Imaging the distribution of the stable isotopes of nitrogen 14N and 15N in biological samples by "secondary-ion emission microscopy".

Thanks to the "secondary-ion emission microscope" (CAMECA IMS 300), we have been able to image the distribution of the stable isotopes of nitrogen 14N and 15N in sections of plant roots (spatial resolution better than 1 micron), as well as to estimate the relative concentrations of these isotopes. The plants used (Lupinus spec.) originated from seeds with natural (i.e., 14N) nitrogen and had been fed for a few days with [15N]-nitrate before sampling. We have found in root sections of 6-day-old plants (prepared at 5 mm from the root tip) a clear-cut regionalization of the distribution of 15N between the vascular cylinder and the cortex. The latter contained approximately 5% 15N (of total nitrogen), whereas the relative concentration of the heavy isotope in the vascular cylinder was significantly lower. The observed concentration difference is probably due to the Casparian strip, which is a barrier for the apoplastic diffusion of solutes from the cortex to the vascular cylinder.