Evidence for the function of the free radical gas — nitric oxide (NO•) — as an endogenous maturation and senescence regulating factor in higher plants

The hypothesis that endogenously-produced nitric oxide (NO•) gas is a natural plant growth regulator was experimentally tested. Experimentation encompassed a wide spectrum of both climacteric and non-climacteric varieties of fruits, flowers, vegetables and legume sprout species. Specific NO• probe monitoring of both vegetative and generative plant organs revealed that temporal progress of maturation and senescence goes hand in hand with a significant decrease of NO• emission. Conversely, exogenous application of the gas either by direct fumigation in an O2-free atmosphere or by means of NO• releasing biochemicals such as N-tert-butyl-α-phenylnitrone and 3 morpholinosyl-nonimine, markedly delayed senescence and maturation. NO• decrease was moreover stoichiometrically related to ethylene upsurge, this phenomenon also being geared to ethylene increment under short-term heat stress. Furthermore, a classic foliage expansion bioassay indicated a concentration-dependent growth effect evidenced as a typical growth regulator-like response where low concentrations promote, and high concentrations inhibit growth. Regarded together, experimental data lends support to the possible role of NO• as a natural senescence-delaying plant growth regulating agent acting primarily, but not solely, by down regulating ethylene emission.