Nitrogen Mobilization and Proteolytic Activities in Germinating and Maturing Bush Beans (Phaseolus vulgarisPhaseolus vulgaris L.)

Abstract

Summary Changes in the nitrogen contents and in the extractable activities of amino-, carboxy- and endopeptidases were analyzed in bush beans (Phaseolus vulgaris L.) throughout the life cycle. Two types of casein digesting enzymes (endopeptidases) were found. One type was the main activity extracted from cotyledons during germination with a pH optimum around pH 5.5 and was stimulated by the presence of mercaptoethanol in the incubation medium. The other type showed high activities in leaf extracts, was most active around pH 7 and was slightly inhibited by the presence of 0.1 % mercaptoethanol in the incubation medium. Beside the main acidic caseolytic activity, the neutral type was also present in cotyledons during germination. In detached cotyledons the acidic activity increased more gradually than in controls, but the neutral type reached higher activities. Aminopeptidase activity decreased during germination. In detached cotyledons this decrease was delayed. Carboxypeptidase activity increased during germination and was not affected by removing the axis. No major differences in nitrogen mobilization or in the pattern of proteolytic enzymes were observed between illuminated and etiolated seedlings. In leaves no major changes in the relative activities of exo- and endopeptidases were detected during the life cycle. In senescing leaves and pods aminopeptidase activity decreased while carboxypeptidase and neutral endopeptidase activities remained stable or increased slightly. The large increase in the neutral endopeptidase activity observed in leaves of maturing cereals could not be found in bean leaves. Also the sharp decrease of exo- peptidase activities found in senescing leaves of wheat and corn did not occur in bean leaves. Since there are major differences between the nitrogen metabolism of cereals and legumes, a different regulation of nitrogen mobilization and of the proteolytic activities appears reasonable.