Abscisic Acid Is Not the Only Stomatal Inhibitor in the Transpiration Stream of Wheat Plants

Xylem sap was collected from the transpiration stream of wheat (Triticum aestivum L.) plants and assayed for the presence of an inhibitor of transpiration using leaves detached from well-watered plants. Transpiration of detached leaves was reduced by nearly 60% by sap collected from plants in drying soil, and to a lesser extent (about 25%) by sap from plants in well-watered soil. As the soil dried the abscisic acid (ABA) concentration in the sap increased by about 50 times to 5 x 10(-8) molar. However, the ABA in the sap did not cause its inhibitory activity. Synthetic ABA of one hundred times this concentration was needed to reduce transpiration rates of detached leaves to the same extent. Furthermore, inhibitory activity of the sap was retained after its passage through an immunoaffinity column to remove ABA. Xylem sap was also collected by applying pressure to the roots of plants whose leaf water status was kept high as the soil dried. Sap collected from these plants reduced transpiration to a lesser extent than sap from nonpressurised plants. This suggests that the inhibitory activity was triggered partly by leaf water deficit and partly by root water deficit.