DOES XYLEM SAP ABA CONTROL THE STOMATAL BEHAVIOR OF WATER-STRESSED SYCAMORE (ACER-PSEUDOPLATANUS L) SEEDLINGS

Sycamore seedlings were grown with their root systems divided equally between two containers. Water was withheld from one container while the other container was kept well-watered. Effects of soil drying on stomatal behaviour, shoot water status, and abscisic acid (ABA) concentration in roots, xylem sap and leaves were evaluated. At 3 d, root ABA in the drying container increased significantly, while the root ABA in the unstressed container of the same plants did not differ from that of the control. The increase in root ABA was associated with the increase in xylem sap ABA and with the decrease in stomatal conductance without any significant perturbation in shoot water status. At 7 d, despite the continuous increase in root ABA concentration, xylem sap ABA showed a marked decline when soil water content was depleted below 0.13 g g-1. This reduction in xylem sap ABA coincided with a partial recovery of stomatal conductance. The results indicate that xylem sap ABA is a function of root ABA as well as the flow rate of water from roots to shoots, and that this ABA can be a sensitive indicator to the shoot of the effect of soil drying.