In vivo Nitrate Reductase Activity in Leaves of Pearl Millet, Pennisetum americanum (L.) Leeke

Abstract

In the in vivo assay of nitrate reductase (NR) in P. americanum leaves, addition of 1% (v/v) Triton X-100, potassium phosphate buffer (80 mM, pH 7.4) and 1.13 mM NADH to the assay medium resulted in maximum activity. With increasing concentration of NADH, saturation-type kinetics were observed. Based on this data metabolic pool concentration for NADH and apparent K_m for nitrate reductase were determined.

In field studies with cultivars BJ-104, J-104 and 5141-A of P. americanum, the relative limitation of NO₃^-, NADH and nitrate reductase in NO₃^- assimilation was determined. NR activity was measured by four modifications of the in vivo assay technique (with NO₃^-, with NADH, without NO₃^- and NADH and with both NO₃^- and NADH additions to the reaction mixture) and with one in vitro technique. For all the cultivars, NADH was the major rate-limiting factor for in vivo assay during early growth stages, while at later stages, NO₃^- was limiting. At no stage was NR rate-limiting. It is concluded that NR activity alone may not serve as biochemical marker for improved efficiency of utilisation of nitrogen in P. americanum.