Effects of Postharvest Treatment with 1-Naphthaleneacetic Acid on Chlorophyll and Carotenoid Metabolism in Citrus Fruit

Abstract

‘Miyagawa-wase’ (Citrus unshiu Marcow.), an early-season citrus variety, is rich in multiple nutrients and widely consumed in Japan. In ‘Miyagawa-wase’, when the pulp reaches maturity and is ready to eat, the peel is still a greenish color because of the relatively high temperature in the harvest season. In this study, to improve the coloration of ‘Miyagawa-wase’ peel, we treated the fruit with 1-naphthaleneacetic acid (NAA) after harvest. The results showed that postharvest treatment with NAA was effective to induce chlorophyll degradation and carotenoid accumulation in ‘Miyagawa-wase’ peel during storage. In the NAA treatment, the reduction in the chlorophyll contents after harvest was in parallel with decreases in the expression of chlorophyll biosynthetic genes (CitGGDR, CitCHLH, CitCHL27, CitPORA, and CitCAO) and an increase in chlorophyll degradation gene CitPPH. The contents of the major carotenoids, β-cryptoxanthin and 9-cis-violaxanthin, were increased by the NAA treatment through upregulation of the expression of carotenoid biosynthetic genes (CitPSY, CitPDS, CitZDS, CitLCYb2, and CitHYb) after harvest. In addition, it was found that the positive effect on degreening in the NAA treatment was inhibited by the ethylene antagonist 1-MCP. In the combination treatment using NAA and 1-MCP, the total chlorophyll content was much higher, while the contents of β-cryptoxanthin and 9-cis-violaxanthin were lower compared with NAA treatment alone, indicating that the acceleration of degreening by NAA may be caused by ethylene. The results presented in this study suggest that postharvest NAA treatment is an effective method for improving the peel coloration in early-season citrus varieties.