Silicon isotopic fractionation under water stress in Sorghum bicolor: evidence from in situ morphotype-specific phytolith analysis

Abstract

Background and Aims Phytolith studies are still facing numerous challenges regarding the available notions of Si absorption from soil and its deposition in aerial organs. This study shows how plant water availability affects the biosilica content and silicon isotopic composition of phytoliths in sorghum (Sorghum bicolor).

Methods Phytoliths were extracted from different plant parts of crops grown experimentally in lysimeters under water stress (WS) and well-watered (WW) conditions and analysed for silicon isotopic composition using femtosecond laser ablation multi-collector inductively coupled plasma-mass spectrometry (fsLA-MC-ICP-MS). This method provided precise isotope ratios of individual phytolith morphotypes.

Results Results indicate that while Si isotopic composition largely reflects watering conditions, single morphotypes present major differences with Bulliform phytoliths demonstrating superior predictive capability for water availability. The distinct Si isotopic signatures observed in Bulliform, Elongate, and Stoma suggest that variations in Si fractionation among morphotypes could be linked to differences in Si absorption and deposition processes, likely mediated by water stress.

Conclusions Our findings align with prior research suggesting that water stress affects Si uptake, potentially altering the Si-water movement relationship. The significant variability in the isotopic data measured indicates the potential involvement of additional environmental, and consequently physiological factors influencing silicon isotope composition in phytoliths, especially Bulliform, which we suggest should be the focus of future research. Our model offers a solid foundation for research in several fields, from agronomic studies aimed at using Si to improve drought-resistance, to palaeoenvironmental and archaeological studies aimed at reconstructing past climate change and human¿environment interactions.