The Effect of Defoliation on the Composition of Xylem Sap from Cabernet franc Grapevines

Abstract

The composition of bleeding sap from grapevines during the periods of budburst and shoot growth were analyzed. The concentration of several elements, pH, acids, and carbohydrates were determined. Trials were carried out in 1983 and 1984 on six twelve-year-old vines (cv. Cabernet franc) grafted onto Kober 5BB rootstock, trained to the Friuli system. Bleeding sap samples collected over a period of 24 hours were obtained weekly from March until May. The daily discharge (mL/day/plant) reached the maximum (208 mL/day) at budburst and subsequently decreased and stopped just prior to bloom. Vines that were defoliated daily from budburst and thereafter had lower concentrations of total nitrogen and ammonia in the sap after budburst than vines that were not defoliated. The daily amount of sap, the pH, and the concentration of nitrate, phosphorus, calcium, potassium, and magnesium were not significantly affected by defoliation.