【摘要】 不同氮源对红萍（Azolla imbricata）的生长和固氮作用都有影响。尿素的抑制作用最为严重,铵态氮次之,硝酸态氮最轻,硝酸铵作用介于铵态氮和硝酸态氮之间。这几种氮源对萍体内游离氨基酸的组分和含量也有影响,变化较大的为苏氨酸和精氨酸。在0.05～51.2 mM NO₃^-浓度范围内,红萍的固氮活性随着NO₃^-浓度变化呈双抛物线形变化,在高浓度下,活性急剧下降。硝酸还原酶活性则相反,与固氮活性相互补。应用¹⁵NO₃^-实验结果表明,在红萍体内¹⁵NO₃^-能迅速还原成¹⁵NH₄⁺。NO₃^-浓度对萍体内叶绿素含量和a/b比影响不大。更多还原

【Abstract】 The growth rate of Azolla （Anabaena azollae） and the nitrogenase activity of Anabaena azollae in symbiosis with Azolla imbricata are dependent upon the symbiont for its nitrogen sources. Inhibition of nitrogenase activity by nitrogen sources in the sequences of urea, ammonia, ammonium nitrate, and nitrate has been demonstrated. The composition and amount of free amino acids, especially threonine, and arginine are different with different nitrogen sources. Growth rate and nitrogenase activity are severely decreased by high levels of nitrates. However, nitrogenase activity of Anabaena azollae in symbiosis with Azolla imbricata exhibits two peak values with increasing of nitrate concentration between 0.05-51.2 mM. On the contrary, the nitrate reductase activity and the plot of nitrogenase activity are mutually compensating at some NO₃^- concentrations. Using ¹⁵N-assay it has been shown that the ¹⁵NO₃^- reduces to ammonia rapidly in Azolla. No effect of nitrate salt on the content and a/b ratio of chlorophyll was observed in our experiments.更多还原