Cytokinins (CKs) coming from the roots via the xylem are known to delay leaf senescence, and their decline may be important in the senescence of soybean (Glycine max) plants during pod development (monocarpic senescence). Therefore, using radioimmunoassay of highly purified CKs, we quantified the zeatin (Z), zeatin riboside (ZR), the dihydro derivatives (DZ, DZR), the 0-glucosides, and DZ nucleotide in xylem sap collected from root stocks under pressure at various stages of pod development. Z, ZR, DZ, and DZR dropped sharply during early pod development to levels below those expected to retard senescence. Pod removal at full extension, which delayed leaf senescence, caused an increase in xylem sap CKs (particularly ZR and DZR), while depodding at late podfill, which did not delay senescence, likewise did not increase the CK levels greatly. The levels of the 0-glucosides and the DZ nucleotide were relatively low, and they showed less change with senescence or depodding. The differences in the responses of individual CKs to senescence and depodding suggest differences in their metabolism. Judging from their activity, concentrations and response to depodding, DZR and ZR may be the most important senescence retardants in soybean xylem sap. These data also suggest that the pods can depress CK production by the roots at an early stage and this decrease in CK production is required for monocarpic senescence in soybean.CK2 appears to be the major senescence-retarding hormone in plants, and its role in leaves is particularly important (30). Nonetheless, there is little integrated information on the CK hormone systems regulating senescence or other processes (21). A wide variety of studies have shown that leaf senescence is usually correlated with a decrease in CK activity levels in the leaves and have implicated roots as the major sources of CKs in mature leaves (30). These root-produced CKs are carried through the xylem into the leaves with the transpiration stream.In soybean, the developing pods, specifically the seeds, cause the plant to degenerate (monocarpic senescence) and die (14,15,19,20). Removal of the pods before, but not during, late podfill can prevent the dramatic yellowing and death of the plant (15, 19). How does CK fit into this correlative control picture? Early in reproductive development, the foliar CK-like activity (16) declines. This decrease is due neither to diversion of the flux from the leaves to the pods (22, 23) nor to an increase in the metabolism of CKs (Z and ZR and their metabolites), which is quite rapid anyhow in mature leaves (22,23,27). Thus, a decline in CK production by the roots could account for the decrease in foliar CK levels. In order to test further the connection between CKs and leaf senescence and to fill in a gap in our understanding of the role of CK in the control of senescence, this study examines the xylem sap levels of CK as a relative index of CK flux in plants allowed to develop fruit and senesce normally as well as in depodded plants.
MATERIALS AND METHODS
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