Passioura has made a number of substantial and highly innovative contributions to understanding of the water relations of plants. Developing techniques for collecting the xylem sap of intact transpiring plants, he has shown for the first time that roots sense water and osmotic relationships in the soil and relay such information through their xylem to sites of control of water loss in leaves. Passioura has evaluated the significance of such a signalling system to the overall responses of plants to the water status of soil and plant and the functioning of their cellular processes of growth under various forms of stress.