The nitrogen metabolism of Spirodela oligorrhiza : II. Control of the enzymes of nitrate assimilation.

Nitrate reductase (NO3R) in Spirodela oligorrhiza has a specific requirement for NAD, and no activity could be detected when reduced NADP was supplied as a cofactor. NO3R and nitrite reductase (NO2R) are adaptive enzymes, being present only when the plant is supplied with NO3 or NO2. There is significant correlation between the concentration of NO3 in plants, and the levels of NO3R that they contain. When plants not containing NO3R or NO2R are supplied with NO3, there was a rapid increase in the levels of the two enzymes and in the concentration of NO3 in the plants. These increases were retarded but not prevented by NH4 in the medium. When NO3-grown plants were depleted of N, there was a rapid decrease in the levels of NO3R and NO2R.The utilization of NH4 prevented the assimilation of NO3 even by plants that contained high levels of NO3R and NO2R. It was therefore concluded that NH4, or the processes or products of its assimilation must inhibit the activity of NO3R. NH4, arginine, asparagine or glutamine, tested separately or together, had little effect on the in vitro activity of NO3R.