Debating the Meaning and Utility of the 'climacteric': Should Ethylene Be the Essential Property?

When the term ‘climacteric’ was first coined by Kidd and West in 1925, they referred to the increase in respiration that they had observed during the ripening of apple fruit.  Kidd and West (1933) subsequently showed that ethylene stimulated the climacteric in apples, and Gane (1934) shortly thereafter provided conclusive evidence that ripening apples produce ethylene.  Although numerous theories were put forth to explain the climacteric, the usefulness of the concept as a frame upon which to hang the physiological and biochemical changes associated with fruit ripening was immediately recognized.  It was assumed that climacteric respiration supplies the energy and carbon skeletons required for the various anabolic and catabolic processes associated with fruit ripening.  Soon a veritable cottage industry arose among fascinated postharvest scientists who categorized virtually all commercially important (and unimportant) fruit species as climacteric or nonclimacteric according to either the presence or absence of a respiratory rise during ripening and whether or not ripening occurs in response to ethylene exposure.  The response of climacteric respiration to temperature and atmosphere changes and the coordination of various physical and chemical changes with the climacteric have been thoroughly explored.  For many years following the initial description of the climacteric respiratory pattern, ethylene was widely considered to be a by-product of ripening since the relatively insensitive techniques then available could not detect ethylene until after the onset of ripening.  In the 1960s, that view began to wane as the use of gas chromatography showed clearly that very low levels of ethylene and increased tissue sensitivity to ethylene precede ripening.  It is now universally acknowledged that the primary feature distinguishing climacteric plant organs is their capacity for autocatalytic ethylene production.  Recent availability of 1-methylcyclopropane (1-MCP) to inhibit ethylene action has reinforced the concept of ethylene primacy.  The question before us, then, is to continue using the term ‘climacteric’ as a respiratory descriptor or to shift its meaning to refer to the behavior of, and plant responses to, the hormone ethylene.  As we debate this conceptual re-orientation, we should not lose sight of the fact that some three-quarters of the fruits and vegetables important in postharvest handling are not climacteric in nature; for most crops, control of global metabolism, for which respiratory activity is the most useful indicator, remains the postharvest scientist’s primary strategy for maintaining quality.