UPTAKE OF (NO2)-N-15 BY SUNFLOWER (HELIANTHUS-ANNUUS) 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 (NO2) -N-15 at concentrations between 5.0 and 72.9 ppb resulted in its uptak e by the plants during illumination as well as during darkness. The NO 2 flux increased linearly with increasing NO2 concentrations in both t reatments. Evolution of NO2 by sunflowers was not observed. During ill umination, measured and potential NO2 uptake were equal, which indicat es that the uptake of NO2 was only limited by stomatal resistance, whe reas in darkness, an additional internal resistance to NO2 cannot be e xcluded. After (NO2)-N-15 exposures, the highest proportions of the N- 15 tracer were fixed as protein nitrogen or as heterocyclic nitrogen c ompounds and glucosamine. Under both treatments there were remarkably high N-15 enrichments in the fraction 'soluble amino acid nitrogen', i ndicating a rapid assimilation of the nitrogen derived from (NO2)-N-15 . A linear relation was found between the delta(15)N values of nitroge n pools investigated and the (NO2)-N-15 concentrations after exposures in light as well as after night-time fumigation. Apparently, all the enzyme systems involved in the assimilation of the NO2 nitrogen must b e available in sufficient amounts in plant tissues during exposures. E ven after exposures in the dark, about 95% of the absorbed (NO2)-N-15 nitrogen was fixed in reduced organic nitrogen compounds, which indica tes that the assimilation of atmospheric NO2 might not depend on the p roduction of reduction equivalents by photosynthesis. The mechanism of NO2 assimilation via nitrate/nitrite reductase within the exposed sun flowers is discussed.