School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemA nutrient culture study on the effect of ammonium and nitrate nitrogen on the growth and development of plants( 1968)Five nutrient culture experiments were designed to study various aspects of ammonium and nitrate nutrition in plants. Test plants were zinnia, Iceland poppy, cabbage and parsnip, and these were selected because of their suspected sensitivity to ammonium nitrogen when grown commercially in sterilized soils. Only in one experiment was any attempt made to stabilize the pH of the nutrient culture with very sparingly soluble calcium salts. In all experiments with unbuffered nutrient cultures, the ammonium fed plants made very poor growth. Nitrate fed plants were the most vigorous, and consistantly outyielded those fed both ammonium and nitrate. The order of decreasing sensitivity to ammonium-nitrogen was Iceland poppy, zinnia, parsnip and cabbage. Symptoms of ammonium damage were stunting, loss of turgidity, leaf chlorosis, tip burn, sap extrudation, epinasty, and a severely restricted root system. It was demonstrated with zinnia that ammonium was equally toxic, at all concentrations studied (14 to 84 p.p.m. N). However with nitrate, and ammonium + nitrate,plant growth increased with increase in substrate nitrogen concentration. The presence of nitrate in a culture containing ammonium, delayed or completely prevented the appearance of ammonium toxicity symptoms. However when nitrate was removed, or substituted by ammonium, symptoms rapidly developed. (Iceland poppy). When the pH of the nutrient cultures was stabilized with CaCO3 or CaHPO4, ammonium toxicity symptoms were prevented. The ammonium fed parsnip plants grew equally well as those fed nitrate, and both were outyielded by those fed ammonium + nitrate. Chemical analysis of plants grown in unstabilized cultures, showed the nitrate fed plants to have the highest calcium, magnesium and lowest nitrogen concentration. The reverse was true for ammonium fed plants. The study with zinnias revealed rapid accumulation of nitrate within the plant before the optimum substrate nitrate concentration for growth had been reached. A trace of nitrate was detected in "ammonium" plants. On stabilizing the solution pH, highest plant nitrogen was found in parsnips fed nitrate, and the lowest in those fed ammonium + nitrate. The calcium content in plant tops was comparable between all nitrogen treatments. The pH of the nutrient cultures fell wherever ammonium was present, irrespective of the nitrate concentration. The pH of the nitrate solutions rose following plant growth. A chromatographic study on free amino acids and amides in young cabbage leaves showed that ammonium fed plants had the highest, and nitrate fed plants the lowest concentration. The amino acid composition was not changed with nitrogen treatment, although with the ammonium nutrition, there was a marked build up of proline, serine, glutamic acid and glutaiaine. The possible mechanisms Involved in the toxicity of the ammonium ion are discussed.