Carbon Concentration Mechanisms In Two Ottelia Species

The unstirred layer between plant and water, lacking of functional stomas, low rate of CO2diffusion etc., these factors result in low concentration of inorganic carbon of aquatic plants. Biochemical, physiological and morphological mechanisms which concentrate inorgan carbon are known for freshwater macrophytes, furthermore, these can be turned off under conditions of carbon sufficiency. These mechanisms include thin blades, C4pathway, Crassulacean acid metabolism (CAM) and HCO3--use. Two freshwater macrophytes, Ottelia alismoides and Ottelia acuminata, were grown at low (mean5μmol L-1) and high (mean400μmol L-1) CO2concentrations under natural conditions. The ratio of PEPC to RuBisCO was1.8in O. acuminata in both treatments. In O. alismoides, this ratio was2.8and5.9when grown at high and low CO2, respectively, as a result of a2-fold increase of PEPC activity. The activity of PPDK was similar to and changed in-line with PEPC (1.9-fold change). The activity of the decarboxylating NADP-malic enzyme (ME) was very low in both species while NAD-ME activity was high and increased with PEPC activity in O. alismoides. These results suggest that O. alismoides might perform a type of C4metabolism with NAD-ME decarboxylation, despite lacking Kranz anatomy. The C4-activity was still present at high CO2suggesting that it could be constitutive. O. alismoides at low CO2showed diel acidity variation of up to34μequiv g-1FW, while the content of Malic acid and PEPC activity also showed deil changes that indicating it may also operate a form of Crassulacean Acid Metabolism (CAM). pH-drift experiments showed that both species were able to use bicarbonate. Thus the two species appear to regulate their carbon concentrating mechanisms differently in response to changing CO2. The Hydrocharitaceae have many species with evidence for C4, CAM, or a metabolism involving organic acids, and are worthy of further study.