Modulation of the root-sourced ABA signal along its way to the shoot in Vitis riparia×Vitis labrusca under water deficit

The intensity of the root-sourced abscisic acid (ABA) signal has long been thought to decrease along its long-distance transport pathway, and hence the shoot responses to the ABA signal would be expected to become less sensitive with the increase in plant height. It is reported here that there is a significant modification of the ABA signal intensity in its pathway to leaves in grapevine (Vitis riparia×Vitis labrusca), but in contrast to the expectation that the ABA signal intensity may decrease along its long-distance transport pathway, it was found that the root-sourced ABA signal is gradually intensified along a vine for as long as 3 m under both water-stressed and non-stressed conditions. Consistent with the alterations in ABA signal intensity, stomatal sensitivity to a root-sourced ABA signal was also gradually increased from the base to the apex. Leaf stomatal conductance near the apex was more severely inhibited than in the leaves at the base of the vine. It was observed that xylem pH was significantly increased from the base to the apex, and that artificially changing the xylem sap pH to be more alkaline by feeding with buffers increased the xylem ABA concentration. Our results suggest that the pH gradient along the stem may play a role in the modification and enhancement of ABA signal intensity such that the stomata at the top of canopy can be more sensitively regulated in response to soil drying.