INTERACTION OF ETHYLENE, OXYGEN AND CARBON DIOXIDE IN CONTROL OF FRUIT RIPENING

Ethylene production during the climacteric in many fruits, in vegetative tissue treated with IAA, and in flowers which have been emasculated, pollinated or treated with IAA applied to their stigma, increases rapidly and then returns to a low rate. In root sections exposed to IAA this timing apparently is due to the fact that the rate of ethylene production reflects the internal IAA content; the latter increases initially after auxin is applied but rapidly decreases again due to induction or activation of the enzyme systems conjugating and destroying IAA. A similar mechanism may be involved in the induction of ethylene production in fruits. In vivo ethylene is derived from methionine, possibly after the amino acid is activated to form S-adenosyl methionine, by decarboxylation of C1, transfer of C2 possibly as a folic acid derivative, formation of ethylene from C3-C4, and transfer of the S-methyl to a suitable receptor molecule. Analogue studies indicate that ethylene attaches (KA=6×10-10M) to a metal containing receptor by one end through a non-covalent bond. This attachment is competitively inhibited by CO2 (KI=4.9×10-4M) thus explaining the ability of this gas to antagonize most biological responses to ethylene, including fruit ripening. CO2 does not inhibit ethylene production, but may influence fruit ripening and induce certain physiological disorders by binding to metal containing enzymes such as catalase. Ethylene production is inhibited at low O2 concentrations; the receptor has an O2 affinity closely similar to that of cytochrome oxidase. In addition O2 is required for ethylene action, and the kinetics best describing the situation are those in which O2 binds to the receptor (KS=4×10-5M) or indirectly oxidizes it before ethylene can attach. This effect of low pO2 is not dependent upon a respiratory inhibition for it occurs at O2 concentrations which are not low enough to inhibit respiration and cannot be duplicated by respiratory poisons. Thus low concentrations of O2 retard fruit ripening both by inhibiting ethylene production and action. The rate of gas exchange and hence the internal concentration of ethylene within a fruit depends upon the diffusion coefficient of ethylene in air. As this is a function of atmospheric pressure, storing fruits at a subatmospheric pressure reduces their ethylene content. In addition this condition automatically decreases the O2 partial pressure thus greatly extending the storage life of preclimacteric fruits. © 1969 Dr. W. Junk N. V.