Secondary ion mass spectrometry imaging of the fixation of 15N-labelled NO in pollen grains

We used secondary ion mass spectrometry to image cellular targets of nitrogen oxides (widespread air pollutants) in pollen grains of birch ( Betula verrucosa Ehrh.) and cockfoot ( Dactylis glomerata L.). The pollen samples were exposed to air supplemented with high doses of ¹⁵NO. The pollen grains were then fixed, dehydrated using a newly developed 'vapour phase' preparation method and embedded in LRW resin. Semithin sections were then analysed. Imaging was performed in scanning mode. As usual, the two isotopes ¹⁴N and ¹⁵N were imaged as ¹²C¹⁴N⁻ and ¹²C¹⁵N⁻, respectively. The isotopic percentages of ¹⁵N were quantitatively determined either by image processing or by direct analysis. We show that the preferential areas of NO fixation in the pollen cell are the sporoderm and discrete intracytoplasmic structures that we tentatively describe as globoid-like structures similar to those encountered in seeds.