Hydrogen Sulfide Delays Cottony Softening of Postharvest Wax Apple Fruit via Regulating Respiration Metabolism and Energy Status

To clarify the regulatory mechanism of hydrogen sulfide (H₂S) delaying the cottony softening of harvested wax apple fruit, the effects of H₂S fumigation on the respiratory metabolism and energy status were investigated. 0.9 mmol/L sodium hydrosulfide solution was used as H₂S donor, and the fruit was stored at 4±1 ℃ for 12 d. The results showed that, compared to the control, the wax apple fruit treated with H₂S showed higher firmness and significantly (P<0.05) lower respiration rate, and slower cottony softening during the storage, and the cottony softening index was 17.56% lower at the end of storage. Meanwhile, the contents of ATP and glucose and the energy charge were maintained at higher levels, particularly the energy charge was 7.89% higher on storage for 12 d. Moreover, the H₂S treatment increased the mitochondrial membrane H⁺-ATPase activity significantly (P<0.05), while it inhibited the activities of phosphofructokinase, pyruvate kinase, pyruvate dehydrogenase, succinate dehydrogenase and cytochrome coxidase of wax apple fruit. Furthermore, H₂S treatment reduced the contents of NAD and NADH, but significantly (P<0.05) increased the NADH/NAD ratio, NAD kinase activity, and the contents of NADP and NADPH. These results suggested that H₂S treatment could inhibit the glycolysis-tricarboxylic acid cycle pathway and electron transport chain of harvested wax apple fruit, which contributed to a lower substrate consumption and a higher synthetic capacity of energy and metabolite. These effects jointly conduced to the delayed senescence of harvested wax apple fruit, which led to the inhibited cottony softening.