Does Cytokinin Transport from Root-To-Shoot in the Xylem Sap Regulate Leaf Responses to Root Hypoxia?

We have examined the hypothesis that cytokinins transported from roots to shoots affects leaf growth, stomatal conductance, and cytokinin concentration of leaves of Phaseolus and a hybrid poplar ( Populus trichocarpa × Populus deltoides ) with hypoxic roots. Because cytokinins may interact with other substances, potassium and calcium concentrations were determined in xylem sap of Populus plants with hypoxic and aerated roots while gibberellin (GA) concentrations were measured in shoot tissues. Root hypoxia decreased leaf growth and closed stomata in both species. In both species, fluxes of cytokinins out of the roots were reduced, but no differences in bulk leaf concentrations were measured between the hypoxic and aerated plants. Shoots with aerated roots contained slightly higher concentrations of GA1 and GA3 than shoots from hypoxic plants. There were no differences in calcium or potassium concentrations in xylem sap between aeration treatments. Exogenously applied cytokinins did not alleviate the growth or stomatal responses caused by root hypoxia. Information on the site(s) and mechanism(s) of cytokinin action and the ways in which cytokinins are compartmentalized within plant cells will be required to understand the physiological significance of cytokinin transport in the transpirational stream.