Diurnal and temporal changes in the chemical profile of xylem exudate from Vitis rotundifolia

Positive root pressure in Vitis rotundifolia Michx. cv. Noble was employed to quantify diurnal and temporal changes in the chemical profile of xylem exudate. Xylem fluid osmolarity (7.2 to 16.8 m M ), water flux (8.2 to 18.5 ml h⁻¹) and solute flux (0.7 to 2.2 mmol h⁻¹) from a cut spur exhibited a diurnal pattern with maxima during midday and minima at night. Total osmolarity was similar to the sum of all organic and inorganic entitites quantified, indicating that the major solutes have been identified. Total amino acid and organic acid concentration were about equal (2 to 7 m M ), and sugars accounted for a minor fraction of the total profile (<0.2 m M ). Glutamine represented ca 80% of the organic N and 70% of the total N transported in the xylem fluid. A circadian rhythm in water flux and net flux of most organic and inorganic entities was observed with maxima during midday and minima at night. The increase in xylem fluid osmolarity occurring during midday was primarily a consequence of increased organic acid (oxalic, citric, tartaric, malic and succinic acids) and ion (NH₄⁺, No₃⁻, P and Ca) concentration. A diurnal cycle in amino acid concentration was less clear. The concentration of individual organic and inorganic entities varied asynchronously with time. Xylem solute was comprised of 80% organic and 20% inorganic components when collected 5 min to 2 h after the commencement of bleeding, but the ratio of organic to inorganic components fell to about 50% after 7 days.