Root hairs and mycorrhiza represent alternative phylogenetically conserved strategies for belowground absorptive surface maximization

Plants take up nutrients from the soil while investing in absorptive root surface or mycorrhizal partners. Root hairs - a major structure for nutrient uptake and cheap to build - increase the absorptive root surface. As such they are an important component of plant resource economics but largely neglected in root economic concepts so far.

This is mainly due to data scarcity, which we set out to overcome by measuring root-hair traits on 82 European grassland species in a greenhouse experiment. Using fluorescence and light microscopy, root-hair length and incidence was measured along with mycorrhizal colonization.

We found a phylogenetically conserved trade-off between plant investment into root hairs and mycorrhiza. A similar trade-off between root-hair incidence and mycorrhiza occurred at the intraspecific level, while patterns were heterogeneous among species. Plant species with high colonization rates showed the highest variability in root-hair incidence.

We conclude that plants vary along a gradient ranging from investment into root hairs as part of a “do-it-yourself” strategy to collaboration with mycorrhizal fungi while showing intraspecific variation in root-hair incidence. These findings demonstrate that root hairs play a fundamental role in fine-root trait variation and need to be considered when studying belowground plant economic strategies.