Effect of phosphorus application on the activities of some dehydrogenases related to carbon metabolism in moong nodules and roots

The activities of isocitrate dehydrogenase (ICDH), malate dehydrogenase (MDH), glucose-6-phosphate dehydrogenase (G6PDH) and lactate dehydrogenase (LDH) in bacteroids, nodule cytosol and roots of Vigna radiata were determined under three phosphorus (P) treatments: control (P₀), 20 kg P₂O₅ ha^-1 (P₂₆) and 40 kg P₂O₄ ha^-1 (P₄₆) at 35 (pre-flowering), 46 (flowering) and 54 (pod-initiation) days after sowing (DAS). The peaks of N₂ ase and nodule cytosol ICDH and MDH activities coincided under each P level at 46 DAS. These activities increased with P₁₀ but decreased with P₄₀ and were greater In nodules than in roots at 46 DAS. At 35 DAS, G6PDH activity was maximum but it decreased from 35 to 46 DAS and was much less in nodule cytosol compared to ICDH and MDH. Nodule cytosol LDH activity was comparable to or lower in roots at each P level. It is, therefore, suggested that ICDH and MDH serve as source of reductant to bacteroids to support N₂-fixation.