Effects of different nitrogen forms and ratios on nitrogen metabolism in Ipomoea batatas (L.) Lam. and their physiological mechanisms

Three different types of sweet potato (Ipomoea batatas (L.) Lam.) were selected as test materials and five nitrogen ratio treatments were established according to three nitrogen forms. Treatments were carried out at 15, 25, and 35 d after planting seedlings. The nitrogen content in different organs, activity of functional leaf nitrogen metabolism enzymes, and expression of enzyme-regulated genes of I. batatas were determined. Results showed that in the same growth period, nitrogen accumulation in plants under the N4 and N5 treatments was significantly higher than that under other treatments; in the early stage of I. batatas growth and development, nitrogen content in the leaves first decreased and then increased, whereas nitrogen content in the stem, fibrous and enlarged roots, and whole plant increased steadily. Moreover, N4 treatment significantly increased the activities of NR, GS, and GDH, while N3 treatment significantly increased the activity of GOGAT. Furthermore, NR activity increased with the increase in the proportion of nitrate nitrogen in the fertilizer. NR activity-related genes were up-regulated when the proportion of nitrate nitrogen increased in the fertilizer. GS and GOGAT-related genes were up- and down-regulated, respectively, by N4 and N5 treatments. Amide nitrogen had no significant effect on the regulation genes of nitrogen metabolism-related enzymes in the early stage. These results imply that in the early stage of I. batatas growth and development, the combined application of nitrate nitrogen and ammonium nitrogen can significantly increase nitrogen accumulation, metabolic enzyme activity, and regulatory gene expression. The nitrogen:nitrate nitrogen:amide nitrogen ratio of 1:2:0 is the best combined application under the experimental conditions.