Initial Stages in the Onset of Senescence in Tobacco Leaves

Abstract

A marked loss of leucine "4C incorporation occurred in chloroplasts isolated from Nicotiana rustica L. leaves exposed to 24 hours of darkness. This loss is not due to an initial decline in RNA-synthesis potential of the chloroplasts, as was inferred from the extent of UTP incorporation by the isolated chloroplasts. Upon reillumination of the leaves, leucine incorporation by the isolated chloroplasts reverted to its original level within 3 to 4 hours, hence it is doubtful whether the period of 24 hours after detachment should be regarded as the initial phase of leaf senescence.After 48 and 72 hours of darkness, however, complete recovery of the incorporation activity was not achieved by reillumination of the leaves, representing the apparent onset of an irreversible process. Treatment with kinetin, which markedly delayed the symptoms of senescence in these tobacco leaves, did not prevent the dark-induced decline in chloroplast protein synthesis activity. Nor, up to 24 hours of darkness, did it have any effect on the light-induced complete recovery of this synthesis. Nevertheless, after reilluminating kinetin-treated leaves that had been exposed to darkness for 48 and 72 hours, leucine incorporation in the isolated chloroplasts was resumed at a faster rate and reached a higher level than did the untreated controls.Detached leaves maintained in darkness provide a standard system for the study of leaf senescence. Various senescenceinduced modifications in such leaves have been documented.These include structural alterations in the volume and shape of the chloroplast (5, 6, 9) as well as changes in several metabolic activities. The latter are reflected in the well known decline in the contents of RNA, protein, and Chl of leaf cells (3,8). These senescence symptoms become clearly observable usually some 48 hr after leaf detachment. However, Richmond et al. (8) reported that in detached tobacco leaves kept in darkness, a marked change in the protein synthesis potential of the chloroplasts isolated from these leaves takes place within 24 hr.This study aims at a more detailed characterization of the decline in the protein synthesis activity of chloroplast isolated from senescing leaves. MATERIALS AND METHODSLeaves of Nicotiana rustica L. grown in a greenhouse in half-strength Hoagland solution were washed thoroughly under a stream of running tap water and sterilized in a 70% ethanol solution for 10 sec. They were then dipped in 1% calcium hypochlorite for 60 sec and finally washed in sterile H20. The major veins were removed, and the pieces of the blade were placed in a plastic bag, weighed, and cooled in ice for 5 min. The leaf pieces were scissors-cut into an ice-cooled mortar and the chloroplasts were extracted according to Spencer and Wildman (10). Two ml Honda solution (containing 25 mg of Ficoll, 50 mg of Dextran T-40, 250 ,tmoles of sucrose, 100 ,ug of BSA, 42 ,umoles mercaptoethanol fatty-acid poor [Cal-biochem] 100 ,moles of magnesium acetate, 100 ,umoles of manganese acetate) were added for each g of l...

Regulation of photosynthesis and transcription factor expression by leaf shading and re-illumination in Arabidopsis thaliana leaves

“…The phenomenon of regreening of senescing leaves or cotyledons after shading, cytokinin treatment or removal of the shoot apex or reproductive organs is known since long in Nicotiana rustica, Phaseolus vulgaris, Pisum sativum, flax, soybean, wheat and other species (Dyer and Osborne, 1971;Greening et al, 1982;Mothes, 1960;Tuquet and Newman, 1980;Vonshak and Richmond, 1975). It appears that there are two phases of senescence, an early reversible phase followed by an irreversible one.…”

Microcalorimetric Study of Iodized and Noniodized Cells and C-Phycocyanin of Spirulina platensis

“…23 On the basis of literature [19][20][21][22][23] and our experimental data, we concluded that total heat evolved in the temperature range 5-52°C at a heating rate less than 0.083°C/min equal to 0.380 kJ g -1 dry biomass (Figures 3 and 4) is the heat of vital capacity maintenance (it includes respiration, mobility, osmotic work of cells, opposition to inhibition influences, etc.) of Spirulina.…”

“…cells is reached at 45°C, and only a temperature increase of several degrees causes it to drop to zero. This effect 22 is explained by supposition about denaturation of proteins responsible for respiration.…”

“…20 The processes of photosynthesis and respiration are also characterized in detail. 19,20,22 In the presented experiments, the influence of photosynthesis on thermal characteristics of Spirulina pl. cells is minimum.…”

Protein Metabolism in Senescing Wheat Leaves

“…It has been suggested that the loss of RuBPCase activity is due to a cessation of chloroplast protein and nucleic acid synthesis, coupled with a continued protein turnover (27). Darkness induces a marked loss in protein synthesis potential of chloroplasts isolated from Nicotiana leaves (23). Hence Table I.…”