Nitrogen Metabolism in Plant Cell Suspension Cultures: II. Role of Organic Acids during Growth on Ammonia.

Tobacco cells (Nicotiana tabacum) are capable of growth on ammonia as a sole nitrogen source only when succinate, malate, fumarate, citrate, alpha-ketoglutarate, glutamate, or pyruvate is added to the growth medium. A ratio between the molar concentrations of ammonia to succinate (as a complementary organic acid) in the growth medium of 1.5 was optimal. Succinate had no effect on the rate of uptake of ammonia from the medium into the cells although it did affect the intracellular concentration of ammonia. However, the changes were not sufficient to explain inhibition of growth as being due to ammonia toxicity. The radioactivity from (14)C-succinate was incorporated into malate, glutamate, and aspartate within 2 minutes.It appears that the role of organic acids is neither connected to ammonium transport nor to relief of ammonia toxicity, but may be related to the need for additional carbon skeletons for synthesis of amino acids.