Production and Action of Ethylene in Senescing Leaf Discs

Abstract

Supraoptimal concentrations of indoleacetic acid (IAA) stimulated ethylene production, which in turn appeared to oppose the senescence-retarding effect of IAA in tobacco leaf discs. Kinetin acted synergistically with IAA in stimulating ethylene production, but it inhibited senescence. Silver ion and CO2, which are believed to block ethylene binding to its receptor sites, delayed senescence in terms of chlorophyll loss and stimulated ethylene production. Both 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. These hormones, alone or in combination, can, in several instances, induce ethylene production (18).CO2 and Ag+ oppose the effect of ethylene, presumably by blocking ethylene action at its receptor sites (6-8, 10). A previous study showed that the senescence-retarding effect of Ag+ and CO2 was associated with stimulation of ethylene production during early stages of senescence (2). In the present study we probed interactions between Ag+, C02, kinetin, and IAA, as they relate to ethylene biosynthesis and action in senescing tobacco leaf discs. To prevent bacterial contamination, penicillin and streptomycin were added (3). The flasks were sealed with rubber serum caps and incubated in darkness at 28 C. When required, appropriate amounts of ethylene or CO2 were injected into the flasks. Accumulation of CO2 evolved by leaf discs was avoided by absorption in KOH (2). Gas samples were withdrawn with a hypodermic syringe for determination of ethylene, as previously described (3). After sampling, the flasks were flushed with sterile fresh air and,. when required, ethylene and CO2 were reintroduced. MATERIALS AND METHODSFor tracer studies 0.75 or 1.50 ,tCi of L-[3,4-'4C]methionine (53 mCi/mmnol) was added to 10-ml Erlenmeyer flasks, each containing 1 ml of test solution and six leaf discs. The labeled ethylene from methionine was transferred via an argyl extension tube into an evacuated 70-ml jar (12) containing the bottom two-thirds of a plastic miniscintillation vial. Ethylene was trapped in 2 ml of 0.1 M mercuric acetate in methanol contained in the plastic vial. By this method, during a 3-h period, over 95% of the ethylene in the incubation flasks was transferred to mercuric acetate. After transfer the plastic vial was placed in a glass scintillation vial to which 10 ml of Aquasol scintillation fluid was added. In experiments where exogenous C2H4 was added (e.g. Table III), measurements were made to confirm total absorption of C2H4 in mercuric acetate before the jars were opened.Chl was extracted from leaf discs with dimethylformamide (3) and determined spectrophotometrically at 665 nm. Concentrations are expressed in optical density (O.D.) units.Treatments within each experiment were tested in triplicate flasks. The standard errors for both ethylene production and Chl content were generally in the ...

Auxin-Induced Ethylene Production as Related to Auxin Metabolism in Leaf Discs of Tobacco and Sugar Beet

“…This assumption is supported by the data showing that AgC, an antagonist of ethylene, could stimulate ethylene production in IAAtreated leaf discs (Fig. 5), probably because this agent could overcome the autoinhibitory effect of ethylene (3). The existence of an autoinhibitory mechanism in the ethylene biosynthesis pathway in the sugar beet system also can be demonstrated by the effect of propylene, an analog of ethylene.…”

Ethylene as a Regulator of Senescence in Tobacco Leaf Discs

“…Combination treatment of CO2 with Ag+ resulted in a strong synergistic effect on both ethylene production and retention of Chl ( Fig. 8 and Table I (5). The conclusion that the climacteric-like rise of ethylene production in senescent leaf discs could not have triggered leaf senescence was derived from previous studies (4, 6, 7), which showed that this rise occurs considerably after the commencement of Chl loss.…”

“…Concentrations above 50 mg/l AgNO3 markedly increased ethylene production and rates of respiration and yellowing; it also severely damaged the leaf discs, blackening the cut surface (data not shown). Apparently, induction of ethylene synthesis caused by relatively low concentrations of AgNO3 and the decrease in respiration are evidence not of Ag+ toxicity but of the antiethylene property of Ag+ (5).…”

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).…”