No abstract

The regulatory role of ethylene in leaf senescence was studied with excised tobacco leaf discs which were aflowed to senesce in darkness.Exogenous ethylene, applied during the first 24 hours of senescence, enhanced chlorophyil loks without accelerating the cHimacteric-like pattern of rise in both ethylne and CO, which occurred in the advanced stage of leaf senescence. Rates of both ethylene and C02 evolution increased in the ethylene-treated leaf discs, especially during the first 3 days of senescence.The rhizobitoxine analog, amnnoethoxy vinyl glycine, markedly inhibited ethylene production and reduced respiration and chlorophyll loss. Pretreat At appropriate time intervals gas samples were withdrawn with a hypodermic syringe for determination of ethylene and CO2 as previously described (6). After sampling, flasks were flushed with sterile fresh air, and when required, ethylene or CO2 was reintroduced.Chl was extracted from leaf discs with dimethylformamide (6), determined spectrophotometrically at 665 nm, and expressed in O.D. units.Number of replicates, leaf discs per replicate, and repeat experiments were as detailed previously (6). RESULTS AND DISCUSSIONThe rapid Chl breakdown phase in tobacco leaf discs senescing in darkness was accompanied by a rise and then a decline in both respiration and ethylene production (6). In fruit these phenomena characterize the climacteric stage and represent the onset of ripen- AHARONI AND LIEBERMAN ing (13, 28). Tests with leaf discs led us to conclude that these phenomena do not indicate the start of senescence in leaves but are associated with a stage rather late in the process. The aging process in morning glory flowers also commenced before ethylene production began (19). Applying exogenous ethylene to precimacteric fruit (27) or flowers (25) hastened the rise in ethylene production. However, treatment of tobacco leaf discs with 10 ,ul/ 1 ethylene for the first 24 h did not significantly hasten the climacteric-like rise in ethylene production (Fig. 1). Ethylene evolution was somewhat greater in ethylene-treated than in control leaf discs, and could represent either increased synthesis and/or release of the exogenous absorbed ethylene. Continuous treatment with AVG markedly inhibited ethylene synthesis during senescence (Fig. 1). The inhibition indicates that methionine is the principal precursor of ethylene biosynthesis in leaves (3, 4, 21) as in fruits (21) Further evidence of a regulatory role for ethylene in leaf senescence was obtained with Ag+, which has been reported to oppose effects of ethylene on growth, senescence, and abscission (9-11). Increasing concentrations of Ag+ from 0 to 20 mg/l completely nullified the ability of ethylene to enhance both Chl loss (Fig. 4) and rate of respiration (Fig. 5). The level of Ag+ that prevented Chl loss in ethylene-treated or ethylene and AVGtreated leaf discs was 20 mg/l or 10 mg/l, respectively. The senescence-retarding effect of AVG was not completely nullified by exogenous ethylene (Fig. 4), probably because tre...

No abstract

Changes in the patterns of ethylene production, chlorophyll content, and respiration were studied in relation to the senescence of intact leaves and leaf discs. The primary leaves of pinto bean, which abscise readily during natural senescence, and tobacco and sugar beet leaves, which do not abscise, were used. A decrease in the rate of ethylene production and respiration, during the slow phase of chlorophyl degradation, was observed in leaf-blade discs cut from mature leaves and aged in the dark. During rapid chlorophyl loss both ethylene production and respiration increased and then decreased. These climacteric-like patterns were shown by leaf discs of all three species. Discs taken from leaves that had been senescing on the plant also showed a climacteric-like rise in ethylene production but not in respiration, which decreased continuously with leaf age. Climactericlike patterns in the rise of ethylene and respiration for leaf discs were also shown by the petioles of both bean and tobacco leaves. This indicates that the rise of ethylene and respiration is characteristic of the general process of senescence in leaves and is not restricted to the abscission process. In contrast to the ethylene-forming systems in climacteric fruits and many flowers, the one in leaves declines sharply in the early stages of senescence. The subsequent rise of ethylene production appears to be associated with the rapid phase of chlorophyl breakdown, and may indicate the final stage of the senescence process during which ethylene could be actively involved in inducing leaf abscission.Ethylene is known to be involved with both Chl degradation in leaf blades and with the abscission process (9,24, 25). Beyer (6) demonstrated that the leaf blade is the primary receptor site for exogenous ethylene in leaf abscission. Aharoni (1) found that the principal site of ethylene production in response to a short period of water stress was also the blades rather than the petioles of leaves. McAfee and Morgan (21) found that ethylene levels in cotton leaf petioles were two to six times as high as those in leaf blades. Less attention was paid to the changing pattern of ethylene production by leaf blades during their senescence. Some studies in which ethylene production by the whole leaf was measured revealed a decrease in ethylene production with leaf age (12,13,20,23) climacteric in fruits. This study did not distinguish between the contribution of the tomato petiole and leaf blade to the ethylene production pattern observed. Since abscission is directly concerned with the petiole it seemed important to determine patterns of ethylene production in leaf blades and petioles in abscising and nonabscising leaves during senescence. In this study we examined patterns of ethylene production by senescing leaf blades and by leaf petioles with and without abscission zones. The capacity of leaves at various ages to produce ethylene in response to IAA and kinetin was also studied. MATERIALS AND METHODSTobacco (Nicotiana tabacum L. cv. Xanthi), pinto be...