Fine root morphology, anatomy and tissue nitrogen and carbon contents of the first five orders in four tropical hardwood species in Hainan Island, China
海南岛4个热带阔叶树种前5级细根的形态、解剖结构和组织碳氮含量

Aims Knowledge of fine root morphology, anatomy and tissue chemistry is critical to understanding root functions (e.g., longevity), but little is known about these root traits and their relationships in woody plants. We investigated root morphology, anatomy and tissue chemistry of the first five orders in four tropical tree species (Altingia obovata, Cryptocarya chinensis, Elaeocarpus sylvestris and Endospermum chinense) in Jianfengling of Hainan Island, China. Our objectives were to: 1) examine how root morphology (diameter, length, specific root length (SRL) and tissue density), anatomy (cortex thickness and stele to root diameter ratio (V/R)) and tissue chemistry (N and C content) changes with root branch orders and 2) reveal the relationships between anatomical structures and root diameter or tissue N or C concentrations in the four tree species. Methods Tree roots of the four species were sampled in August 2009, and root samples were sorted into different orders. Root morphology of the first five orders was analyzed by the Win-RHIZO system. Root tissue C and N concentrations in roots of each order were analyzed by the Vario MACRO Element Analyzer. Individual roots in each order were made into paraffin slices stained by safranin and fast green to observe root anatomical structures and to calculate cortical thickness, stele diameter and V/R. Important findings From the first to fifth order, root diameter, length and tissue density as well as stele diameter and V/R increased, and SRL and cortex thickness decreased in all species. The first two or three orders exhibited primary development with an intact cortex and lower V/R ratio, whereas higher order roots showed secondary development with no cortex and higher V/R ratio. Correlation analysis indicated that cortex thickness can explain 97% of the variations of root diameter and 70% of stele diameter. In all species, tissue N concentration decreased and tissue C concentrations increased with ascending root order. Moreover, C/N ratio in roots was mainly affected by tissue N rather than tissue C concentrations. These results suggest that there are systematic differences in root morphology, anatomy and tissue chemistry among different orders, and root morphology and tissue chemistry are closely linked to root anatomical traits such as cortex thickness in these tropical tree species.