Ethylene and the Respiration Climacteric

Abstract

MANY theories have been proposed to explain the nature of the climacteric rise in respiration seen in most ripening fruits. They include changes in tissue organization which lead to increased metabolism¹, or effects of mitochondria which cause a loss of respiratory control². The role of ethylene in the induction of ripening has now been confirmed but, in spite of much attention, the physiological nature and significance of the climacteric have remained elusive³. Although fruit tissues undergo many changes during ripening⁴, much biochemical organization is retained; mitochondria do not lose their respiratory control² and protein synthesis continues up to the climacteric maximum^5–7. Ripening has therefore been interpreted as a process requiring considerable cellular work, so that the climacteric is merely the respiratory summation of cellular energy requirements³. In many fruits, however, the energy requirements for the largely catabolic events of ripening must be very small. In the banana, for example, which shows a classical respiration climacteric, starch makes up as much as 20 per cent of the fresh weight before ripening⁸. During ripening, the starch is almost completely hydrolysed by phosphorylase to sugars (an exergonic reaction). Moreover, it has been shown that the respiratory climacteric can occur in the absence of protein synthesis⁶.