Understanding the influence of 2,4-dichlorophenoxyacetic acid and melatonin treatments on the sweet and acidic flavors and citric acid metabolism of ‘Olinda’ orange (Citrus sinensis (L.) Osbeck)

The regulatory mechanism by which 2,4-dichlorophenoxyacetic acid (2,4-D) and melatonin (MT) affect the sweet and acidic flavors of citrus fruit was investigated by comparing the fruit quality and gene expression related to citric acid metabolism of ‘Olinda’ oranges (Citrus sinensis (L.) Osbeck). 2,4-D increased the fruit firmness, total soluble solids, soluble sugar content and solid-acid ratio and decreased the respiration rate, titratable acidity and citric acid content during storage, and a similar but weaker effect was found in the MT treatment before 70 days of storage. 2,4-D and MT maintained postharvest quality and improved the fruit sweetness and acidity flavor of ‘Olinda’ Valencia oranges. Genes encoding of PEPC1/2/4 and CS1 in the citric acid biosynthesis pathway and Aco2/3, IDH2/3, ACLa2/β, PEPCK1 and FBPase2 in the citric acid degradation pathway were upregulated by 2,4-D or MT. However, genes related to citric acid degradation, such as GS2, Aco1, IDH1, GAD4/5 and SSADH in the γ-aminobutyric acid (GABA) pathway and ACLa1 and FBPase1 in the acetyl-CoA pathway, were oppositely regulated by 2,4-D and MT. Principal component analysis showed that the decrease in citric acid content was significantly negatively correlated with the expression of genes related to the GABA and acetyl-CoA degradation pathways. Moreover, the ACLα1/β and FBPase1 genes in the acetyl-CoA pathway might be the key genes causing differences in the acidity of ‘Olinda’ oranges among the three groups.