Studies of the hormonal regulation of leaf senescence

This thesis describes an investigation into the role of abscisic acid (ABA) in the regulation of foliar senescence in radish and bean. The effect of ABA on the decline of protein and RNA pre-labelled with radioactive precursors, and on the loss of chlorophyll during the senescence of leaf discs of radish (Raphanus sativus L.) were studied. ABA stimulated the decline of chlorophyll during the early period of incubation, but subsequent loss of chlorophyll was retarded by ABA. ABA also enhanced the loss of protein and RNA from the leaf discs. Moreover, since the level of radioactive protein or RNA was normally lower in ABA-treated discs than in water-treated discs, it is possible that ABA was acting to stimulate protein and RNA degradation. On the other hand, the data indicate that ABA might act by limiting synthesis of radioactive protein or RNA in the post-labelling period. Thus, protein and RNA synthesis inhibitors were used in order to limit this period of incorporation. The limitations and possible disadvantages of the use of such inhibitors are discussed. The uptake of [14]C-ABA by radish leaf discs was highest 1 to 2 days after excision and then declined to 6 days. This pattern of uptake was not mirrored by uptake of [14]C-sucrose. The uptake of [14]C-ABA and [14]C-sucrose was substantially reduced by anaerobic conditions. Radioactive ABA is metabolised to 2 other major radioactive products by radish leaf discs, even after 6 days of pre-ageing in water. A similar pattern of metabolism for [14]C-ABA was observed in whole leaves of radish. Metabolism of [14]C-ABA in leaf discs was largely dependent on aerobic metabolism and was not affected by the addition of antibiotics to control any microbial contamination. Studies were also made on the pattern of metabolism during a 24 hour time-course and on the fate of partially purified metabolites re-supplied to radish leaf discs. The extraction and identification of the metabolic products of ABA has been attempted. The presence of endogenous ABA in primary leaves of bean (Phaseolus vulgaris L.) has been demonstrated. Using circular dichroism procedures, quantitative determinations of ABA in bean leaves at various stages of development indicated little consistent change in the amount of ABA with the onset and progress of senescence. ABA levels in extracts of wilted and non-wilted leaves of bean were compared in a preliminary experiment. In ultrastructural studies of radish leaf tissue, differences in the pattern of senescence-associated changes in cellular organelles were noted between leaf tissue aged in water, tissue aged in ABA and naturally senescent leaf material. The problems resulting from the possible variation in leaf material and to the asynchronous nature of senescence are discussed. The benefits and disadvantages associated with the use of leaf discs as an assay system for leaf senescence are also discussed