Plant growth and development, which determinate plant form, require the integration of a variety of environmental signals with the intrinsic genetic program. Fundamental to this process are several growth regulators called the plant hormones or phytohormones. In accordance with definition, the plant hormones are signal molecules produced within the plant and occur in extremely low concentrations.

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.