Ripening of citrus and other non-climacteric fruits: A role for ethylene

Recent studies indicate that citrus and other non-climacteric fruits reveal changes in gene expression similar to those observed in climacteric fruits, justifying the use of the term 'ripening' with regard to both types of fruit. Non-climacteric fruits are not uniform. Some (citrus, grape) reveal ripening-related symptoms in response to exogenous ethylene (or ethephon), while others (strawberry, bell pepper) do not show any response to such treatments. As to the possible role of the low levels of endogenous ethylene in the ripening of non-climacteric fruits, experiments with inhibitors of ethylene biosynthesis or action are most meaningful. Such inhibitors do not interfere with the ripening of strawberry but the ripening of citrus fruit peel is strongly inhibited. Following the experimental evidence, a division of the non-climacteric fruits into two classes may be proposed: a) Fruits which do not show any response to ethylene. b) Fruits which lack a climacteric, autocatalytic rise in ethylene evolution but reveal ripening-related responses to exogenous ethylene. Furthermore, it may not be unreasonable to assume that endogenous ethylene plays a role in the ripening of the ethylene-responsive, non-climacteric fruits. Many of the ripening-related responses to ethylene are opposed by the senescencedelaying growth regulators, gibberellins and cytokinins. The mechanism underlying this antagonism is discussed in relation to the possible role of endogenous ethylene in the regulation of fruit ripening and senescence.