Stimulation of dark carbon fixation in rice and tomato roots by application of ammonium nitrogen

Dark carbon fixation was investigated in roots of rice plants, an ammonium-tolerant species and tomato plants, an ammonium-sensitive one during periods of growth on nutrient media containing ammonium, ammonium nitrate, or nitrate as the nitrogen source. Application of ammonium nitrogen immediately stimulated the dark carbon fixation in the roots of both plants. The stimulation was greater in the plants grown on ammonium media than in those grown on ammonium nitrate media. The increase in the rates of dark carbon fixation continued in rice plants for 7 d whereas in tomato plants the rates reached the maximum value at day 1, and then gradually decreased. Most of the ¹⁴C fixed for 3 h remained in the roots: 75–90% in rice plants and 65–80% in tomato roots. Soluble fixation products were composed of amino acids and organic acids. Organic acids were more labelled than amino acids. Amino acids were more labelled in both plants grown on ammonium and ammonium nitrate media than in those grown on nitrate media. The proportion of ₁₄C located in soluble metabolites was not consistently affected by the nitrogen sources. Aspartate and asparagine were more labelled in rice plants, and malate was more labelled in tomato plants. These results show that rice is superior to tomato in the function of dark carbon fixation for the continuous detoxification of excess ammonium.