Exogenous nitric oxide inhibits the respiratory metabolism of postharvest wax apple fruit and its role in the delayed cottony softening

This study investigated the impacts of nitric oxide (NO) fumigation on the respiratory metabolism of postharvest wax apple fruit and its impact on fruit cottony softening. NO treatment inhibited the respiration rate of wax apple fruit and delayed the development of cottony softening; NO treatment suppressed the activities of hexokinase, phosphohexose isomerase (PGI), pyruvate dehydrogenase, and succinate dehydrogenase (SDH) in fruit pulp, but improved the activities of pyruvate kinase and citrate synthase (CS), and maintained the contents of glucose and pyruvic acid, indicating a reduction in the EMP-TCA pathway. Moreover, the NADH content and the activities of SDH and cytochrome C oxidase (CCO) were decreased with NO treatment, while the NADH/NAD ratio increased, suggesting that the electron transport chain complexes of wax apple fruit were reduced. Besides, NO treatment enhanced the activity of NADK in wax apple fruit, which lowered the levels of NAD and NADH but increased levels of NADP and NADPH, indicating an increase in pentose phosphate pathway. Additionally, CATPCA and heatmap analysis show that the PGI, CS, SDH, and CCO played a crucial role in NO-inhibited respiration. Collectively, these findings indicate that NO treatment suppressed the respiratory metabolism of postharvest wax apple fruit via regulating respiratory pathways, which reduces fruit senescence and delays cottony softening.