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

✍ Scribed by V. K. Rajasekhar; H. Mohr

• Publisher: Springer-Verlag
• Year: 1986
• Tongue: English
• Weight: 588 KB
• Volume: 169
• Category: Article
• ISSN: 0032-0935
• DOI: 10.1007/bf00392112

✦ Synopsis

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 (NH2). 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 nitrateinducible NR and NIR whereas in continuous farred light -which operates exclusively via phytochrome without significant chlorophyll formation -NH 2 (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 NO~-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~, exogenous NH2 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~-. The adverse effect of NH2 on 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.