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NO is an inorganic free radical gaseous molecule that has been shown to play an unprecedented range of roles in biological systems. Taking into account these numerous functions and the reports indicating that NO can regulate processes related to plant growth and development, endogenous sources of NO need to be clarified as well as the effect of the variations of NO levels upon the plant life cycle. We consider here the main endogenous sources of cellular NO in plant tissues, and the contribution of enzymatic sources upon seed germination. Non-enzymatic generation of NO from NO2− under conditions of low pH could be of considerable importance since significant amounts of NO2− can be found in plant tissues. However, at least under the reported experimental conditions, enzymatic activity seems to be more relevant to total NO generation, allowing a strict control of NO steady state concentration. The complexity of the overall scenario presented here shows the need for further studies into NO production and consumption pathways in germinating seeds exposed to fluctuating environmental conditions. [ABSTRACT FROM AUTHOR]