Nitrate assimilating system in general is known to show increase in nitrate uptake system and nitrate reductase system in the presence of nitrate. In other words, nitrate assimilatory system is induced by the presence of nitrate. Similarly, cells or organisme assimilating NH3 as nitrogen source show lack of nitrate assimilatory system. Such control of nitrate is called repression control. Thus nitrate is an inducer while ammonia is repressor of nitrate assimilatory system. Red light is known to enhance synthesis of nitrate assimilatory system and this light effect is mediated by well-known photomorphogenic pigment called phytochrome.