J. Gerendas et al., PHYSIOLOGICAL AND BIOCHEMICAL PROCESSES RELATED TO AMMONIUM TOXICITY IN HIGHER-PLANTS, Zeitschrift fur Pflanzenernahrung und Bodenkunde, 160(3), 1997, pp. 239-251
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.