ABSTRACT Pierce's disease (PD) is one of the major factors limiting the expansion of the wine industry in the southeastern United States. This research is initiated to test the hypothesis that Pierce's disease (PD) resistance/tolerance is due to the presence of unique biochemical components in the xylem fluid.The quantity and composition of xylem fluid is known to vary with time of day, season of the year, location in the plant, plant age, and plant health. In this study protein, sugars and amino acids levels of the xylem fluid were determined to evaluate their role in supporting Xylella fastidiosa (Xf)growth in the xylem tissue. The objective of this study was to determine differences in xylem sap composition among the Muscadines, Florida bunch hybrids and California bunch grape cultivars.For this purpose xylem sap was collected from each cultivar during different seasons and developmental periods and analyzed. The pH of the California bunch and Florida hybrids was found to be slightly higher than that of the muscadine, 6.14, 5.95 and 5.78, respectively.The conductivity also varied greatly among the grape cultivars tested. Xylem sap is being analyzed for proteins, amino acids and sugars to determine differences in the metabolite levels among the grape cultivars.Preliminary data indicated presence of wide differences in these components among the cultivars.

KEY WORDS-Xylem, Proteins, Resistance, Sugars