The effect of potentially toxic concentrations of ammonium on root development of Scots pine seedlings raised on Perlite was investigated during growth periods of 3 or 10 weeks after sowing. It was shown that imbalanced ammonium nutrition led to conspicuous changes of root morphology provided the pH value in the medium was allowed to decrease to 3.9 due to the NH+4‐dependent proton excretion into the rhizosphere. Ammonium toxicity could not be observed with seedlings treated either with ammonium nitrate or with ammonium chloride at pH 5.3 − 6.8. While the supply of NH+4 considerably inhibited root development the biomass production of the shoot was increased. Determination of the endogenous level of ammonium in roots and the leaf whorl exclude a simple causal correlation between ammonium toxicity and accumulated ammonium as has been postulated for herbaceous plants.
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Vollbrecht et al. (1992) studied this question.