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). In short-term experiments between 60 and 78 h after sowing it was found that in darkness NH-simultaneously given with NO-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(simultaneously given with NO) 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 NOappearance of NR is induced by NH 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, exogenous NHstrongly 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. The adverse effect of NHon growth ('NH-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. [ABSTRACT FROM AUTHOR]