Fruit are classified as climacteric or nonclimacteric, based on the presence or absence of a respiratory increase during ripening. The climacteric rise is associated with increases in internal concentrations of carbon dioxide and ethylene, and of respiration and autocatalytic ethylene production. For example, in apples, respiration and ethylene production may increase by 50 to 100 percent and 1,000-fold, respectively. Climacteric fruit also can be differentiated from nonclimac-teric fruit by their responses of respiration and/or ethylene production to exogenous ethylene or its analogues, such as propylene. In a climacteric fruit, ethylene advances the timing of the climacteric, autocatalytic production continues after removal of ethylene, and in contrast to a nonclimacteric fruit the magnitude of the respiratory rise is independent of the concentration of applied ethylene. Thus, timing of the respiratory increase and ripening of climacteric fruit is advanced by exposure to ethylene. The initiation of ripening and subsequent development of positive quality factors and negative storability factors are often associated with the climacteric. Most temperate tree fruit are climacteric, the major exception being the cherry. In the cherry, the respiration rate declines during growth and development and remains at low levels during ripening. Nonclimacteric fruit, in contrast to climacteric fruit, do not ripen after harvest.
As with any classification system, these categories are an oversimplification. Some fruit, such as nashi pears, have cultivars that are climacteric or nonclimacteric. Nonripening mutants of nectarines and other fruit have been identified, as have "suppressed climacteric" plums that do not produce sufficient ethylene to coordinate ripening but show characteristic responses to propylene. Also, differences in physiology can occur within climacteric fruit—early season apple cultivars, for example, tend to have much higher rates of ethylene production and respiration and ripen faster than late-season cultivars.
Fruit-ripening classifications are not measures of perishability, as evidenced by cherries that deteriorate quickly after harvest compared with apples that can have long storage lives. However, harvest of fruit before the climacteric and application of postharvest handling techniques, such as low-temperature and controlled atmosphere storage, reduce or eliminate the respiratory climacteric and generally reduce respiration rates. Low storage temperatures are the primary means of reducing metabolic rates, but the safe temperature range is influenced by susceptibility of the fruit to chilling injury. Most apple and pear cultivars are resistant to development of chilling injury, while stone fruit are more sensitive to injury.
The effects of temperature on ethylene metabolism are different from those on respiration. Low temperatures delay ethylene production in some apple cultivars but enhance it in others. Some pears require a chilling period to induce ethylene production and proper ripening.
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