The amino acid distribution in rachis xylem sap and phloem exudate of Vitis vinifera 'Cabernet Sauvignon' bunches.

Amino acids are essential to grape berry and seed development and they are transferred to the reproductive structures through the phloem and xylem from various locations within the plant. The diurnal and seasonal dynamics of xylem and phloem amino acid composition in the leaf petiole and bunch rachis of field-grown Cabernet Sauvignon are described to better understand the critical periods for amino acid import into the berry. Xylem sap was extracted by the centrifugation of excised leaf petioles and rachises, while phloem exudate was collected by immersing these structures in an ethylenediaminetetraacetic acid (EDTA) buffer. Glutamine and glutamic acid were the predominant amino acids in the xylem sap of both grapevine rachises and petioles, while arginine and glycine were the principal amino acids of the phloem exudate. The amino acid concentrations within the xylem sap and phloem exudate derived from these structures were greatest during anthesis and fruit set, and a second peak occurred within the rachis phloem at the onset of ripening. The concentrations of the amino acids within the phloem and xylem sap of the rachis were highest just prior to or after midnight while the flow of sugar through the rachis phloem was greatest during the early afternoon. Sugar exudation rates from the rachis was greater than that of the petiole phloem between anthesis and berry maturity. In summary, amino acid and sugar delivery through the vasculature to grape berries fluctuates over the course of the day as well as through the season and is not necessarily related to levels near the source.