FIGURE 2 Roots in the culture medium secrete the activity of enzymes related to nitrogen metabolism. (a), (b), and (c) respectively represent the changes in nitrogen metabolism-related enzymes under the superimposed environmental factors of nitrogen, phosphorus and nitrogen-phosphorus deficit stresses. (d) indicates the changes in nitrogen metabolism-related enzymes under the superimposed drought and high temperature. * indicating a significant difference to the N, P, NP, D, and CK treatment, p < 0.05.