Cell walls dominate the cellular allocation of silicon in sedges.

The degree of plant silicification is classically proxied by organ-scale silicon concentration, [Si], which ignores where Si is allocated at the cellular scale. Bridging this scale gap is crucial to elucidate the physiological and ecological roles of silicification in high-Si taxa and its influence on biogeochemical cycles. We used high-resolution X-ray microtomography and cellular-scale element mapping to study silica allocation and silicification patterns in 15 species from the second most-abundant Si-accumulating family worldwide, namely Cyperaceae (sedges). Despite considerable variation in deposition sites and cellular [Si] between species and anatomical areas, we show that at least 78±17% of silica is localized in the epidermis, mostly as a silica layer in the outer tangential cell wall (OTW). Both the thickness and cellular-scale [Si] of this continuous layer, which is up to 18 µm thick, were highly correlated to organ-scale [Si] among species. Our findings demonstrate that bulk silica content is largely governed by deposition in the OTW in sedges, rather than by deposition in cell lumina. These findings open new perspectives for Si-related plant physiological and biogeochemical processes and pave the way for the use of X-ray microtomography not only to visualize, but also to quantify biogenic silica allocation in plants.