We conducted water culture experiments to elucidate the mechanisms of plant stress induced by excess supply of NH_4-N and NO_3-N using tomato and wheat plants. The application of NH_4-N at levels of 40, 200, 1,000 and 2,000 ppm to the culture medium resulted in the growth retardation compared with the plants with 200 ppm NO_3-N and at high NH_4-N levels necrosis or chlorosis appeared on the leaves. The ammonium concentration in leaves increased along with increase of medium NH_4-N. The application of NO_3-N at 1,000 and 2,000 ppm to tomato plants and at 2,000 ppm to wheat plants retarded the plant growth as compared with the plants grown in 200 ppm NO_3-N, but no visible injury appeared on the leaves. High medium NO_3-N caused marked decrease of nitrate reductase activity in tomato leaves but small effect on the activity of wheat leaves. The effect of high medium NO_3-N on nitrite reductase activity was small, and nitrite accumulation was negligible in all treatment with NO_3-N. However, amino acids accumulated to large extents in both plants with high medium NO_3-N. Treatment with high NO_3-N or NaCl in culture medium resulted in accumulation of amino acids, particularly glutamic acid, glutamine, alanine, glycine and proline. The extents of growth retardation and leaf amino acid accumulation were larger in NaNo_3-treated plants than NaCl-treated plants if compared at the same electric conductivities in medium. The growth retardation was closely related with stomatal resistance. It is concluded that NH_4-N stress is caused by large ammonium accumulation in plant tissues, and that NO_3-N stress causes the growth retardation partly due to the disturbance of plant nitrogen metabolism and also to the decrease of stomatal aperture.
