Seasonal Variation in pH, Carbohydrate and Nitrogen of Xylem Exudation of Vitis vinifera

Over two seasons, 1989 and 1990, pH and carbohydrate concentration of the xylem exudate of Vitis vinifera L. var. Waltham Cross were examined around the time of budburst. During this period in 1990, weekly determinations of NH4+ and NO3- concentrations in xylem exudate were also performed. Over the 2 years, exudate pH varied inversely with daily exudate flow, falling from plateau levels of approximately pH 6.0 to lower than pH 5.5 around the date of greatest daily exudation, then rising again to a plateau of about pH 7.5. Exudate carbohydrate concentration variations were also consistent over the 2 years of the study, falling from plateau values of approximately 120 mg glucose equivalents L-1 (approximately 660 μM) at or immediately prior to the date of maximal exudation flow, to zero values within 4 weeks. These data, as well as reflecting a mobilisation of stored carbohydrates to apical tissues prior to budburst, also concur with previously observed activities of an apparently carbohydrate plant growth inhibitor in grapevine exudate. Exudate NH4+ and NO3- concentrations both increased directly with exudate flow up to the date of maximal daily exudation, then fell again to lower levels. Unlike seasonal pH variation, NH4+ and NO3- concentrations were not significantly correlated to daily exudate volume. The seasonal variations in pH infer a flow-dependent mobilisation from storage tissues, the reasons for which at budburst are discussed.