| In order to explore the effects of exogenous nitric oxide on metabolism process of carbon and nitrogen of pumpkin seedlings, a pot experiment was conducted to study the effects of 80 μmol·L-1 SNP on the growth of seedlings, the contents of chloroplast pigments, the activities of carbon and nitrogen metabolism related enzymes and the amount of metabolites of pumpkin (cv. Yinhui No.2) seedlings under the stress of 60 mmol·L-1 Na2CO3. The results showed that aboveground and underground dry weight, relative growth rate, root shoot ratio, Rubisco carboxylase activity, light enzyme activity of photosynthetic pigment contents and nitrogen metabolism were decreased in different degrees, while Rubisco oxidation activity, total free amino acids and proteolytic enzyme activity were significantly increased when the seedlings exposed to 60 mmol·L-1 Na2CO3 for 12 d. Application of 80 μmol·L-1 SNP obviously alleviated the inhibition effects of Na2CO3 stress on the seedling growth and photosynthesis, as it increased the plant dry matter and leaf chlorophyll content. Compared with Na2CO3 stress treatment, 80 μmol·L-1 SNP considerably increased dry matter accumulation, relative dry weight, chlorophyll content, carotenoid content, nitrate reductase, glutamine synthetase, carboxylase, amylase activity and soluble protein accumulation by 26.72%, 30.45%, 17.69%, 46.15%, 46.43%, 30.70%, 38.75%, 70.0%, 65.37%, respectively, while decreased protease, oxygenase activity, free amino acid content by 18.29%, 31.76%, 28.57%. Meanwhile, exogenous NO maintained the stability of total soluble sugar content by increasing the activity of amylase and Rubisco carboxylation and decreasing the oxidative activity. Exogenous NO also inhibited the activity of proteolytic enzymes and glutamate dehydrogenase (GDH), decreased the content of free amino acids in leaves, and increased the accumulation of protein by increasing the activity of NR, GS and glutamate synthase (GOGAT). Under alkaline salt stress, 80 μmol·L-1 SNP promoted the keto acid translating into amino acids through the enhancement of pumpkin seedlings photosynthetic carbon metabolism. Meanwhile, by improving the activity of related enzymes NR, GS, GOGAT, 80 μmol·L-1 SNP caused the switch of carbon to nitrogen, maintaining the balance between them under salt stress of Na2CO3. |