During the early stages of growth when the primary leaves of beans (Phaseolus vulgarisL., cv. Kinghorn) are expanding, there is an increase in chlorophylls, carotenoids and protein levels as well as superoxide dismutase (SOD), catalase and peroxidase activities. However, following the initiation of flowering on day (d) 21 there is a steady decrease in all of these parameters except levels of peroxidase activity, and by d 35 the primary leaves are also showing pronounced morphological symptoms of senescence. Following the onset of senescence, primary leaves were also leaking and showed increased levels of malondialdehyde (MDA). In addition, there was an increase in the lipid phase transition temperature of a membrane fraction consisting predominantly of chloroplast membranes, which reflected the formation of gel phase lipid. By d 56, the leaves had abscinded. A single application of benzyladenine (BA) to the primary leaves or decapitation of the shoot above the primary leaf on d 9 retarded all of these senescence symptoms, with decapitation being more effective than BA.On d 35 when the primary leaves were pale green-yellow, a single application of BA to the leaves or decapitation of the shoot above them reversed the symptoms of senescence. Decapitation was more effective than BA as a means of reversal, causing chlorophyll, carotenoid and protein levels to rise to values higher than those present in the control leaves. Decapitation also reversed the senescence-related changes in membrane phase properties, decreased membrane leakiness and reduced lipid peroxidation.
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