Nitrogen Metabolism in Canavalia ensiformis (L.) DC. : II. Changing Activities of Nitrogen-Assimilating Enzymes during Growth

Abstract

Summary The ontogeny of glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 1.4.1.14), asparagine synthetase (EC 6.4.5.4) and glutamate dehydrogenase (EC 1.4.1.2) has been followed in cotyledons, roots, stems and leaves of young plants of Canavalia ensiformis . The four enzymes were found in all regions. Glutamine synthetase activity increased in the leaves with time and remained low in the cotyledons. Initially glutamate synthase activity in the cotyledons was NADH-dependent but later became Fd-dependent. Asparagine synthetase specific activity was highest in the roots until day 5 after which activity was similar in all regions. While glutamate dehydrogenase specific activity decreased with time in the leaves it increased in the cotyledons and roots. We suggest that in the early stages of seedling growth asparagine synthetase and glutamate dehydrogenase both play a fundamental role in the reassimilation of ammonium released from canavanine. When the plant acquires full photosynthetic capacity the glutamine synthetase/glutamate synthase pathway becomes the principal pathway of reassimilation in the aerial parts of the plant.