Relative Activities of Xylem-Supplied Cytokinins in Retarding Soybean Leaf Senescence and Sustaining Pod Development

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Abstract
Soybean explants (leaf, pod and subtending stem segment) excised at early-mid podfill and cultured on a mineral nutrient solution senesce much sooner than comparable intact structures. Pod development is also advanced, but seed yield is reduced. Cytokinin added to the mineral nutrient medium retards leaf yellowing, blade abscission, petiole abscission and to a lesser extent pod development. At 3×10−7m, dihydrozeatin riboside (diZR), benzyladenine, dihydrozeatin (diZ), zeatin riboside (ZR), zeatin (Z), isopentenyladenine or isopentenyladenine riboside retard leaf yellowing by 18, 15, 12, 4–6, 2–5, 2 and 1 days, respectively. These cytokinins show a similar hierarchy of activity on abscission. Cytokinin also retards changes in stomatal resistance and transpiration rate. The activity hierarchy for the different cytokinins on stomatal resistance and pod development differs from that on leaf yellowing. When cytokinins are given as a long pulse (about 3 days), they show a somewhat different activity pattern compared with a continuous supply of cytokinin. Clearly, the cytokinins coming up through the xylem from the roots play an important role in maintaining the foliage, and a decline in the supply of cytokinins from the roots could be a major factor in monocarpic senescence of soybeans. If Z. ZR, diZ and diZR are normally supplied by the roots via the xylem, they must be important factors in sustaining leaf function and pod development.

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