Postharvest NAA treatment seems an effective method for improving the peel coloration in early-season citrus varieties

In citrus fruit, peel color turns from green to orange?gradually during the ripening process. The color development in citrus fruit is closely associated with the?seasonal temperature. It is well known that low?temperature is a key factor to stimulate color break in?citrus fruit peel (Alqu?zar et al., 2008; Rodrigo et al.,?2013), while high temperature inhibits peel color development during the ripening process. In general, the?color development in citrus fruit requires a night time?temperature below 13?C and a large difference in temperature between day and night (Mitalo et al., 2020).?However, in some early-season citrus varieties, the temperature does not drop low enough for peel color development in the harvest season, and the peel is often a?greenish color when the pulp reaches maturity. The peel?color is the main index of fruit quality, and greenish peel color affects the acceptance of citrus fruit by consumers. Thus, postharvest degreening treatment is of?great importance for early-season citrus varieties to?improve coloration and gain more acceptance for marketing. In most citrus varieties, chlorophyll and carotenoid are the major pigments determining the fruit color. During the ripening process, the rapid loss of chlorophylls?and massive accumulation of carotenoids lead to fruit?degreening (Rodrigo et al., 2013). How degreening occursIn the past decades,?the metabolism of chlorophyll and carotenoids have?been extensively investigated in citrus fruit, and the key?enzymes involved in chlorophyll and carotenoid accumulation have been well characterized (Alqu?zar et al., 2008; Kato et al., 2004; Kaewsuksaeng et al., 2015;?Rodrigo et al., 2004; Yin et al., 2016). In plants, the?biosynthesis of carotenoids is catalyzed by a series of?desaturation, cyclization, hydroxylation, and epoxidation enzymes, including phytoene synthase (PSY),?phytoene desaturase (PDS), ?-carotene desaturase?(ZDS), lycopene ?-cyclase (LCYb), lycopene ?-cyclase?(LCYe), ?-ring hydroxylase (HYb), and zeaxanthin?epoxidase (ZEP). Previous studies suggested that?CitPSY, CitLCYb2, and CitHYb were the key genes regulating carotenoid accumulation in the flavedo and?juice sacs of citrus fruit (Alqu?zar et al., 2009; Fujii et al., 2021; Kato et al., 2004; Ma et al., 2016, 2023;?Peng et al., 2013; Zhang et al., 2012). During the ripening process, significant increases in the expression of?CitPSY, CitLCYb2, and CitHYb led to massive accumulation of ?,?-xanthophylls, which were the major?carotenoids responsible for the yellow color of flavedo?and juice sacs. As shown in Figure 1 (here, the 2nd picture), carotenoid and?chlorophyll share a common biosynthetic pathway,?using geranylgeranyl diphosphate (GGPP) as a precursor. The phytol chain of chlorophyll is synthesized from?the reduction of GGPP catalyzed by geranylgeranyl?reductase (GGDR). In citrus fruit, metabolic flux?changing from chlorophyll biosynthesis to carotenoid?biosynthesis occurred during the ripening process along?with rapid decreases in the expression of chlorophyll?biosynthetic genes (CitGGDR, CitCHLH, CitCHLM,?CitCHL27, CitPORA, CitCS, and CitCAO; Al?s et al.,?2006; Ma et al., 2021a, b). In addition, the degradation of chlorophylls to non-fluorescent chlorophyll catabolites catalyzed by CitCLH, CitPPH, CitPAO, and?CitRCCR also caused the loss of chlorophylls during?the ripening process in citrus fruit (Yin et al., 2016).Auxin rolAuxin is one of the most important plant hormones?regulating plant growth and development. In citrus,?auxin has been reported to be effective to induce fruit?set, stimulate plant growth, and maintain fruit quality?by retarding calyx abscission during storage (Agust??et al., 2002; Bermejo et al., 2018; Carvalho et al.,?2008). In previous studies, we found that auxin treatment enhanced the carotenoid accumulation and?improved the poor coloration in gibberellin (GA) and?prohydrojasmon (PDJ)-treated puffy fruit (Ma et al.,?2021c). Moreover, the expression of ethylene biosynthetic genes and ethylene production were induced by?auxin treatment, which indicated that the regulation of?carotenogenesis by auxin in citrus fruit may be exerted?via the stimulation of ethylene biosynthesis. The research?Miyagawawase? is an early-season citrus variety?widely consumed in Japan. The fruit of ?Miyagawawase? are harvested at the beginning of autumn when?the pulp reaches maturity while the peel is still a greenish color. To improve the coloration of the ?Miyagawawase? fruit, the effects of postharvest treatment of?1?naphthaleneacetic acid (NAA) on chlorophyll and?carotenoid metabolism were investigated in this study. In addition, to further verify the interaction between?auxin and ethylene in the regulation of citrus fruit?degreening, we investigated the effects of a combination treatment of NAA and 1-methylcyclopropene?(1?MCP), which is an ethylene antagonist, on the?degreening of citrus fruit in this study. The results will?contribute to elucidating the roles of auxin in plants,?and provide new insights into the improvement of coloration in early-season citrus fruit AbstractThe 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, ?-cryptoxanthin and 9-cisviolaxanthin, 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 ?-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. Pictures?1 - Fig. 2 of the original paper - Fig.?2.?Changes in the appearance of citrus fruit after postharvesttreatment with NAA or NAA?+?1-MCP. 2 - Fig 1 of the original paper - Metabolic pathway involved in the metabolism of carotenoid and chlorophyll via the MEP pathway in citrus. MEP pathway,?methylerthritol-4-phosphate pathway; GGPP, geranylgeranyl diphosphate. The enzymes investigated in this study were: CHLH, magnesium?chelatase; CHLM, magnesium-protoporphyrin IX methyltransferase; CHL27, Mg-Proto IX monomethyl ester cyclase; PORA, protochloro?phyllide oxidoreductase A; CS, chlorophyll synthase; CAO, chlide a oxygenase; PSY, phytoene synthase; PDS, phytoene desaturase; ZDS,???carotene desaturase; LCYb, lycopene ?-cyclase; LCYe, lycopene ?-cyclase; HYb, ?-ring hydroxylase; ZEP, zeaxanthin epoxidase. SourceEffects of Postharvest Treatment with 1-Naphthaleneacetic Acid on?Chlorophyll and Carotenoid Metabolism in Citrus FruitGang Ma, Lancui Zhang, Kan Murakami, Masaki Yahata, and Masaya KatoThe Horticulture Journaldoi: 10.2503/hortj.QH-082https://www.jstage.jst.go.jp/article/hortj/advpub/0/advpub_QH-082/_pdf