Rapid screening of wood and leaf tissues: investigating silicon-based phytoliths in Populus trichocarpa for carbon storage applications using laser-induced breakdown spectroscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy

Phytoliths, which are noncrystalline particles of amorphous silica that form inside plant cells, contribute to the global carbon cycle through their ability to occlude organic carbon. The organic carbon within phytoliths is less susceptible to decomposition; thus, a better understanding of the relationship between phytolith formation and silicon levels in plant tissues may enable bioengineered species to maximize carbon capture during litter decomposition. To establish a high-throughput capability for the phenotypic characterization of phytolith formation in plants, this study used laser-induced breakdown spectroscopy (LIBS) and scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS) to investigate the relationship between silicon concentration, silicon distribution, and phytolith formation in Populus trichocarpa. The results demonstrated the ability to use LIBS with a standard addition approach to calibration to quantify silicon concentrations in pelletized wood and leaf plant tissues with limits of detection of 28.9 and 150 ppm, respectively. This technique enabled rapid testing to rank samples based on silicon levels to be used in genome-wide association studies or to screen samples prior to SEM-EDS testing. SEM-EDS mapping of leaves revealed that phytolith formation occurred primarily at moderate silicon concentrations, estimated as surface silicon concentrations between 0.5-6 wt%. In contrast, phytoliths were not observed at very low concentrations (<0.5 wt%) or at high concentrations (>6 wt%) where silicon was instead dispersed across the leaf. Additionally, phytolith size distributions could be quantified using image analyses of silicon maps derived from SEM-EDS. Neither LIBS nor SEM-EDS results indicated a significant relationship between the silicon levels in the leaves and high- and low-silicon expressing genotypes. However, silicon concentrations were strongly associated with the geographical origin of the sample, indicating that seasonality (i.e., plant phenology) or environmental factors, such as the silicon availability in soil, may play an important role in phytolith formation.