Collection of xylem sap at flow rate similar to in vivo transpiration flux

We have explored a method to collect xylem sap using a Scholander pressure chamber for potted plants. Intact root system in pots which fitted the pressure chamber was pressurized at a pneumatic pressure numerically equal to the absolute value of shoot water potential. The rate of xylem flow obtained from the stem stump under such pressure was found similar to the rate of transpiration before detopping. The rate of pressurized flow from detopped roots was linearly related to the pressure applied in both well-watered and soil-dried plants. The osmotic concentration of the xylem sap was negatively related to the rate of volume flow, suggesting the necessity to collect xylem sap at in vivo flow rate if original solute concentration is to be evaluated. The concentration of ABA in the xylem sap, however, did not show such a relationship with water flux. Both well-watered and soil-dried plants showed the concentration of ABA in xylem sap largely stable with a range of volume flow rate, indicating a linear relationship between the rate of ABA delivery through xylem and that of volume flow. We also compared the concentrations of ABA in xylem sap sequentially collected from pressurized roots with that from detached shoots of the same plants. The concentration of ABA in the initial saps from shoots showed to be similar to that from roots. However, a decrease in the concentration of ABA in the xylem sap collected from detached leaf or twig was observed when more volume of sap was collected, which might also be dependent on the plant species and the volume of xylem vessels concerned.