wall swelling within the flesh. Indirect evidence suggests that decreased conductance of the xylem accounts for the decrease in flow during stage III. These findings render the possibility of a significant role for xylem transport in fruit cracking unlikely.