Ethylene as a Regulator of Senescence in Tobacco Leaf Discs

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 Pretreat At appropriate time intervals gas samples were withdrawn with a hypodermic syringe for determination of ethylene and CO2 as previously described (6).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...

Bicarbonate/CO2-Facilitated Conversion of 1-Amino-cyclopropane-1-carboxylic Acid to Ethylene in Model Systems and Intact Tissues

“…Increased production of ethylene in response to enhanced levels of CO2 has been demonstrated for detached leaves and leaf discs of cocklebur (15), intact sunflower plants (6,12), tobacco leaf discs (1,2,18), and detached senescing oat leaves (14). The effect is obtained when CO2 is supplied either directly as a gas or indirectly as bicarbonate, and no increase in ethylene production ' is obtained when CO2 is removed with a KOH trap (15,18).…”

Effects of Carbon Dioxide on Ethylene Production and Action in Intact Sunflower Plants

“…For example, carbon dioxide stimulated ethylene production of the senescing tobacco leaf discs (2). Similarly, when CO2 was removed from the atmosphere with KOH, ethylene production by infected sweet potato roots decreased (1 1).…”

Production and Action of Ethylene in Senescing Leaf Discs

“…Ethylene plays a considerable role in regulation of fruit ripening (21) and senescence of flowers (15) and leaves (2). Generally, the senescence-enhancing effect of ethylene is opposed by effects of other growth hormones-auxins, cytokinins, and gibberellins.…”