EFFECTS OF HUMIC SUBSTANCES ON NITRATE UPTAKE AND ASSIMILATION IN BARLEY SEEDLINGS

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Abstract
Barley seedlings (Hordeum vulgare L. ’Astrix’) were incubated with humic substances extracted with 0.1 M Na4P2O7 from a grassland soil and with their fractions obtained either by gel-permeation chromatography on Sephadex G100 or by dialysis. After incubation the seedlings were transferred into an assay medium, in order to evaluate the rate of nitrate uptake and the extractable activities of nitrate reductase (NR), glutamate dehydrogenase (GDH), and glutamine synthetase (GS). Nitrate uptake was inhibited for up to 4 h incubation with unfractioned humic extract (UH), while it was progressively stimulated after 4–16 h incubation. The maximum stimulation was obtained at 100 mg L−1 humus concentration. The above enzyme activities were stimulated by UH: NR to the same degree, GDH and GS to a lower degree than nitrate uptake. Sephadex fractions stimulated nitrate uptake to a level decreasing in the following sequence of apparent molecular weight: < 9000; > 100 000; 9000–100 000. None of the fractions affected significantly GDH and GS activities. The humic fraction of molecular weight > 9000 separated by dialysis, differently from the Sephadex fractions of the corresponding molecular size, was effective on the uptake and also on the enzyme steps of nitrate assimilation. Therefore, the regulatory properties of humic substances appeared to be dependent not only on their molecular size, but also on the reciprocal interactions of the different constituents, which were affected by the fractionation procedure. Key words: Humic fractions, nitrate uptake, barley seedlings

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