Abstract
Nitrogen is a vital nutrient element for plants. The two main forms of nitrogen (N) are nitrate N (NO3 −) and ammonium N (NH4 +). The effects of the different NO3 −:NH4 + ratios (0:100, 25:75, 50:50, 75:25, and 100:0) in the nutrient solution on the growth, root morphology, ultrastructure and ion flux rate of lettuce roots were studied. The results showed that the biomass and leaf area were greatest when the ratio of NO3ˉ:NH4 + was 50:50 or 75:25. The shoot NO3 − content was highest when the ratio of NO3ˉ:NH4 + was 50:50 or 75:25. The shoot NH4 +-N content was highest when the ratio of NO3ˉ:NH4 + was 50:50. The NO3 − flux was greatest when the ratio of NO3ˉ:NH4 + was 50:50 or 75:25. Superoxide dismutase (SOD) activity was lowest when the ratio of NO3ˉ:NH4 + was 0:100, and peroxidase (POD) activity was highest when the ratio of NO3ˉ:NH4 + was 100:0. The malonic dialdehyde (MDA) content was lowest when the ratio of NO3ˉ:NH4 + was 50:50, and the MDA content was higher when the ratio of NO3ˉ:NH4 + was 0:100 or 25:75. It was smooth and complete when the ratio of NO3ˉ:NH4 + was 50:50 or 75:25.
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