Autar K. Mattoo, Morris Lieberman 

Several chemicals were used to probe the in situ ethylene forming enzyme systems in apple tissue and Penicillium digilatum. 2,4-Dinitrofluorobenzene, a membrane permeant probe, inhibited ethylene production effectively in apples but far less effectively in P. digitatum. In contrast, salicylaldehyde, another membrane permeant probe, effectively inhibited the P. digitatum system but, except at 0.1 mM concentration, little influenced the apple system. l,5-Difluoro-2,4-dinitrobenzene (DFDNB), a membrane permeant probe which cross-links proteins with proteins and with phospholipids, strongly inhibited ethylene biosynthesis in both apple and P. digitatum, whereas dimethyl suberimidate, the protein cross-linking reagent, inhibited slightly the apple system but not P. digitatum system. Picrylsulfonate (TNBS), a non-permeant membrane probe, up to 0.1 mM, did not inhibit any of the two systems studied. However, in the presence of exogenous methionine in the apple system and glutamate in P. digitatum, TNBS at 0.1 and 1 mM caused inhibition of ethylene synthesis. These probes did not affect respiration of apple slices under similar incubating conditions, excepting for DFDNB which on longer incubation did inhibit respiration, but the effect on ethylene synthesis was 15 times greater. Divalent cation ionophores, A23187 and X537 A, had no effect on ethylene synthesis in both the systems. The water soluble iron chelating agent, o-phenanthroline, was a more potent inhibitor of the apple system but minimally affected P. digitatum. In contrast, the lipophilic chelator, bathophenanthroline, was a more potent inhibitor of the P. digitatum system. Assay of the fatty acid composition of polar lipids from crude membrane fractions showed considerably greater linoleic to linolenic ratio in P. digitatum than in apple. We suggest that the ethylene formations in apple and P. digitatum are sensitive to a modification of membrane structure and that specific chelator-sensitive metals (perhaps iron and copper) are involved in ethylene synthesis in both these systems.