Different Ammonium (NH₄<SUP>+</SUP>) Sensitivities by Korean cabbage and Lettuce May Be Linked to Nitrite (NO₂<SUP>-</SUP>) Content and Nitrite Reductase (NIR) Activity

다국어 초록 (Multilingual Abstract)

Ammonium (NH₄<SUP>+</SUP>) sensitivity of plants is an issue of agricultural and ecological importance. Excessive NH₄<SUP>+ </SUP>triggers widely-integrated impact, conferring the toxicity symptoms. Despite plant species differ in their NH₄<SUP>+</SUP> toxicity tolerance, current knowledge suggested that all plant species employ an identical pathway in nitrogen (N) uptake and assimilation. This study was conducted to get a new insight toward NH₄<SUP>+</SUP> tolerance in plants that drew comparisons with activities of key enzymes as well as contents of chemicals assessed in this pathway in two popular leafy vegetables, Korean cabbage (Brassica campestris L.) ‘Ssamchu’ and lettuce (Lactuca sativa L.) ‘Caesar green’ as the model plants. When these plants were supplied with one of three ammonium (NH₄<SUP>+</SUP>):nitrate (NO₃<SUP>-</SUP>) ratios, 100:0, 50:50 and 0:100, during a hydroponic cultivation, lettuce was relatively more tolerant, while Korean cabbage was highly sensitive, to NH₄<SUP>+</SUP>. Ammonium sensitivity was characterized by certain detrimental visual symptoms developing on the plant such as reduced plant growth, leaf chlorosis and stunted roots, and also biochemical attributes such as enhanced activities of antioxidant enzymes, decreased contents of starch, sugar and vitamin C, and decreased photosynthesis. Subsequent quantification of activities of key enzymes and contents of chemicals showed remarkably higher contents of nitrite (NO₂<SUP>-</SUP>) and lower activities of nitrite reductase (NIR) in Korean cabbage as compared to lettuce, suggesting that these two parameters may have contributed to the differences in NH₄<SUP>+</SUP> sensitivity of these two plant species.