Influence of Ammonium and Nitrate Nutrition on Enzymatic Activity in Soybean and Sunflower

Under conditions of controlled pH, nitrate and ammonium are equally effective in supporting the growth of young soybean (Glycine max var. Bansei) and sunflower (Helianthus annuus L. var., Mammoth Russian) plans. Soybean contains an active nitrate reductase in roots and leaves, but the low specific activity of this enzyme in sunflower leaves indicates a dependency upon the roots for nitrate reduction. Suppression of nitrate reductase activity in sunflower leaves may be due to high concentrations of ammonia received from the roots. Nitrate reductase activity in leaves of nitrate-supplied soybean and sunflower follows closely the distribution of nitrate reductase. For the roots of both species, glutamic acid dehydrogenase activity was greater with ammonium than with nitrate. The glutamic acid dehydrogenase of ammonium roots is wholly NADH-dependent, whereas that of nitrate roots is active with NADH and NADPH. In leaves, an NADPH-dependent glutamic acid dehydrogenase appears to be responsible for the assimilation of translocated ammonia and ammonia formed by nitrate reduction.In soybean roots ammonia is actively incorporated into amides, much of which remains in the roots. Sunflower roots are less active in amide formation but transfer much of it, together with ammonia, into the shoots. Glutamine synthetase activity in leaves is 20- to 40-fold lower than in roots.Glucose-6-phosphate dehydrogenase activity appears to be correlated with the activity of the nitrate reducing system in roots, but not in leaves.