Graduate Thesis Or Dissertation
 

Physiology of senescence of the shoot apex in Pisum sativum L.

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  • Shoot apex senescence is defined as the collective progressive and deteriorative processes which ultimately terminate the functional life of the shoot apex. Senescence of shoot apices of Pisum sativum L. 'Alaska' as measured by cessation of stem elongation was delayed by removal of flowers and by treatment with gibberellin A₃ and was hastened by treatment with AMO-1618 (2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidinecarboxylate methyl chloride). Onto-genetic changes in relative endogenous gibberellin levels and in capability of gibberellin biosynthesis in deflowered and control (flower- and fruit-bearing) plants were determined indirectly by studying time-course changes in the sensitivity, as indicated by the growth response, of these plants to applied gibberellin and AMO-1618. The results of these experiments suggest that the endogenous gibberellin level varies directly with the growth rate. Analyses of total RNA, nitrogen, protein and inorganic phosphorus in shoot tips of deflowered and control plants throughout ontogeny revealed that the levels of all these metabolites declined during senescence. Also throughout ontogeny shoot tips of control and deflowered plants were compared with respect to their ability to enzymically degrade RNA and to take up and incorporate ³²P-ortho-phosphate into RNA. The specific activity of ribonuclease increased as senescence progressed while the absolute activity appeared to decrease in correlation with the decrease in total nitrogen content. When compared with nonsenescing shoot tips, senescing shoot tips accumulated less ³²P, but exhibited an apparent enhancement of ³²P incorporation into RNA, which was attributed to a reduction in the endogenous phosphorus pool causing a smaller dilution of the accumulated ³²P. It is concluded that decreases in the levels of RNA, protein, gibberellin and inorganic phosphorus and in the transpirational uptake of nutrients are factors correlated with senescence of the shoot apex. Thus, while shoot apex senescence is correlated with numerous interrelated processes, senescence apparently results most directly and basically from deterioration of RNA and protein synthesis and concomitant enzymic degradation of these macromolecular species.
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file details EcklundPaulR1969.pdf 2017-08-14 Pubblico Scaricare