Effect of ammonium and nitrate on growth and appearance of nitrate reductase and nitrite reductase in dark- and light-grown mustard seedlings.

Nitrate-induced and phytochrome-modulated appearance of nitrate reductase (NR; EC 1.6.6.1) and nitrite reductase (NIR; EC 1.7.7.1) in the cotyledons of the mustard (Sinapis alba L.) seedling is strongly affected by externally supplied ammonium (NH 4 (+) ). In short-term experiments between 60 and 78 h after sowing it was found that in darkness NH 4 (+) -simultaneously given with NO 3 (-) -strongly inhibits appearance of nitrate-inducible NR and NIR whereas in continuous far-red light-which operates exclusively via phytochrome without significant chlorophyll formation -NH 4 (+) (simultaneously given with NO 3 (-) ) strongly stimulates appearance of NR. The NIR levels are not affected. This indicates that NR and NIR levels are regulated differently. In the absence of external NO 3 (-) appearance of NR is induced by NH4 in darkness as well as in continuous far-red light whereas NIR levels are not affected. On the other hand, in the absence of external NO 3 (-) , exogenous NH 4 (+) strongly inhibits growth of the mustard seedling in darkness as well as in continuous far-red light. This effect can be abolished by simultaneously supplying NO 3 (-) . The adverse effect of NH 4 (+) on growth ('NH 4 (+) -toxicity') cannot be attributed to pH-changes in the medium since it was shown that neither the growth responses nor the changes of the enzyme levels are related to pH changes in the medium. Non-specific osmotic effects are not involved either.