Nitric oxide (NO) gas is a small molecule with a high reactivity of the resulting radical character. NO in plant cells can be enzymatically synthesized from L -arginine in a reaction catalysed by the enzyme nitric oxide synthase activity or nitrite in a reaction catalyzed by nitrate reductase . It is also possible via non-enzymatic synthesis of NO . The results of the past years have shown a role in the regulation of signal range of physiological processes ranging from seed germination and ending on aging plants .