Responses of Leaf Anatomy and CO2 Concentrating Mechanisms of the Aquatic Plant Ottelia cordata to Variable CO2

Abstract

Acclimation to variable CO₂ was studied in floating leaves of the freshwater monocot Ottelia cordata grown in either low or high CO₂. 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, and the stomatic chamber area, showed no significant response to high CO₂. pH-drift experiments indicated that floating leaves of O. cordata were able to use bicarbonate regardless of CO₂ concentrations. Photosynthetic enzyme activities and patterns of organic acids fluctuation confirmed that floating leaves of O. cordata can operate CAM only at low CO₂, and perform C₄-like metabolism at both high and low CO₂. Overall, the present results imply that the floating leaves of O. cordata does not just rely on the atmospheric CO₂ for its inorganic carbon, but is also dependent on CO₂ and bicarbonate in the water. By showing these effects of CO₂ variation, we highlight the need for further experimental studies 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, as well as studies on the detailed biochemical pathway for C₄ and CAM metabolism in this species.

Keywords: C4; CO2 availability; CO2 concentrating mechanisms; bicarbonate use; crassulacean acid metabolism; organic acids.

Figure 1

Leaf anatomy and chloroplast ultrastructure of O. cordata floating leaves grown at low and high CO₂. (A–C) Anatomy of O. cordata floating leaf grown under low CO₂. (D–F) Anatomy of O. cordata floating leaf grown under high CO₂. (G) Chloroplast in epidermal cells under low CO₂. (H) Chloroplast in mesophyll cells under low CO₂. (I) Chloroplast in epidermal cells under high CO₂. (J) chloroplast in mesophyll cells under high CO₂. a, air space; le, lower epidermal cell; lm, lower mesophyll cell; m, mitochondria; s, starch; st, stoma; sc, stomatic chamber; ue, upper epidermal cell; um, upper mesophyll cell; w, cell wall.

Figure 2

Influence of CO₂ concentrations on activities of enzymes from O. cordata floating leaves collected at dusk (21:00 h, before the lights turn off) and dawn (07:00 h, before the lights turn on), as well as the correlations between activity of Rubisco, pyruvate phosphate dikinase (PPDK) and activity of phosphoenolpyruvate carboxylase (PEPC) at dusk and dawn, grown at low and high concentrations of CO₂. (A) Rubisco activity. (B) PEPC activity. (C) Ratio of PEPC to Rubisco activity. (D) PPDK activity. (E) NADP-malic enzyme (NADP-ME) activity. (F) Correlation between activity of Rubisco and activity of PEPC. (G) Correlation between activity of PPDK and activity of PEPC. In (A~E), the mean values (n=3~4) with their s.d. are shown. Different letters show significant differences (p<0.05, post hoc Duncan’s test) among treatments. In (F~G), each point represents the activity measured from an individual sample from high CO₂ and low CO₂ acclimated plants. Pearson correlation was used to test the correlation between activity of PEPC and Rubisco, PPDK and PEPC.

Figure 3

Influence of CO₂ concentrations on acidity and concentrations of organic acids from O. cordata floating leaves collected at dusk (21:00 h, before the lights turn off) and dawn (07:00 h, before the lights turn on). (A) Acidity. (B) Malic acid. (C) Citric acid. (D) Aspartic acid. (E) trans-Aconitic acid. (F) Fumaric acid. LC means low CO₂; HC means high CO₂. The mean values (n=3) with their s.d. are shown. Different letters indicate significant differences (p<0.05, post hoc Duncan’s test) among treatments.