Ag(I), applied foliarly as AgNO3, effectively blocked the ability of exogenously applied ethylene to elicit the classical "triple" response in intact etiolated peas (Pisum sativum cv. Alaska); stimulate leaf, flower, and fruit abscission in cotton (Gossypium hirsutum cv. Stonevilie 213); and induce senescence of orchids (Hybrid white Cattleya, Louise Georgeianna). This property of Ag(I) surpasses that of the well known ethylene antagonist, C02, and its persistence, specificity, and lack of phytotoxicity at effective concentrations should prove useful in defining further the role of ethylene in plant growth.Ethylene is widely recognized as an important ubiquitous plant hormone involved in many developmental processes including fruit ripening, abscission, senescence, growth, flowering, and sex expression (1). However, the initial biochemical event triggered by ethylene which ultimately causes these diverse developmental changes is unknown. During studies (3, 5, 6) aimed at better understanding this key event, Ag(I) was found to block ethylene action effectively. This property of Ag(I) surpasses that of the well known ethylene antagonist, CO2 (1), and therefore, should provide a new experimental tool for studying ethylene action and defining further its role in plant growth. Here, the author reports the ability of Ag(I) to block specifically the action of exogenously applied ethylene in such classical ethylene responses as abscission, senescence, and growth retardation. MATERIALS AND METHODSThe general experimental approach was to treat foliarly intact pea, cotton, and orchid plants with various concentrations of AgNO3 and observe the protection afforded by such treatments following a given ethylene exposure. Although silver nitrate was the preferred salt, because of its water solubility and immediate laboratory availability, other salts such as silver acetate and silver lactate have also been found to be effective. A variety of other metal ions including Ni, Hg, Co, Cd, Cu, Pd, Pt, Rh, Zn, and Ru were evaluated for their ability to block ethylene action, but none were effective when applied foliarly to intact plants with the exception of palladium (K2PdCl4) and mercury (HgNO3 and Hg(NO3)2, whose effects were marginal.Alaska peas were planted in vermiculite in 10-cm plastic pots, watered with distilled H20, and grown for 4 days in a dark room at a constant 23 C and 75% relative humidity. treatment, control (solvent-treated) and treated pots were placed in two dark chambers. One chamber was purged with air at 15 liters/min, while the other was purged at the same flow rate with 0.25 pl/liter of ethylene.Cotton plants (Gossypium hirsutum cv. Stoneville 213), were grown in an environmental growth room as previously described (4), and at 5 weeks of age, they were treated with aqueous solutions containing 0, 25, 50, 100, or 200 mg/liter of AgNO3 plus 0.01 % Tween 20 as a surfactant. The following day, treated and control plants were placed in two large chambers (4). One chamber was purged with 12 Al/liter of ethy...

The relationship between ethylene action and metabolism was investigated in the etiolated pea seedling (Pisum sativum L. cv Coliectively these data suggest that the metabolism of ethylene may be related to its action.The early work with 1'C-labeled ethylene resulted in the general view that ethylene undergoes no permanent chemical change either before, during, or after it accomplishes its biological function (1,2,20,22). More recent work with highly purified 14C2H4(5) applied to plant tissues under aseptic conditions (3, 4) has made it necessary to change this view since an active ethylene metabolic system has been found in several plant tissues including etiolated pea seedlings (3, 4, 17), carnation (10), and morning glory flowers (12), cotton and bean abscission explants (6) tomato fruit (unpublished data). This metabolic system exhibits similar characteristics in all tissues so far examined and leads to the incorporation of ethylene into water-soluble tissue metabolites and the oxidation of ethylene to CO2. The activity of this system and the rate of tissue incorporation relative to oxidation depend on the stage of development (3, 10, 12) and are influenced by such factors as temperature, homogenization, seedling fractionation, 02, C02, COS, and CS2 (4, 10). Propylene (11) and acetylene (unpublished data) are also metabolized by pea seedlings but at substantially different rates than ethylene. Furthermore, the neutral propylene metabolites formed are chromatographically distinct from those derived from ethylene (1 1).Based on this work with purified "C2H4 it was suggested (4) that ethylene metabolism is an integral part of the ethylene action mechanism. Specifically, the constant metabolic turnover of ethylene in terms of oxidation and/or incorporation at a Cu+-containing receptor site(s) was proposed as the initial biochemical event in the ethylene action sequence. Ag+ (7-9), high CO2 (2,14), and low 02 (2,14,15,19)

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The role of ethylene-mediated reduction of auxin transport in natural and ethylene-induced leaf abscission was studied in the cotton (Gossypium hirsutum L., cv. Stoneville 213) cotyledonary leaf system. The threshold level of ethylene required to cause abscission of intact leaves was between 0.08 and 1 u/l with abscission generally occurring 12 to 24 hours following ethylene fumigation. The threshold level of ethylene required to reduce the auxin transport capacity in the cotyledonary petiole paralleled that required for stimulation of abscission. In plants where cotyledons are allowed to senesce naturally there is a decline in auxin transport capacity of petioles and increase in ethylene synthesis of cotyledons. The visible senescence process which precedes abscission requires up to 11 days, and increases in ethylene production rates and internal levels were detected well before abscission. Ethylene production rates for entire cotyledons rose to 2.5 m,ul g-' hr1and internal levels of 0.7 Il/l were observed. These levels appear to be high enough to cause the observed decline in auxin transport capacity. These findings, along with those of others, indicate that ethylene has several roles in abscission control (e.g., transport modification, enzyme induction, enzyme secretion). The data indicate that ethylene modification of auxin transport participates in both natural abscission and abscission hastened by exogenous ethvlene.The ability of ethylene to inhibit auxin transport is well documented (9-11, 16, 24, 39, 40, 46). Equally well documented (see " Reviews" 17,18,29,31,48) is the ability of auxin to retard or prevent abscission. The amount of auxin reaching the abscission zone and the distribution of auxin both proximal and distal to the abscission zone appear to play an important role in this preventive action of auxin (8,17,29,48). In view of the ability of ethylene to reduce the basipetal auxin transport capacity and to function as a potent accelerator of abscission (see "Reviews" 13, 47, 50), the gas may function indirectly to regulate abscission through its effect on basipetal auxin transport. Morgan and Gausman (40) first suggested such a role for ethylene in abscission, and this idea has been supported by others (14, 16 (9-11, 16, 24, 39, 40, 46) and synthesis (52) No evidence has yet been provided, however, which establishes a causal relationship between abscission and any of these auxin-ethylene interactions.This paper presents a series of experiments designed to test the hypothesis that either endogenous or exogenously applied ethylene influences or controls abscission, in part, through its effect on basipetal auxin transport. MATERIALS AND METHODSPlant Culture. Cotton plants (Gossypiutm hirsutum L., var. Stoneville 213) were grown in a greenhouse or in a controlled environment room (2000 ft-c; 15-hr photoperiod; relative humidity 56 + 5% day, 60 ± 5% night; temperature 29.4 ± 1 C day, 23.9 + 1 C night). Plants were watered with a modified Hoagland's solution (41) and were grown in 15.2-cm pl...