Ellen A. Burg

Ethylene Action and the Ripening of Fruits: Ethylene influences the growth and development of plants and is the hormone which initiates fruit ripening

Recent studies employing gas chromatography show that an amount of ethylene large enough to stimulate ripening is always present within a fruit before the respiratory climacteric begins. This fact and data from experiments in which fruits were exposed to a partial vacuum or varying concentrations of O2, CO2, and ethylene oxide reinforces the view that ethylene is a ripening hormone. The respiratory climacteric begins soon after the fruit is harvested because the tissue no longer receives from the shoot system a substance which inhibits ripening; this substance may act by lowering the sensitivity of the fruit to ethylene. The threshold for ethylene action is also influenced by the composition of the atmosphere, for O2 is a substrate in the reaction activated by ethylene and CO2 inhibits the action of ethylene by competing with the olefin for the receptor site. Experiments indicate that ethylene is derived from acetate or acids of the Krebs cycle and acts by binding to a metal receptor site in the tissue.

Fruit Storage at Subatmospheric Pressures

The storage life of bananas and other fruits is prolonged by ventilating them with air at less than atmospheric pressure. This procedure accelerates the escape of the ripening hormone ethylene from the tissue; by reducing the oxygen tension it also lowers the fruits sensitivity to the hormone.

Relationship between Ethylene Production and Ripening in Bananas

The ethylene content of bananas (Musa sapientum L., var. Silk fig) is constant throughout their growth and development until ripening commences, when an abrupt increase in ethylene synthesis precedes the onset of the climacteric rise in respiration. This accelerated production rate may be required to raise the internal content of ethylene to a stimulatory level, or it may be an autocatalytic response initiated when the tissue becomes sensitive to the low level of ethylene present throughout its developmental period. Following harvest, the time required to respond to a low concentration of applied ethylene decreases steadily as the fruit ages until, just prior to the onset of natural ripening, the fruit becomes able to react rapidly to a quantity of gas closely similar to what it contained throughout the preclimacteric period.

Interaction of ethylene, oxygen and carbon dioxide in the 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.