Foliar ammonium accumulation as an index of stress in plants

Foliar ethylene evolution is used as a bioassay to index physiological stress in plants. Accumulation of uncombined ammonium frequently precedes or coincides with ethylene evolution by stressed plants and also may serve as an index of stress. The objective of this research was to assess the relationship between ethylene evolution and ammonium in plants stressed by toxicity. Ethylene evolution and ammonium accumulation by tomato (Lycopersicon esculentum Mill.) foliage were measured for plants grown in a greenhouse with nutrition from ammonium- or nitrate-based nutrient solutions. Foliar symptoms of stress from ammonium toxicity appeared on plants growing on ammonium-based solutions. Ethylene evolution increased in response to a progressive array of foliar ammonium accumulation, which varied with source of N and duration of the treatments. Above a threshold of about 0.2 mg NH4- N g(-1) fresh weight, ethylene evolution reset sharply and linearly with ammonium accumulation. Ammonium accumulation in foliage is suggested as an index to assess environmentally induced stresses in plants.