Effect of cytokinins supplied via the xylem at multiples of endogenous concentrations on transpiration and senescence in derooted seedlings of oat and wheat

This study was conducted to determine whether naturally occurring xylem cytokinins, when supplied to leaves via the xylem at approximately endogenous concentrations, increase transpiration and delay senescence in selected monocot species (oat and wheat), The concentrations of some of the major cytokinins (zeatin, dihydrozeatin, cis-zeatin and their ribosides, the O-glucosides and nucleotides) were determined in the xylem exudate of oat and wheat seedlings by radioimmunoassay, Evidence is presented that the small volume of exudate (4-5 mm(3)) collected per plant was xylem sap in transit at the time of shoot excision, Using the data on cytokinin levels, the individual bases and ribosides (and a base/riboside mixture), at multiples of concentrations determined in xylem sap, were tested in transpiration and senescence bioassays, The individual O-glucosides (and mixtures of the O-glucosides) were similarly tested at (i) multiples of the molar concentrations of the corresponding bases and ribosides, and/or at (ii) multiples of the endogenous concentrations, Similarly, zeatin and dihydrozeatin nucleotides were tested at multiples of the molar concentration of zeatin riboside and, in some instances, at multiples of endogenous concentrations. Our results suggest that, at least in oat and possibly in wheat, zeatin-type bases, ribosides and O-glucosides supplied to the leaf in xylem sap are likely to play a role in regulating transpiration in vivo, O-glucosides in oat xylem sap may be important regulators of leaf senescence in the intact plant, The nucleotides were present in xylem sap at lower concentrations than most of the bases, ribosides and O-glucosides, The nucleotides appear likely to play a lesser role than the bases, riboside and O-glucosides in controlling transpiration and senescence in the intact plant.