11.5.2 Climacteric behaviour
Ethylene production is closely associated with ripening of many fruits. Typically, fruit will generate barely detectable amounts until ripening when there is a burst of production. Fruit fall more or less into two classes of behaviour (Table 11.2) with respect to ethylene physiology. In type 1, as fruit progress towards edibility, respiratory rate increases followed by a decline as fruit senesce. Pear, banana and avocado (Figure 11.13) show an especially strong response. Ethylene pro-duction also increases sharply to a maximum at this time, and then declines before fruit rots intervene and lead to a renewed output. The major rise in ethylene production may take place before, just after or close to the respiratory peak. Such fruit are classed as climacteric, with apple, avocado, banana, ﬁg, mango, papaya, passionfruit, pear and tomato being classic examples. Climacteric fruit ripen after harvest, and need not remain on the tree or vine. Type 2 fruit, exempliﬁed by blueberry, cherry, citrus, cucumber, grape, pineapple and strawberry (Table 11.2) do not show such sharp changes. Respiration rate either remains unchanged or shows a steady decline until senescence intervenes, with no increase in ethylene production; these are called non-climacteric fruits. Paradoxically, both unripe climacteric and non-climacteric fruit do increase their respiration rate when exposed to exogenous ethylene.
All fruits were once classiﬁed under this either/or nomenclature but many variations between the two types became apparent, so the original classiﬁcation is better seen as two extremes of a continuum. For example, kiwifruit progress through most of the ripening changes in the absence of any rise in ethylene and CO2 production; this occurs only towards the end of ripening and softening as fruit are undergoing middle lamella dissolution. Ripening is no longer perceived as always being driven by ethylene production or by a rise in respiration.