The present experiments were carried out in order to obtain informations about the mechanism of ammonium injury to plant by investigating changes of metabolism of tomato plants under ammonium excess. Young tomato plants were used after cultured in solutions containing different amounts of NH_4-N. The affected plants were examined for their enzymes activities related to respiration and for the accumulation of NH_4-N in their tissues. Catalase activities of leaves and roots in NH_4-N fed plants were higher than that in NO_3-N fed plants. NH_4-N contents in leaves and roots of the former plants were elevated to 100 ppm. TTC-reducing activity of roots was lower in NH_4-N fed plants than in NO_3-N fed ones, suggesting a decrease of dehydrogenase activity in the roots of the former. Polyphenol oxidase activities of leaves and roots were higher in NH_4-N fed plants than in NO_3-N fed plants, suggesting that the root browning due to lignification observed under ammonium excess was presumably related to the increase of that enzyme activity.
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