23Na NMR microimaging: a tool for non-invasive monitoring of sodium distribution in living plants.

Detailed knowledge of the sodium (Na) distribution within the tissues of highly salt-tolerant Australian native species could help in understanding the physiological adaptations of salt-tolerance or salt-sensitive plants. 23Na nuclear magnetic resonance (NMR) microimaging is presented as a tool to achieve this goal. Maps of the Na distribution in stem tissue were obtained with an in-plane resolution of approximately125 µm and a slice thickness of 4 mm. Simultaneously recorded high resolution 1H NMR images showing water distribution in the same slice with 31 µm in-plane resolution and 1 mm slice thickness, were used as an anatomical reference together with optical micrographs that were taken immediately after the NMR experiments were completed. To quantify the Na concentration, reference capillaries with known NaCl concentrations were located in the NMR probe together with the plant sample. Average concentration values calculated from signal intensities in the tissue and the capillaries were compared with concentration values obtained from atomic emission photometry and optical microscopy performed on digested stem sections harvested immediately after NMR experiments. Results showed that 23Na NMR microimaging has great potential for physiological studies of salt stress at the macroscopic level, and may become a unique tool for diagnosing salt tolerance and sensitivity.