Nitrate reductase activity was positively correlated with thenitrate contribution to the N source in the control plants. Whenplants were grown under CTs, however, the NR activity increasedwhen the ammonium contribution was increased from 0 to 25%. Itwas lower when the N-A ratio was 50:50, but the highest NR activitywas observed at a ratio of 75:25. In general, when the ammoniumion was greater than 25% of the N supplied to the seedling, NR activity was markedly reduced regardless of temperature