Drying of fibrous roots strengthens the negative power relation between biomechanical properties and diameter

Abstract

Aims Test the effects of root drying on biomechanical properties of fibrous roots. Methods Tensile strength and Young’s modulus of Festuca arundinacea roots were tested after full hydration and during progressive drying. Root diameter, water loss, and water content were measured for all treatments. Results Hydrated roots showed weak relations between biomechanical properties and diameter. After only 30 min air-drying, both tensile strength and Young’s modulus increased significantly in thin roots (< 1 mm) and after 60 min drying, both strength and Young’s modulus showed a negative power relation with root diameter. The maximum strength and Young’s modulus values recorded after 60 min drying were respectively three- and four-times greater than in hydrated roots. Strength and Young’s modulus increased rapidly when water content dropped below 0.70 g g−1. These biomechanical changes were the result of root diameter shrinkage of up to 50% after 60 min drying, driven by water loss of up to 0.7 g g−1. Conclusions Strength and Young’s modulus largely increased with root drying. We suggest controlling root moisture and testing fully hydrated roots as standard protocol, given that slope instability is generally caused by heavy rainfall events and loss of matric suction.

Intra‐ and inter‐specific variation in root mechanical traits for twelve herbaceous plants and their link with the root economics space

“…These results contrast with most previous studies showing that tensile strength and stiffness of roots decrease with increasing diameter, not only in herbaceous species, but also in shrubs and trees (Fan and Su 2008, Mao et al 2012, Loades et al 2015). The absence of a relationship between tensile strength and diameter is, however, not surprising, given the number of factors other than diameter that impact strength, such as root age (Boldrin et al 2021), water content (Ekeoma et al 2021), topological order and anatomy (Mao et al 2018). In order to reduce these confounding effects, we normalized root sampling: 1) roots were sampled according to root type and location within the root system, 2) > 99% of roots had a diameter of < 2 mm, 3) roots were fully hydrated to avoid drying, that influences diameter and mechanical properties and 4) in each sampling population, equal or quasi‐equal numbers of replicates were ensured (Table 2).…”

“…low suberisation and lignification) (McCormack et al 2015). Compared to the lignified stele, cortex is usually much weaker mechanically due to its increased content of large, water-filled and thin-walled parenchyma cells (Ekeoma et al 2021). In addition, absorptive roots are generally ephemeral and so do not require a major investment in mechanical quality, whereas transport roots possess both structural and transport functions and so a high mechanical resistance to failure will enhance persistence and lifespan in soil.…”

“…Maximum slope of the quasi-linear part (elastic zone) of the relationship between σ and ε: E r = max(dσ/dε) E r reflects the root material's elasticity or tensile stiffness (Cohen et al 2009, Ekeoma et al 2021). The greater the E r , the stiffer the root tissue.…”

“…Root age or location (e.g. sampling distance from root tip; Dumlao et al 2015, Loades et al 2015, Boldrin et al 2021), water status (Boldrin et al 2018, Ekeoma et al 2021), type (nodal, seminal or lateral; Loades et al 2015), anatomy (Chimungu et al 2015, Mao et al 2018) and topological order (Mao et al 2018) have all been found to better explain mechanical trait variation than diameter alone. Roots of most plant species can be classed into two root types: 1) absorptive roots, which represent the most distal roots dedicated to the acquisition and uptake of resources, and 2) transport roots, that occur at higher branching orders and have mainly transport and mechanical functions (McCormack et al 2015).…”