Jianguo Gao^* 

Department of Ecology, College of Urban and Environmental Sciences, and Key Laboratory for Earth Surface Processes of the Ministry of Education, Peking University, No.5 Yiheyuan Road Haidian District, Beijing 100871, China

*Corresponding author. Email: gaojg@pku.edu.cn

Photo caption: The C. chinensis populations in summer (upper) and early winter (lower). Photos of C. chinensis populations courtesy of Binghua Chen and Binjie Ge.

Although only 1–2% of angiosperms are aquatic species, they are of interest because of their significance in answering the question that puzzled Charles Darwin: how flowering plants evolved. Cladopus chinensis, a submerged perennial herbaceous plant inhabiting tropical and sub-tropical rivers, is useful when studying how the environment influences the morphological evolution and pollination mechanisms of dicotyledonous angiosperms. C. chinensis is affixed onto rocks in fast-flowing water using a haptera, a root-specific anchor. When vigorous growth occurs during summer, these small plants cover the surface of rocks, resembling a green carpet from a distance. C. chinensis flowers in winter, and a bract-like structure, the total bract, grows in the fertile branch. After the fertile branch emerges from the water, the pedicel quickly expands. Once the total bract breaks, dark red flowers bloom from the top of the fertile branch, forming a dark red carpet-like appearance on the stones. C. chinensis grows in rivers with rapid water flow, high oxygen content, and sufficient sunlight. It can only complete its life cycle in rivers with high water quality.

There are several interesting biological questions about C. chinensis. First, C. chinensis relies on haptera, a specialized root for anchored colonization in rapidly flowing rivers. Its flat roots are attached to the rock surface, while shoots grow out from the roots, unlike other angiosperms, with the direction of root growth is vertical to horizontal. The origins of these morphological changes have long interested evolutionary biologists. A recent comparative genomic analysis showed that the C. chinensis genome has many expansions and contractions of both the oxidative phosphorylation and photosynthesis genes, which may be beneficial for their growth under flooded stress conditions (Hortic Res 2020; doi.org/10.1038/s41438-020-0269-5). However, how they overcome the mechanical stimulus caused by rapid flowing water and how this differs from other terrestrial angiosperms in response to gravity still remains unexplored. The second question deals with reproduction. This species carries out vegetative growth in summer but shifts to sexual reproduction during the dry season in winter. The pollination mechanism of such an amphibious flowering plant is unknown. Whether it is pollinated by wind, water currents or raindrops, an animal vector (such as a river arthropod or insect), or through a mixed abiotic–biotic pollination mode remains to be understood (Nat Commun 2016; doi.org/10.1038/ncomms12980). The third question is about how to effectively conserve C. chinensis due to its high-water quality requirements and the completion of reproduction in winter. With accelerated climate warming and the increase of extreme climate events, assessment of the extinction risk faced by this species might become an urgent need.