Pre-harvest nickel application to the calyx of ‘Saijo’ persimmon fruit prolongs postharvest shelf-life

Since nickel ions (Ni2+) have an inhibitory effect on 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase by replacing iron (Fe2+) and forming an enzyme-metal complex. 0.1% (w/v) NiCl2 solution was sprayed onto the calyx of persimmon fruit (Diospyros kaki Thunb. cv Saijo) twice before harvest. Ni2+-treatment effectively inhibited ‘Saijo’ fruit softening, and prolonged the fruit shelf-life for 2 days by delaying ACC accumulation and ethylene production. However, non-treated control fruit rapidly softened on the fifth day due to the immediate increase in ACC levels and ethylene production after the removal of astringency. Relatively firm Ni2+-treated fruit were observed on day 7, at which point the control fruit had completely softened. Northern blots indicated that only slight DK-ACS1 transcription could be observed on day 6 in Ni2+-treated fruit, whereas abundant DK-ACS1 mRNA was transcribed on day 5 in non-treated fruit. Correspondingly, ACC synthase activity was delayed with the Ni2+-treatment, whereas it had little effect on ACC oxidase activity. We propose that Ni2+ could be exploited as an effective new reagent to prolong persimmon shelf-life via pre-harvest treatments.