Nature and Characterization of Ammonium Effects on Wheat and Tomato

Abstract

Abstract The very significant yield increases obtained by adding NH 4 to NO 3 ‐supplying hydroponic cultures have, for unknown reasons, not been duplicated in soil systems. This research was conducted to consolidate and extend our knowledge of the effects of added NH 4 on plant processes, and to define the plant and culture characteristics requisite to optimal utilization of mixed N sources. Wheat ( Triticum aestivum L. cv. Anza) and tomato ( Lycopersicon esculentum Mill. cv. 6718 VF) were grown in solution cultures providing variables of N source (NO 3 , NH 4 , and NH 4 NO 3 ), culture buffering, and concentration of NH 4 and NH 3 (aq). Treatment effects on plant growth and composition were evaluated. Wheat grew best in NH 4 NO 3 culture, equally well in NO 3 or NH 4 cultures buffered at pH 6, and poorest in unbuffered NH 4 cultures. Growth enhancement with NH 4 NO 3 was accompanied by increased N intake and synthesis of protein. Lowered yields in unbuffered NH 4 cultures were associated with H + injury to the roots and reduced intake of N and nutrient cations. Toxic effects of NH 4 per se occurred when culture concentrations exceeded 16 mmol L −1 for roots. Severity of toxicity was associated with abnormally high root and shoot amide concentrations, and tissue levels of unassimilated NH 4 exceeding 50 mmol kg −1 . In alkaline cultures, NH 3 (aq) concentrations greater than 0.028 mmol L −1 for tomato and 0.15 mmol L −1 for wheat reduced top growth, and destroyed the structure and functioning of roots. These detrimental effects of NH 3 (aq) were related to its concentration in the culture, not to tissue NH 4 .