Abstract: Aims Aquatic plants play an important role in freshwater ecosystems. Previous works have largely focused on the functional significance of plant above ground parts, with much less attention on the root structures of aquatic plants. In this study, we divided 21 aquatic plants (including five introduced plants) into multiple plant groups (different life forms, monocot/eudicot and introduced/native) with the goal of addressing two questions: (i) what root structures do aquatic plants exhibit, and (ii) are there differences among these plant groups?
Methods Twenty-one aquatic plants belonging to four life forms (free-floating, emergent, floating-leaved and submerged) were collected at the near flowering stage from a typical macrophyte-dominated lake in the Yangtze River Basin, China. The following root topological parameters were quantified: altitude (a), path length (p e), magnitude (M), mean topological length (b), topological index (TI) and normed indices q a and q b .
Important findings The root topological indices TI, q a and q b for the 21 aquatic plants were 0.724 ± 0.013, 0.290 ± 0.031 and 0.152 ± 0.024 (means ± S.E.), respectively, revealing a general pattern of dichotomous branching, except for the aquatic root of Myriophyllum aquaticum (Vell.) Verdc., which displays herringbone branching. All three topological indices were significantly lower for monocots (TI = 0.700 ± 0.130, q a = 0.191 ± 0.149 and q b = 0.086 ± 0.236) than eudicots (TI = 0.752 ± 0.206, q a = 0.405 ± 0.569 and q b = 0.229 ± 0.393), indicating that the roots of monocots are typically more dichotomous-like than those of eudicots. Among the four life forms, the three topological indices for emergent plants (TI = 0.832 ± 0.006, q a = 0.616 ± 0.018 and q b = 0.381 ± 0.014) were significantly higher than those of the other three life forms. Overall, there was no difference between the topological indices of introduced and native aquatic plants, but the introduced species M. aquaticum and Alternanthera philoxeroides (Mart.) Griseb. had both aquatic and edaphic roots as well as unusual functions, which may help explain their strong viability.

Key words: aquatic plants, biological invasion, introduced species, root structure, topological index