Regulation of ascorbic acid metabolism in postharvest navel orange fruit during storage by exogenous hydrogen sulfide

Hydrogen sulfide (H₂S) is an important gaseous signaling molecule in plants. It has been shown in our previous studies to maintain the storage quality of postharvest fruits at low concentrations. Navel orange, a fruit relatively high in vitamin C (ascorbic acid), experiences a rapid decline in this nutrient during postharvest storage, which reduces its nutritional value. However, the effect of H₂S on ascorbic acid (AsA) metabolism of postharvest navel orange from molecular level has not been reported. This study investigates the application of exogenous H₂S to regulate AsA metabolism in navel orange.

Postharvest navel orange fruits were fumigated with 25 μl L^-1 H₂S (air as control) for 30 min, and stored at 20 °C for 18 days. Fruit samples were collected every three days for analysis. The content of total AsA, and AsA was quantified. Concurrently, general quality parameters including weight loss, soluble solids content (SSC), and titratable acidity (TA) were also monitored. The expression levels of key genes involved in AsA metabolic pathway were analyzed using quantitative real-time PCR.

Fumigation with 25 μl L^-1 H₂S significantly inhibited the increase in weight loss, maintained SSC, and delayed the decline in TA, total AsA, and AsA content in navel oranges during the 18-day storage period. The expression level of genes involved in the ASA metabolic pathway was modulated by H₂S treatment. Up-regulated genes of AsA biosynthesis (CitPGI, CitPMM, CitGME, CitGGP, CitGPP, CitGalDH, CitGalLDH, and CitGalUR) and regeneration (CitMDHAR and CitGR), as well as down-regulated genes of degradation (CitAO), all contributed to the increased AsA content during storage.

Application of exogenous 25 μl L^-1 H₂S effectively preserved the postharvest quality and nutritional value of navel orange. Further studies showed that H₂S primarily regulated the expression of genes involved in the AsA metabolism pathway in fruits via the L-galactose pathway, while cooperating with D-galacturonic acid, AsA cycle and degradation pathways to accelerate the synthesis and accumulation of AsA in fruits. These findings demonstrate that H₂S treatment helps maintain higher vitamin C content in navel orange by regulating key genes in the AsA metabolic pathway.