Nitrogen fixing efficiency and enzymes of CO assimilation in nodules of ureide and amide producing legumes

Abstract

Levels of nitrogenase, hydrogenase, 02 uptake, and some enzymes of C02 assimilation, viz. phosphoenolpyruvate carboxylase (PEP carboxylase; EC 4.1.1.31), NAD+ malate dehydrogenase (MDH, EC 1.1.1.37), NAD + malic enzyme (ME, EC 1.1.1.39), and aspartate aminotransferase (AAT, 2.6.1.1.) were compared in the root nodules of pigeonpea (Cajanus cajan) and clusterbean (Cyamopsis tetragonoloba) at different stages of plant growth, starting from 25 days after sowing (DAS) till pod maturity. Nitrogenase and hydrogenase activities which were maximum at the time of flowering (71-78 DAS) were 2-5 times more in clusterbean than those in pigeonpea. Specific activities of PEP carboxylase, AAT, ME, and MDH were maximum at the vegetative stage in both the crops. However, the activity expressed on per g fresh weight basis followed the pattern of nitrogenase activity. Compared to pigeonpea, AAT and PEP carboxylase activities were 2-4 times higher in the nodules of clusterbean. At the flowering stage when photosynthetic supply to the nodules is restricting, the activity of ME and MDH in relation to CH2 reduction increased in pigeonpea, indicating that cluster bean and pigeonpea besides differing in the form of transporting fixed nitrogen also have different enzymic routes for providing carbon skeleton for ammonia assimilation.