Circadian rhythmicity of nitrate reductase activity in barley leaves

Nitrate reductase (EC 1.6.6.1) activity showed circadian rhythmicity in the first leaf of 8–11 days old barley ( Hordeum vulgare L. cv. Herta) plants. Circadian rhythms were found using both the in vitro and in vivo method for testing the enzyme activity. When the light intensity was reduced from 65 to 20 W m^?2, the amplitude was smaller and the oscillations were damped sooner. In continuous darkness nitrate reductase activity decreased in a two step process. Three different light qualities were tested which all gave the same results.