The role of ethylene in kiwifruit softening

Premature softening limits storage life of kiwifruit at 0 degrees C. Exogenous ethylene induces rapid softening at 20 degrees C and at 0 degrees C and a concentration of 0.01 mu l/L will enhance softening at 0 degrees C. The influences of low temperature, an ethylene synthesis inhibitor (AVG) and application of ethylene at different maturities, were investigated to elucidate ethylene's role in initiating kiwifruit softening. Fruit response to ethylene became more pronounced and fruit softened more as maturity advanced. Exposing fruit to 0 degrees C for 2-9 weeks hastened ethylene production compared with fruit maintained at 20 degrees C continuously after harvest. However, fruit softening occurred without changes in ethylene production, ACC concentration or ACO activity. AVG treated fruit softened slightly more slowly and had lower ACC concentrations, ACO activity and ethylene production than control fruit, both at 20 degrees C and after 2 months at 0 degrees C. Kiwifruit softened from about 90 N to 12 N when endogenous ethylene production was low (below 0.2 mu l/kg/h) and constant; we suggest that this is System 1 ethylene production. Increased ethylene production (System 2?) only occurred as fruit softened from 12 N to eating ripe (6-8 N), suggesting that kiwifruit become more sensitive to ethylene with time during maturation and at 0 degrees C, possibly because ethylene receptors become more sensitive or more numerous.