The involvement of cytokinins and abscisic acid (ABA) in the monocarpic senescence (foliar yellowing following fruit development) of soybeans was examined. Foliar sprays of cytokinin (10−4M zeatin or 10−5M benzyladenine), begun when the plants first set fruit and repeated every other day, significantly delayed, but did not prevent, monocarpic senescence. Foliar sprays of 10−4M ABA, applied in the same manner, significantly hastened senescence of fruiting soybeans but apparently had no effect on depodded plants. Leaf and stem material from pre-senescent and senescent plants was extracted, chromatographed, and bioassayed for cytokinin-lilce activity (Amaranthusbetacyanin production assay) and ABA-like activity (oat coleoptile straight growth assay for inhibitors). ABA-like activity increased, and cytokinin-like activity decreased in shoot tissue before the plants began to senesce. Cytokinin-like activity in the fruit also declined during this period. These results implicate a decrease in cytokinins and an increase in ABA-like inhibitors in the control of monocarpic senescence of soybeans, but neither alone is c
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