Comparative analysis of fruit ripening and associated genes in two kiwifruit cultivars ('Sanuki Gold' and 'Hayward') at various storage temperatures

Kiwifruit exhibit a peculiar ripening pattern, as extensive softening is known to occur in the absence of any detectable ethylene. We previously demonstrated that this softening is regulated by low temperature independent of ethylene. However, there are no reports that provide comparisons of the ripening patterns among different kiwifruit cultivars at various storage temperatures. The purpose of this study was to compare the ripening responses and associated gene expression in 'Sanuki Gold' (Actinidia chinensis var. chinensis) and 'Hayward' ((Actinidia chinensis var. deliciosa) fruit, two kiwifruit cultivars differing in on-vine maturity dates and postharvest storability, during storage at 5 °C, 10 °C, 15 °C and 22 °C. Fruit softening, soluble solids concentration (SSC) increase and reduction of titratable acidity (TA) occurred in the absence of any detectable ethylene, and treatment with an ethylene inhibitor 1-methylcyclopropene (1-MCP) failed to suppress the changes, suggesting that they were independent of ethylene. 'Sanuki Gold' fruit showed a higher sensitivity to low temperature supported by accelerated fruit softening and TA reduction, and induction of several genes such as AcACO3, AcXET2, AcPG, AcEXP1, AcPMEi, AcGA2ox1, AcMADS2, AcNAC5 and AcbZIP2 at 5 °C, 10 °C and 15 °C within 28 d. By contrast, 'Hayward' fruit exhibited a lower sensitivity to low temperature as accelerated softening, TA reduction and induction of most ripening-associated genes were recorded only at 5 °C and 10 °C. These differences in sensitivity to low temperature, between 'Sanuki Gold' and 'Hayward' fruit, would account for the dissimilarities observed in on-vine maturity dates and postharvest storability.