The chemical composition of xylem sap in Vitis vinifera L. cv. riesling during vegetative growth on three different franconian vineyard soils and as influenced by nitrogen fertilizer

Cuttings of grapevine (Vitis vinifera L. cv. Riesling clone B 68) grafted on SO4 (Selection Oppenheim No. 4) rootstocks were grown in pots with three different soils from Franconian vineyards derived from different geological formations (namely, Loess, Muschelkalk (shell lime), or Keuper). Additionally, the influence of N-fertilizer treatment was investigated. From the midrib of leaves six to eight of the sole shoot, xylem sap was collected simultaneously by pressurizing the rhizosphere during the vegetative growth phase. The chemical composition of xylem sap was determined and compared with that of the aqueous soil extract. In Muschelkalk soil, carbon, nitrogen, and calcium were present in the greatest concentrations. Sulfur, boron, magnesium, sodium, and potassium were greatest in Keuper, and the concentrations in Loess soil were intermediate, Aqueous extraction of the soils resulted in a two-fold greater concentration of total solutes in Keuper extract compared with Muschelkalk, and more than threefold than in Loess. The apparent volume flow was greatest in the middle leaves along the shoot and in plants grown on Keuper; additionally there was a tendency for fertilizer treatment to increase flow. The concentrations of mineral ions in xylem sap were the same in all the leaves of a shoot of grapevine. An important exception was the supply to the leaves of amino acids, which increased in concentration along the transpiration stream and were greatest in the youngest leaves (particularry in non-fertilized plants). Potassium was the dominant cation in xylem sap and was greatest in plants grown on Keuper. Concentrations of sodium and calcium were increased in non-fertilized plants, but not significantly in vines grown on Muschelkalk. In xylem sap, nitrate was the major anion, followed by malate. Nitrate concentration was greatest in plants grown on Muschelkalk, while malate was greater in plants grown on Keuper. Chloride, sulfate, and phosphate concentration in sap were increased by fertilizer treatment. Abscisic acid was markedly increased in xylem sap of non-fertilized plants grown on Loess and Muschelkalk and was discussed as a signal for nutrient limitation. If Keuper was the substrate it was also increased by fertilizer treatment. Of the organic N-compounds, glutamine was the largest fraction. On the basis of the relation of nitrate to total N in xylem sap, it could be assumed that about 40% to 75% of nitrate reduction took place in the shoots. In general, soil type had only a moderate effect on the chemical composition of the xylem sap compared with the effect of N-fertilizer.