NATURE AND CHARACTERIZATION OF AMMONIUM EFFECTS ON WHEAT AND TOMATO

The very significant yield increases obtained by adding NH4 to NO3-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 NH4 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 (NO3, NH4, and NH4NO3), culture buffering, and concentration of NH4 and NH3(aq). Treatment effects on plant growth and composition were evaluated. Wheat grew best in NH4NO3 culture, equally well in NO3 or NH4 cultures buffered at pH 6, and poorest in unbuffered NH4 cultures. Growth enhancement with NH4NO3 was accompanied by increased N intake and synthesis of protein. Lowered yields in unbuffered NH4 cultures were associated with H+ injury to the roots and reduced intake of N and nutrient cations. Toxic effects of NH4 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 NH4 exceeding 50 mmol kg-1. In alkaline cultures, NH3(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 NH3(aq) were related to its concentration in the culture, not to tissue NH4.