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Physiological Effects of the Form of Nitrogen on Corn Root Tips: A³¹P Nuclear Magnetic Resonance Study¹

Physiological effects of different N forms(NO^–₃, NH⁺₄, or a combination of both) on corn (Zea mays L.) root tips and leaves were studied by following ³¹P signals with a nuclear magnetic resonance spectrometer. With root tips, both cytoplasmic and vacuolar pH could be measured, whereas with leaves, only vacuolar pH could be determined. The N treatments did not affect the cytoplasmic pH of corn root tips in contrast to proposals of previous workers. Leaf vacuolar pH was higher and root tip vacuolar pH lower with NO^–₃ than with NH⁺₄. Under anaerobic conditions, cytoplasmic pH was reduced because of lactic acid fermentation. Nitrate, an electron acceptor, delayed the acidification of the cytoplasm compartment because it represents an alternative way to reoxidize NADH. In conclusion, for the conditions of these experiments, the pH of the cytoplasm of corn root tips was not modified by the form of N absorbed; however, the pH of this compartment was affected by the form of N presented during development anaerobiosi.

Key Words: Cytoplasmic and vacuolar pH • Nitrate: ammonium ratio • Anerobiosis • Nuclear magnetic resonance spectroscopy • Zea mays L.

² Graduate research assistant, and professor, respectively, Dep. of Agronomy, Iowa State Univ. Ames, IA 50011.

Received for publication December 5, 1984.