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 glasshouse experiment. Using fluorescence and light microscopy, root-hair length and incidence were measured along with mycorrhizal colonization. We found a phylogenetically conserved trade-off between plant investment into root hairs and mycorrhiza, which captures functional differences between obligatory and non-mycorrhizal plants as well as between grasses and legumes. This trade-off defines a novel dimension in fine-root trait variation. We conclude that plants invest in root hairs as part of a 'do-it-yourself' strategy in absorptive surface maximization for soil exploration. 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.