Role of phytohormones in nitrate uptake and in vivo nitrate reductase activity of mustard cotyledons

In vivo nitrate reductase activity and nitrate uptake in gibberellic acid ( GA)and kinetin (KN)-treated mustard (Brassica juncea Coss, cv. T-59 Varuna) seedlings grown in the presence of light were investigated. An attempt was also made to determine the metabolic pool size and in vivo apparent K-m va lue of nitrate reductase (NR) for nitrate (NO3). Both NO3 uptake and in viv o NR activity showed biphasic saturation kinetics, but causal relationship between them was not evident. Kinetin inhibited uptake and promoted in vivo NR activity, while although GA promoted uptake, in vivo NR activity was al most equal to distilled water (DW) control. The hormones affected the K-m v alue differently, GA showed lower K-m value while KN showed higher K-m valu e for NO3. Metabolic pool size (MPS) was also significantly affected by bot h the hormone treatments. The results are discussed in relation to the use of in vivo NR activity for calculating the apparent K-m and MPS for NO3,.