Uptake of NO by sunflower (Helianthus annum) during exposures in light and darkness: quantities, relationship to stomatal aperture and incorporation into different nitrogen pools within the plant.

Exposures of sunflowers (Helianthus annuus L. var. Giganteus) to NO , at concentrations between 5.0 and 72.9 ppb resulted in its uptake by the plants during illumination as well as during darkness. The NO , flux increased linearly with increasing NO , concentrations in both treatments. Evolution of NO by sunflowers was not observed. During illumination, measured and potential N Of uptake were equal, which indicates that the uptake of NO was only limited by stomatal resistance, whereas in darkness, an additional internal resistance to NO. cannot be excluded. After NO , exposures, the highest proportions of the N tracer were fixed as protein nitrogen or as heterocyclic nitrogen compounds and glucosamine. Under both treatments there were remarkably high N enrichments in the fraction 'soluble amino acid nitrogen', indicating a rapid assimilation of the nitrogen derived from NO , A linear relation was found between the δ N values of nitrogen pools investigated and the NO concentrations after exposures in light as well as after night-time fumigation. Apparently, all the enzyme systems involved in the assimilation of the NO nitrogen must be available in sufficient amounts in plant tissues during exposures. Even after exposures in the dark, about 95% of the absorbed NO , nitrogen was fixed in reduced organic nitrogen compounds, which indicates that the assimilation of atmospheric NO , might not depend on the production of reduction equivalents by photosynthesis. The mechanism of NO, assimilation via nitrate/nitrite reductase within the exposed sunflowers is discussed.