Responses of CO ₂-concentrating mechanisms and photosynthetic characteristics in aquatic plant Ottelia alismoides following cadmium stress under low CO ₂

The effects of cadmium (Cd) have been investigated in an aquatic plant Ottelia alismoides grown under low CO₂. Under low CO₂, no Cd treated O. alismoides operated three carbon dioxide-concentrating mechanisms (CCMs) efficiently, including HCO₃^- acquisition, C4 and CAM photosynthesis. After 4 days of treatment with 200 μM and 2000 μM Cd, O. alismoides exhibited an elevated Cd accumulation along with the increasing Cd concentration. Both Cd treatments induced appreciable phytotoxicities in O. alismoides. The leaves showed chlorosis symptoms and the anatomy as well as chloroplast ultrastructure were obviously damaged. Significant decreases in the content of pigments, chlorophyll fluorescence (Fv/Fm and Yield of PS II) and carbon isotope ratio (δ¹³C) were measured in leaf extracts of O. alismoides grown with both concentrations of Cd. In addition, the pH-drift technique showed that both Cd-treated O. alismoides plants could not uptake HCO₃^-. The maximum and minimum acidity in Cd-exposed O. alismoides were greatly decreased and the diurnal change of acidity was absent in both Cd treated plants. Furthermore, significant decreases in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), pyruvate phosphate dikinase (PPDK) and phosphoenolpyruvate carboxylase (PEPC) activities were also found at Cd treated O. alismoides plants, indicating the disturbance within C4 cycle. The alterations in the functionality of CCMs in O. alismoides induced by Cd might be related with the inhibition of the enzymes such as carbonic anhydrase (CA) and PEPC involved in inorganic carbon fixation, and the destruction of chloroplasts, as well as the re-allocation of energy and nutrients involved in CCMs and Cd detoxification.

Keywords: Acidity; Cadmium; Carbon dioxide-concentrating mechanisms; Ottelia alismoides; pH-drift.