Across 33 broad-leaved deciduous woody species, silicon enhances leaf lamina stiffness but not tensile strength whereas cellulose enhances both.

Silicon (Si) has been hypothesized to be a metabolically cheaper substitute for carbon-based cell wall components to support leaves. However, how the biomechanical function of Si, deposited as amorphous silica, differs from cell wall components remains untested. Here, we tested the hypothesis that species with higher leaf Si concentrations have stiffer but more brittle leaf lamina. We measured the mechanical properties, including modulus of elasticity (E), tensile strength (σ), and maximum strain (ε), tissue density, and the concentrations of Si and cell wall components for 33 deciduous broad-leaved woody species. Multiple regression results showed that tissue density, Si concentration, and cellulose concentration contributed positively to E and negatively to ε. By contrast, tissue density and cellulose concentration, but not Si concentration, contributed to σ. No significant contribution of lignin concentration to mechanical properties was detected. These results suggest that Si might function as a substitute for cellulose to increase stiffness but not the strength of a lamina. Greater Si concentration decreased ε without increasing σ, which made the lamina more brittle. The brittleness associated with Si might explain a potential cost or disadvantage of using Si, which would elucidate the trade-offs between species with different leaf Si concentrations.