PHYSIOLOGICAL AND BIOCHEMICAL PROCESSES RELATED TO AMMONIUM TOXICITY IN HIGHER-PLANTS

Nitrate and ammonium have different effects on many biochemical and ph ysiological processes in plants, and at high concentrations this can l ead to markedly different growth responses. Most plant species show re duced growth, smaller leaves and a stunted root system when exposed to high ammonium concentrations, and in severe cases this leads to chlor osis. Although well known, ammonium toxicity is poorly understood and is generally considered to be the result of one more of the following effects: (i) ammonium-induced mineral nutrient deficiency, arising fro m the impaired uptake of metal ions; (ii) secondary growth inhibition arising from the acidification of the rooting medium, (iii) alteration s in intracellular pH and osmotic balance; (iv) uncoupling of photopho sphorylation from electron transport, following the accumulation of am monium in leaves; and (v) altered polyamine and phytohormone metabolis m. These hypotheses are reviewed in the light of the available literat ure and experimental evidence from own experiments. It is concluded th at no mechanism on its own provides an adequate explanation of the ava ilable data.