Root-derived trans-zeatin riboside and abscisic acid in drought-stressed and rewatered sunflower plants: interaction in the control of leaf diffusive resistance?

Four-week-old potted sunflower plants (Helianthus annuus L.) were exposed to drought for up to two days by withholding irrigation. During the stress treatment and after rewatering, xylem sap was collected from decapitated hypocotyls by pressurising the root system. The water potential (Ψ) of the hypocotyl, the diffusive resistance of the second leaf pair, total transpiration and the concentration and flux rates of ABA and trans-zeatin riboside (ZR), identified by combined gas chromatography-mass spectrometry (GC-MS) as the dominant cytokinin in xylem sap, were determined. ABA contents were also analysed in root and leaf tissue. When Ψ of the hypocotyl decreased, the concentration and flux rate of ZR decreased drastically after a transient rise. A significant rise in ABA concentration and flux rate in xylem sap as well as a parallel rise in leaf diffusive resistance occurred as soon as Ψ reached values of -0.4 MPa and lower. Root ABA concentration began to rise at the same water potential parallel to the rise in xylem sap, whereas the ABA concentration in leaves began to rise only at Ψ values lower than -0.6MPa. Treatment of the root system with norflurazon prior to drought stress suppressed the increase in the ABA concentration in xylem sap and caused higher transpiration rates. Watering the drought-stressed plants led to a rapid decrease in ABA content of the xylem sap within three hours, whereas the decrease in leaf diffusive resistance was somewhat slower. The ZR concentration in the xylem sap rose continuously after rewatering, reaching a 60-fold increase after five hours, and declined again afterwards. Studies in which ZR and ABA were applied to cut shoots in concentrations similar to those in xylem sap of well-watered plants (ZR) and drought-stressed plants (ABA) showed that ZR, even in very low concentrations, antagonised the effect of ABA on transpiration. The results are discussed with regard to a possible antagonistic interaction of ZR and ABA as non-hydraulic root-to-shoot signals, and with regard to their interplay with hydraulic signalling.