Using anatomical traits to understand root functions across root orders of herbaceous species in temperate steppe.

Root anatomical traits play crucial roles in understanding root functions and root form-function linkages. However, the root anatomy and form-function linkages of monocotyledonous and dicotyledonous herbs remain largely unknown. We measured order-based anatomical traits and mycorrhizal colonization rates of 32 perennial herbs of monocotyledons and dicotyledons in a temperate steppe. For monocots, relative constant proportion of cortex and mycorrhizal colonization rates, but increased cell-wall thickening of endodermis and proportion of stele were observed across root orders, indicating slight reduction in absorption capacity and improvement of transportation capacity across orders. For dicots, the cortex and mycorrhizal colonization disappeared in the 4th and/or 5th order roots, while the secondary vascular tissue increased markedly, suggesting significant transition of root functions from absorption to transportation across root orders. The allometric relationships between stele and cortex differed across root orders and plant groups, suggesting different strategies to coordinate the absorption and transportation functions among plant groups. In summary, our results reveled different functional transition patterns across root orders and distinct strategies on coordinating the absorption and transportation of root system between monocots and dicots. These findings will contribute to our understanding of the root form and functions in herbaceous species.