Effect of Nitric Oxide on Softening and Cell Wall Metabolism of Postharvest Mango

To elucidate the inhibitory mechanism of nitric oxide (NO) on softening of postharvest mango (Mangifera indica L.) fruit, mango fruits at green mature stage were dipped into 0.25 mmol/L sodium nitroprusside (SNP, a nitric oxide donor) solution for 20 min and storage at (20±2)℃ for 20 d. During storage, fruit firmness, cell wall component, and activities of cell wall degrading enzymes were investigated at regular intervals. The results showed that SNP treatment significantly suppressed the activities of polygalacturonases (PG) (p<0.05) during storage period of 20 d, and significantly inhibited the activities of cellulose (CX) (p<0.05), β-galactosidase (β-Gal) (p<0.01) and α-L-arabinofuranosidase (α-L-Af) (p<0.01) during storage period of 10 d. However, it significantly increased the activities of CX and β-Gal (p<0.05) from 10 d to 20 d, and α-L-Af activity (p<0.05) on the 20 th day, as compared with control fruit. SNP treatment significantly inhibited the increase of pectin methylesterase (PME) activity during storage period of 5 d (p<0.05), but maintained higher PME activity from 10 d to 20 d (p<0.05). In addition, SNP treatment delayed the degradation of propectin and cellulose extremely significantly (p<0.01), and inhibited the increase of soluble pectin content (p<0.05), therefore delayed the loss of fruit firmness, as compared with control fruit. There were extremely significant positive correlations between fruit firmness and the contents of propectin and cellulose (p<0.01), however, there were significant negative correlations between fruit firmness and the activity of CX (p<0.05), soluble pectin content (p<0.01) and the activities of PG, β-Gal and α-L-Af (p<0.01). There were extremely significant negative correlations between soluble pectin content and cellulose content (p<0.01), however, there were significant positive correlations between the soluble pectin content and the activitiy of α-L-Af (p<0.05), the activities of PG and β-Gal (p<0.01). Thus, it could be concluded that SNP treatment could delay fruit softening of postharvest mangoes by regulating cell wall degrading enzyme activities and decreasing the degradation of cell wall component, which would help to maintain the integrity of cell wall structure.