Effect of 1-methylcyclopropene and methylenecyclopropane on ethylene binding and ethylene action on cut carnations

Tobacco leaves were exposed to (14)C-labeled ethylene (3.7 x 10(-2) microliters per liter) in the presence and absence of unlabeled ethylene and other compounds. Most of the [(14)C]ethylene appears to be bound to displaceable sites. Lineweaver-Burk plots for a one-half maximum response in a tobacco leaf respiration test gave a value of 0.3 microliter per liter for ethylene, 50 microliters per liter for propylene, and 266 microliters per liter for carbon monoxide. Scatchard plots for displacement of [(14)C]ethylene from the site gave 0.27 microliters per liter for ethylene, 42 microliters per liter for propylene, and 746 microliters per liter for carbon monoxide. At 2%, CO(2) displaces about 35% of the bound ethylene, but increasing the concentration to 10% does not displace the remaining [(14)C]ethylene. A value of 3.5 nanomolar was calculated for the concentration of ethylene-binding sites available to exogenous ethylene. This does not account for the sites occupied by endogenous ethylene, and the total number of binding sites is probably somewhat higher. Using tissue culture material, the system was shown to be stable to freezing and thawing; and the pi-acceptors, carbon monoxide, cyanide, n-butyl isocyanide, phosphorous trifluoride, and tetrafluoroethylene, were shown to compete with ethylene for binding.

The inhibitory effects of the cyclic olefin 2,5-norbornadiene (NBD) on ethylene action were tested in carnation (Dianthus caryophyllus L. cv White Sim) flowers. Treatment of flowers at anthesis with ethylene in the presence of 500 microliters per liter NBD increased the concentration of ethylene required to elicit a response (petal senescence), indicating that NBD behaves as a competitive inhibitor of ethylene action. Transfer of flowers producing autocatalytic ethylene and exhibiting evidence of senescence (petal in-rolling) to an atmosphere of NBD resulted in a rapid reduction in ethylene production, petal 1-aminocyclopropane-1-carboxylic acid synthase activity, 1-aminocyclopropane-1-carboxylic acid content, and ethylene forming enzyme activity. Removal of NBD resulted in recovery of ethylene biosynthesis. These results support the autocatalytic regulation of ethylene production during the climacteric stage of petal senescence and suggest that continued perception of ethylene is required for maintenance of ethylene biosynthesis. The inhibition of ethylene action by NBD after the flowers had reached the climacteric peak was associated with interruption of petal senescence as evidenced by reversal of senescence symptoms. This result is in contrast to the widely held belief that the rate of petal senescence is fixed and irreversible once petals enter into the ethylene climacteric.