Nitric oxide (NO) is an inorganic free radical, that plays an important role in regulation of variety processes in living organisms. Soils are an important source of NO, produced during denitrifica- tion, nitrification and reduction of NO 3 - to NH 4 +. In plants NO production depends on the activity of nitric oxide synthase and/or nitrate reductase as well as non enzymatic generation of NO can be detected. Similarly as in animals, in plant cells NO is widely regarded as a signaling molecule.