Low-temperature, nonfreezing, storage induces pitting and necrosis in the flavedo tissue of chilling susceptible citrus fruits. In this study the role of ethylene and phenylalanine ammonia-lyase (PAL; EC 4.3.1.5) in the cold-induced citrus peel damage has been investigated. It has been shown that increasing PAL activity by applying ethylene at a nonchilling temperature did not cause fruit damage or reduce the incidence of this peel disorder when fruits were subsequently held at a chilling temperature (2 degrees C). The cold-induced peel damage was enhanced by applying inhibitors of PAL activity and ethylene synthesis and action. These results indicate that the induction of PAL and ethylene during fruit cold storage, but not before, plays a role in reducing the development of chilling symptoms. The cold-induced PAL activity was reduced by inhibitors of ethylene production, but inhibitors of ethylene action exerted little effect on the activation of this enzyme. Therefore, the activation of PAL may be dependent on ethylene but also an independent cold signal apparently related to the cold-induced peel damage.
1-Methy1cyclopropene (l-MCP), formerly designated as Sis-X, has been shown to be an effective inhibitor of ethylene responses in carnation flowers in either the light or the dark. The binding appears to be to the receptor and to be "permanent". A 6 h treatment at 2.5 nll-1 is sufficient to protect against ethylene, and 0.5 nll-1 is sufficient if exposure is for 24 h. As carnation flowers age, a little higher concentration appears to be needed. Most of the natural increase in ethylene production during senescence is prevented by treatment with 1-MCP. A closely related compound, methylenecyclopropane shows ethylene activity. A tritium labelled 1-MCP (60 mCi mmol-1 ) has been prepared. A higher specific activity is needed for more critical studies.
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