Evidence for Xylem Discontinuity in Pinot noir and Merlot Grapes: Dye Uptake and Mineral Composition During Berry Maturation

Abstract

To estimate changes in xylem flow during greenhouse-and field-grown grape (Vitis vinifera L.) berry maturation, apoplastic dye (aqueous Eosin Y or azosulfamide) was allowed to suffuse through cut pedicel ends. Dye uptake in colored Pinot noir berries was significantly less than that in pre-colored berries. Dye distribution in soft-green berries was uneven in comparison to that in firm-green berries, suggesting that xylem discontinuities occur as the berries soften. Merlot clusters, collected from the field near veraison and the berries separated by deformability and color, showed distinct differences in dye uptake and distribution in soft-green versus firm-green berries. Pinot noir and Merlot berry deformability increased sharply before berry coloring (approximately 3 days before for Pinot noir), and in some cases preceded the second rapid berry expansion. In Pinot noir, total K⁺ per berry increased rapidly after veraison, but total Ca²⁺ per berry did not. Dye uptake and Ca²⁺ accumulation patterns suggested increased flow resistance in the xylem after veraison. Berry softening coincided with a change in dye uptake patterns and sometimes occurred before the resumption of rapid berry growth.