Abstract
Plants grow better when they are supplied with a combination of ammonium (NH4+) and nitrate (NO3−) than when either one is supplied as the sole N (nitrogen) source. However, the effects of N forms on N metabolism and major N assimilation enzymes in different plants, especially vegetables, are largely neglected. This study was conducted on two plants with distinct NH4+ tolerances to compare the responses of two popular leafy vegetables, Korean cabbage (Brassica campestris L.) ‘Ssamchu’ and lettuce (Lactuca sativa L.) ‘Caesar green’, to the N source. To this end, plant growth and quality, photosynthesis, carbohydrate, N contents (in the forms of NO3−, NO2−, NH4+, total protein), and key N assimilation-related enzyme (NR, NIR, GS, GDH) activities were investigated. When plants were subjected to one of three NH4+:NO3− regimes, 0:100, 50:50, or 100:0, lettuce was relatively more tolerant while cabbage was extremely sensitive to high NH4+. Both plants benefited more from being grown with 50:50 NH4+:NO3−, as evidenced by the best growth performance, ameliorated photosynthesis, and enriched carbohydrate (C) stock content. In addition, as compared to cabbage, the GS and GDH activities were reinforced in lettuce in response to an increasing external NH4+ level, resulting in low NH4+ accumulation. Our findings suggested that boosting or maintaining high GS and GDH activities is an important strategy for the ammonium tolerance in vegetables.
Highlights
In order to understand how the carbohydrate © status in plants is affected by enhanced ammonium nutrition (EAN) or enhanced nitrate nutrition (ENN), we investigated the soluble sugar and starch contents in plants after+ harvest
This study provided evidence that Korean cabbage ‘Ssamchu’ was extremely sensitive while lettuce was relatively tolerant to high concentrations of ammonium
In comparison to sole NH4 + or NO3 − supply, a combination of the two forms of N appeared to be more beneficial to both vegetables, as characterized by the best growth performance, ameliorated photosynthesis, and enriched carbohydrate (C) stock content
Summary
Many higher plant species acquire their N in the form of nitrate (NO3 − ) and ammonium (NH4 + ) from the soil solution [1]. Both can always be absorbed and used via roots, but they vary greatly in the biochemical, energetic, and molecular features for assimilation. Excessive NO3 − is carried away through leaching, which pollutes the environment; edible crops, especially leafy vegetables, have been found to accumulate an intermediate product nitrite (NO2 − ) during the nitrogen assimilation, which has toxic effects on both the plant growth and human health [2]
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