Inorganic carbon fixation and metabolism in maize roots as affected by nitrate and ammonium nutrition

The effects of NO − 3 and NH + 4 nutrition on the rates of dark incorporation of inorganic carbon by roots of hydroponically grown Zea mays L. cv. 712 and on the metabolic products of this incorporation, were determined in plants supplied with NaH 14 CO 3 in the nutrient solution. The shoots and roots of the plants supplied with NaH 14 CO 3 in the root medium for 30 min were extracted with 80%; (v/v) ethanol and fractionated into soluble and insoluble fractions. The soluble fraction was further separated into the neutral, organic acid, amino acid and non‐polar fractions. The amino acid fraction was then analyzed to determine quantities and the 14 C content of its individual components. The rates of dark incorporation of inorganic carbon calculated from H 14 CO − 3 fixation and attributable to the activity of phosphoenolpyuvate carboxylase (EC 4.1.1.31), were 5‐fold higher in ammonium‐fed plants than in nitrate‐fed plants after a 30‐min pulse of 14 C. This activity forms a small, but significant component of the carbon budget of the root. The proportion of 14 C located in the shoots was also significantly higher in ammonium‐fed plants than in nitrate‐fed plants, indicating more rapid translocation of the products of dark fixation to the shoots in plants receiving NH + /sp 4 nutrition. Ammonium‐fed plants favoured incorporation of 14 C into amino acids, while nitrate‐fed plants allocated relatively more 14 C into organic acids. The amino acid composition was also dependent on the type of nitrogen supplied, and asparagine was found to accumulate in ammonium‐fed plants. The 14 C labelling of the amino acids was consistent with the diversion of 14 C‐oxaloacetate derived from carboxlyation of phosphoenolpyruvate into the formation of both asparatate and glutamate. The results support the conclusion that inorganic carbon fixation in the roots of maize plants provides an important anaplerotic source of carbon for NH + 4 assimilation.