Abstracts Biological Sciences

The role of ethylene in kiwifruit softening

by Hyun Ok Kim

Premature fruit softening during storage at 0°C is a serious and costly problem for the New Zealand kiwifruit industry. Ethylene gas (C2H4) is a potent promoter of fruit softening; it is involved in regulation of fruit ripening and influences a number of processes, including ethylene production, respiration rate and fruit softening. Exogenous ethylene increases softening rate in kiwifruit at 20°C and at 0°C; a concentration of 0.01µl/1 will enhance softening at 0°C. The precise relationship between kiwifruit softening and endogenous ethylene concentration is not known. The influences of low temperature, an ethylene synthesis inhibitor (aminovinylglycine (AVG)), an inhibitor of ethylene action (1-methylcyclopropene (1-MCP)) and application of ethylene at different maturities were investigated in an attempt to elucidate ethylene's role in initiating kiwifruit softening. Exposing fruit to 0°C for more than 52 days hastened ethylene production upon removal to 20°C compared with fruit maintained at 20°C continuously after harvest; this enhanced ethylene was associated with increased 1-aminocyclopropane-1-carboxylic acid (ACC) concentration and ACC oxidase (ACO) activity. Kiwifruit softening at 0°C occurred even though ethylene production was low and constant between 0.02 to 0.06µl/kg/h (corresponding to internal ethylene concentrations (IEC) 0.2 to 0.6µl/1). This softening was associated with low ACC concentrations varying between 0.2 to 0.5nmole/g and ACO activity varying between 0.01 to 0.66nl/g/h These results indicate that very low concentrations of ethylene may play an essential role in kiwifruit softening. In kiwifruit treated with AVG (500ppm) 4 weeks before harvest, ethylene biosynthesis, manifested as reduced ACC concentration, ACO activity and ethylene production, was significantly inhibited both at 20°C and after storage at 0°C. AVG application resulted in a slower softening rate and firmer fruit than in untreated controls. Rates of softening were 0.4N/day and 1.9N/day in the AVG treated fruit and control fruit respectively during 14 days at 20°C. However, the AVG effect was reduced after storage at 0°C for 14 days, indicating that the AVG effect was only temporary and may not be sufficient to warrant possible commercial use for longer storage. Application of 250ppm AVG 2 or 4 weeks before harvest, and 500ppm AVG 2 weeks before harvest had no effect on ethylene production or fruit firmness. Fruit infected with B. cinerea produced more ethylene than non-infected fruit at 0°, 4°, 10° and 20°C. An increase in ethylene production, induced by B. cinerea infection, occurred in tissue slices from the invasion zone (the infection front containing both infected tissue and sound tissue immediately ahead of the infection front) and adjacent zone (sound tissue ahead of the invasion zone) of kiwifruit. The increased ethylene in infected fruit was associated with increased ACO activity in tissue from adjacent and distal (sound tissue at the distal end of the fruit) zones. Application of 500ppm AVG to kiwifruit vines before…