Xylem, Phloem and Transpiration Flows in a Grape: Application of a Technique for Measuring the Volume of Attached Fruits to High Resolution Using Archimedes' Principle

Abstract

The minute changes in volume of a grape berry which occur from hour to hour were measured non-destructively in the field using readily available and cheap laboratory equipment and a modern electronic balance. The method, applicable even to small (approximately 10 g) fruits, is based on Archimedes' principle and gave a resolution of about 1 part in 1 000 by measuring the buoyant upthrust experienced by a berry when immersed in water. Volume data from control, pedicel-steamed, and detached berries were used to calculate the magnitudes and directions of the fluid flows which took place through the stalk of the phloem and xylem streams and through the skin in the transpiration stream. In the latter stages of fruit development, after the onset of ripening, net volume growth more or less ceases in grapes although their rate of sugar import is at its strongest. Cessation of volume growth comes about because the strong inflow of sugary water in the phloem is closely balanced in part by transpirational water loss through the skin and in part by the backflow of xylem water to the parent vine. This xylem backflow appears to persist throughout the diurnal cycle. The net backflow direction of the xylem stream, together with the inability of the phloem stream to carry certain ions (notably calcium), may explain how some mineral imbalance disorders arise in the later stages of fruit development. The intense manner in which fruiting sinks compete with vegetative sinks in Vitis finds its explanation in the breakdown of apoplast: symplast compartmentation in the berry which occurs around the time of onset of ripening. The breakdown exposes the terminal sieve tubes of the berry to a highly negative water potential environment, serving to increase both the speed and the concentration of the translocation stream.