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.

Effect of Temperature on Nitrate Uptake, Translocation and Metabolism in Pennisetum americanum

Daily cycles of net nitrate uptake, in vitro and in vivo nitrate reduction, and nitrogen transport from roots to tops were studied in two cultivars of Pennisetum americanum growing at day/night temperatures of 18/13°C and 30/25°C or 33/28°C. Rates of net nitrate uptake, xylem exudation and loading of amino acids into the xylem usually peaked once in the light period, whereas rates of in vitro nitrate reductase activity and in vivo reduction of NO3- to NO2- showed two peaks, one in the light and one in the dark period. Mean hourly rates per gram dry weight root of net nitrate uptake, xylem exudation and loading of amino acids into the xylem were 20, 44 and 40% respectively lower at 18/13°C than at 30/25°C. A whole-plant budget showed that both genotypes reduced the same amount of nitrate per day when they were grown at the same temperature. The proportion of nitrate which was reduced in roots was higher at 30/25°C (50%) than at 18/13°C (20%). At 30/25°C there was net export of reduced nitrogen from roots to tops whereas at 18/13°C there was import of reduced nitrogen into roots. Temperature-induced changes in daily cycles of nitrate uptake, rates of nitrate reduction and transport are interpreted as adaptations toward better carbon economy at low temperature.