Influence of Ammonium on the Development Patterns of Proteolytic Enzymes and Glutamine Synthetase in Vigna radiata Seedlings

The influence of ammonium on the development patterns of proteolytic enzymes and glutamine synthetase was examined during the germination and early seedling growth of mung bean. Protease and carboxypeptidase activities in the cotyledons of seedlings grown on water increased 15-fold and 7-fold, respectively over a 5-day germination period. The provision of 10 mM NH₄⁺ to the seedlings decreased the enzyme activities by 30% in the cotyledons. A similar decrease in both protease and carboxypeptidase was observed with the hypocotyl of NH4 grown seedlings as compared to water control. Germination was accompanied by a marked decline in leucine aminopeptidase activity, and NH₄⁺ did not appear to have a significant effect on the development pattern of this enzyme. A comparison of glutamine synthetase activity on day 5 between control and NH₄⁺ showed that the enzyme level was repressed by 50%, 30%, and 25% in root, hypocotyl, and cotyledon, respectively, due to NH₄⁺ treatment.

The increase in the proteolytic activities were concurrent with a decrease in the protein content of cotyledon and hypocotyl tissues, whereas the proteolytic activities in the root decreased as the protein content declined during development. There was a progressive accumulation of soluble amino acids in the cotyledons of NH₄⁺ grown seedlings with relatively less accumulation in the hypocotyl. The reverse occurred for the seedlings grown on water. These results suggest that exogenously supplied NH₄⁺ during the germination of mung bean seeds slows down protein hydrolysis and the synthesis of glutamine synthetase.