Anatomical Studies of Ammonium-Induced Stem Lesions in Tomato1

Abstract Tomato plants develop dark, pitted stem lesions when grown on ammonium nutrition in soil culture with insufficient potassium, (1,4,5). Ammonium utilization by plants requires a higher potassium level than nitrate utilization (10). Furthermore, when ammonium salt solutions are applied to soils, potassium may be fixed in the clay lattices by ammonium ions (2). Hence, the potassium supplying power of the soil is lowered, and plants are unable to obtain enough potassium to meet their needs. In sand culture, stem lesions occur only when potassium salts are excluded from the nutrient solution with ammonium as the sole nitrogen source (1). Lesions do not develop during potassium deficiency with nitrate sources of nitrogen.