NATURE AND CHARACTERIZATION OF AMMONIUM EFFECTS ON WHEAT AND TOMATO

The very significant yield increases obtained by adding NH4 to NO3-sup plying hydroponic cultures have, for unknown reasons, not been duplica ted in soil systems. This research was conducted to consolidate and ex tend 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 s olution 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 bes t in NH4NO3 culture, equally well in NO3 or NH4 cultures buffered at p H 6, and poorest in unbuffered NH4 cultures. Growth enhancement with N H4NO3 was accompanied by increased N intake and synthesis of protein. Lowered yields in unbuffered NH4 cultures were associated with H+ inju ry to the roots and reduced intake of N and nutrient cations. Toxic ef fects of NH4 per se occurred when culture concentrations exceeded 16 m mol L-1 for roots. Severity of toxicity was associated with abnormally high root and shoot amide concentrations, and tissue levels of unassi milated NH4 exceeding 50 mmol kg-1. In alkaline cultures, NH3(aq) conc entrations greater than 0.028 mmol L-1 for tomato and 0. 15 mmol L-1 f or wheat reduced top growth, and destroyed the structure and functioni ng of roots. These detrimental effects of NH3(aq) were related to its concentration in the culture, not to tissue NH4.