The worldwide allometric relationship in anatomical structures for plant roots
The anatomical structures, i.e., the cortex and stele, are fundamental for the absorptive function of plant roots. Unraveling how the allometric structures are assembled in absorptive roots is essential for our understanding the plant ecology, physiology and responses to global environmental changes. In this review study, we first compile a globally largest dataset on key root structural traits, i.e., root diameter, cortex thickness and stele radius across 512 species. Using this largest dataset, we confirm an allometric relationship of absorptive root structures in a previous study using a much smaller species pool, i.e., the cortex thickness increased much faster than the stele radius with increasing root diameter. The allometric relationship is further validated within and across different plant growth forms (woody, grass, and liana species), mycorrhiza types (arbuscular mycorrhiza, ectomycorrhiza, and orchid mycorrhizas), phylogenetic gradients (from ferns to Orchidaceae of primitive angiosperms), and environmental change scenarios (e.g., the elevation of atmospheric CO2 concentration and nitrogen fertilization), supporting the universal allometric relationship in plant roots. We then summarized recent proceedings as well as possible issues on mechanisms underlying the root allometric relationship. The ecological and evolutionary implications for this allometric relationship in roots are also discussed. Finally, we propose several directions that should be stressed in future studies regarding the allometric relationship in plant roots.