The characterization and contrasting effects of the nitric oxide free radical in vegetative stress and senescence of Pisum sativum Linn foliage

As monitored respectively by a nitric oxide (NO)-specific probe and gas chromatography, emission of both NO and ethylene from pea foliage increases with duration of stress and/or senescence promoting conditions. Depending on concentration, NO appears to exert a stress-coping or, contrariwise, an inhibitory effect on leaf growth. Stress coping is evidenced by the marked deceleration of stress ethylene by application of the NO releasing compound S-nitroso-N-acetylpenicillamine (SNAP) and possibly also by significant increment of leaf disc expansion upon treatment with low concentrations of chemically generated NO gas. At higher NO concentrations, leaf expansion is inhibited by NO treatment as well as by three different NO releasing compounds: SNAP, 3-morpholinosydnonimine, and N-tert-butyl-alpha-phenyl-nitrone. Further evidence to the assumption of endogenous involvement of NO metabolism is lent by the inhibition reversal induced by the nitric oxide synthase inhibitor N-G-methyl-L-arginine. Reconciliation of the opposite effects is discussed in terms of translocation and cellular binding sites of the free radical gas.