Plants of Urtica dioica L., a very nitrophilous species, were grown in a nutrient solution containing either high (100%) or low (2%) nutrient supply. Part of these plants were subjected to a sudden switch from 100% to 2% or vice versa. Plant weight, sugar and organic nitrogen (both soluble and insoluble) and nitrate content were measured during growth. The activities of two nitrogen assimilating enzymes, nitrate reductase (NR) and glutamine synthetase (GS) were determined. Growth of Urtica dioica was retarded at low nutrient supply. Root growth was limited by another factor than nitrogen. This was shown by a higher protein content. In the first period after a switch from 100% to 2%, redistribution of nitrogen from shoot to root could be demonstrated, and leakage from the root into the nutrient solution. It is suggested that in these conditions GS in the root reacted to this downward flux. Comparison with earlier findings on the less nitrophilous Plantago lanceolata showed that at 100% nutrient supply a correlation occurs between nitrate reduction and glutamine synthetase activity in that plant part which exported reduced nitrogen: the root in P. lanceolata and the shoot in U. dioica. In the importing plant part, glutamine synthetase was influenced by nitrate reduction as well as by imported reduced nitrogen.
Ries de Visserde Visser, R, 1985, Efficiency of respiration and energy requiretnents of N assimilation in roots of Pisiun sativum. Plant, The function of alternative path respiration in roots was investigated in pea plants (Pisum sativum L, cv. Rondo), Plants were grown in symbiosis with Rhizobium leguminosarum (strain PF2), cotnpletely dependent on N^ fijtation, or non-nodulated, receiving nitrate or ammonium at the same rate as Nj was fixed in symbiosis. Under these conditions, relative growth rates of plants grown with N,, NOj or NHJ were the same. This facilitated interpretation of the effect of the N source on the efficiency of root respiration, as determined by the relative activity of the non-phosphorylating alternative path. The 'wasteful' oxidation of carbohydrate via this pathway was defined as the glucose equivalent of the difference between the amounts of ATP (mol O;)"' produced in cytochrome and alternative path respiration, 'Wasteful' carbohydrate oxidation maximally amounted to 4% (NJ, 15% (NOj") and 25% (NH^) of the daily carbohydrate oxidation in the roots. It is concluded that the 'wasteful' oxidation of carbohydrate via the alternative path is of minor importance for the adaptation of root respiratory metabolism to different energy requirements of N assimilation. The total carbohydrate import by roots fixing N, was ca 60 and 30% higher than the import by roots assimilating NO,^ or NHj", respectively. Two factors are shown to account for these differences: the high carbohydrate cost of N, fixation, and the small contribution (30%) of the roots to NO3" reduction by the plant. The high carbohydrate requirements of roots fixing N2 were met by higher rates of photosynthesis as compared with piants utilizing NO3" or NH,,
The influence of shoot transpiration on the rates of growth and nitrogen fixation was investigated in Pisum sativum L. cv. Rondo. In short term experiments, rates of transpiration and acetylene reduction of intact plants were measured simultaneously, using air‐tight perspex vessels enclosing the basal part of the nodulated root. In long term experiments, accumulation of dry matter and reduced nitrogen in the plant were determined as well. Transpiration rate changed diurnally and was varied by manipulating the vapour saturation deficit or the flow rate of the air in the growth cabinet. The rate of acetylene reduction declined after subjecting intact plants to high transpiration rates. This decline was accompanied by a desiccation of the root nodules. Dry matter and reduced nitrogen accumulation were not affected by transpiration rate. At low transpiration rate reduced nitrogen content of the root nodules was higher than at high transpiration rate. However, in these nodules the rate of acetylene reduction was not significantly affected. It is concluded that the nitrogenase activity of pea root nodules is insensitive to changes in the flow rate and the organic N concentration of the xylem sap within a wide range of transpiration conditions under the applied growth conditions.
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