Ottelia cordata (Wallich) Dandy, a member of the Hydrocharitaceae, is a perennial and heterophyllous aquatic macrophyte, which has ovate-cordate floating leaves and linear or lanceolate submerged leaves. O. cordata grows submerged leaves only in the first year, after that, it only grows floating leaves. It has been reported that the floating leaves of O. cordata can use bicarbonate (HCO₃^–) and perhaps operate the Crassulacean acid metabolism (CAM).  

The Laboratory of Aquatic Plant Biology led by Prof. LI Wei from CAS Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden studied the plasticity of the CO₂-concentrating mechanisms (CCMs) and leaf anatomy in this species responding to the varying CO₂ concentrations in the water, to evaluate further possible effects of CO₂ enrichment on this species. 

The most striking anatomical variations responding to high CO₂ included the enlarged upper epidermal cells and the decreased area of epidermal chloroplasts. Stomata that distributed on the upper surface showed no significant response to high CO₂. In addition to the use of atmospheric CO₂ for its inorganic carbon, the floating leaves of O. cordata can use HCO₃^– in the water and perform C₄-like metabolism regardless of high and low CO₂ concentrations, and operate CAM only at low CO₂.  

Further experimental studies are needed to focus on the regulatory mechanisms in O. cordata floating leaves, that prevent futile cycling among the three CO₂ concentrating mechanisms (bicarbonate use, C₄, and CAM metabolism) and the strategy for exploiting atmospheric CO₂. 

Research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences, and the National Natural Science Foundation of China. 

The results have been published in Frontiers in Plant Science entitled “Responses of Leaf Anatomy and CO₂ Concentrating Mechanisms of the Aquatic PlantOttelia cordata to Variable CO₂”. 

Leaf anatomy and chloroplast ultrastructure of O. cordata floating leaves grown at different CO₂ (Image by WBG)