Temperature and ethylene affect induction of rapid softening in ‘Granny Smith’ and ‘Pacific Rose™’ apple cultivars

Abstract

The occurrence of rapid softening in apples (Malus domestica Borkh.) at ambient temperatures (10–20 °C) varies among cultivars, as some cultivars require exposure to low temperatures before autocatalytic ethylene biosynthesis occurs. The influence of time at 0.5 °C on subsequent softening at 20 °C was investigated for the slow softening ‘Granny Smith’ and ‘Pacific Rose™’ apple cultivars. Ethylene (100 μl l−1 for 24 h) was also applied to apples at 20 °C to determine if ethylene treatment could replace the requirement for cold treatment. ‘Granny Smith’ fruit without ethylene or cold treatment softened slowly at 20 °C, while both ethylene- and cold-treated fruit softened more rapidly than non-treated fruit. Non-treated ‘Pacific Rose™’ fruit also softened slowly or not at all at 20 °C, but in contrast to ‘Granny Smith’, did not soften rapidly after either ethylene or cold treatment. The mechanism by which both cold and ethylene treatments initiated rapid softening in ‘Granny Smith’ may be facilitated by ethylene, as these treatments induced maximum internal ethylene concentrations (IEC) that were two to threefold greater than in non-treated fruit at 20 °C. However, rapid softening at 20 °C was often delayed relative to the increase in IEC from basal concentrations for cold-treated ‘Granny Smith’ fruit. This delay for cold-treated ‘Granny Smith’ fruit, and absence of rapid softening in ‘Pacific Rose™’ fruit at 20 °C despite a maximum IEC of more than 100 μl l−1, suggests that fruit sensitivity to ethylene may be more important than IEC for regulating softening. ‘Pacific Rose™’ may be a genotype of apple with reduced capacity for ethylene biosynthesis and action, and hence softening. The role of ethylene in softening of both cultivars at shelf life temperatures may be clarified by research on changes in ethylene sensitivity that occur during maturation and ripening.