EFFECT OF FRUIT ZONE LEAF THINNING ON TOTAL GLYCOSIDES AND SELECTED AGLYCONE CONCENTRATIONS OF RIESLING (VITIS-VINIFERA L.) GRAPES

Selective leaf removal from fruit zones of mature Riesling grapevines grown at two northern Virginia (approximate to 39 degrees N) sites was evaluated over two seasons for its influence on total grape glycoside s. Two to four leaves per shoot were removed three weeks after bloom f rom around fruit clusters grown on a high (1.8 m above ground) bilater al cordon at one vineyard, and a low (1.2 m) bilateral cordon at the o ther. Leaf removal increased the percentage of sunlight penetration in to the canopy fruit zone in three of four measurements. The concentrat ion of total glycosides, estimated by the analysis of glycosyl-glucose , was mostly higher in the fruit from leaf-removed vines than control vines for both the low-trained and high-trained vineyards. The sum of the concentration of selected bound monoterpene alcohols (geraniol, ne rol, and linalool), and bound aromatic alcohols (benzyl alcohol and 2- phenylethanol), was also higher in fruit of leaf-removed canopies than controls at most sampling dates.