Determination of optimum temperature for long-term storage and analysis of ripening-related genes in 'Rainbow Red' kiwifruit

Kiwifruit is considered a climacteric fruit since exogenous ethylene induces ripening-associated changes. However, we previously reported that low temperature modulated ripening in kiwifruit since ripening occurred faster during storage at 5°C compared to 20°C, in the absence of any detectable ethylene. It is therefore not clear which temperature is suitable for long-term storage of kiwifruit. The purpose of this study was to determine the optimum temperature for long-term storage, and analysis of ripening-related genes in 'Rainbow Red' kiwifruit. Kiwifruit were harvested at commercial maturity, and stored at either 0, 2, 5 or 22°C in ethylene-free chambers. During storage, incidence of fruit deterioration and changes in fruit firmness, soluble solids concentration (SSC) and titratable acidity (TA) were monitored at 4-week intervals. Real-time PCR was also conducted to analyze the changes in expression of selected ripening-related genes. Incidence of fruit fast senescence and deterioration was high at 22°C, so kiwifruit could only be stored for a maximum of 8 weeks. Nevertheless, healthy fruit at 22°C remained firm and maintained high TA for 8 weeks. The incidence of ethylene-producing fruit, and consequent fast senescence, was greatly reduced at 5°C, while it was completely suppressed at 0 and 2°C. Fruit at 5°C decreased in firmness and TA faster, attaining eating-ripe quality within 8 weeks. At 2°C, fruit decreased in firmness and TA gradually, achieving eating-ripe quality after 12 weeks. Conversely, fruit at 0°C maintained higher firmness and TA, and did not attain eating-ripe quality even after 12 weeks. SSC increased at all storage temperatures, although the lowest levels were observed in fruit at 0°C. Real-Time PCR analysis revealed that cell-wall-modifying genes (AcPG, AcPL2, AcEXP1 and AcXET2) and carbohydrate-metabolism-associated genes (Acβ-AMY1 and Acβ-AMY2) were markedly induced in fruit at 5°C. Overall, these results indicate that 0°C is the suitable temperature for long-term storage of kiwifruit, while 2°C is suitable for medium-term storage. For short-term storage, 5°C can be recommended.