Regulation of growth and photosynthetic performance in Elodea canadensis in response to inorganic nitrogen

1. Many submerged aquatic macrophytes possess inducible biochemical or biophysical mechanisms that can enhance the internal inorganic carbon concentration above that which can be achieved by diffusive uptake of CO 2 coupled with C-3 photosynthesis. 2. The possible interaction between nitrogen economy and use of a carbon concentrating mechanism was studied in Elodea canadensis, which has a mechanism based on bicarbonate usage. The nitrogen content of the plants was varied from -800 to -3500 μmol N g -1 DW by growing them in hydroponic cultures at a range of inorganic nitrogen concentrations. 3. The relative growth rate increased with increasing tissue-N from 0.020 day -1 at 800 μmol N g -1 DW to 0.045 day -1 at 1600 μmol N g -1 DW. The photosynthetic performance of the plants was also affected by tissue-N. The photosynthetic capacity increased from 172 to 465 μmol CO 2 g -1 DW h -1 and the HCO 3 - uptake capacity from 63 to 310 μmol C g -1 DW h -1 with increasing tissue-N. The CO 2 conductance, a measure of CO 2 affinity, increased four times and the bicarbonate conductance 10 times. 4. High nitrogen concentrations are required in Elodea to maintain an efficient carbon-assimilation apparatus and an efficient carbon-concentrating mechanism. 5. The interdependence of nitrogen-use efficiency for photosynthesis (PNUE) and nitrogen content was dependent on the inorganic carbon concentration at which PNUE was measured. PNUE measured at saturating inorganic carbon concentrations declined with increasing tissue-N, presumably as a result of increased allocation of nitrogen into non-photosynthetic pools at high tissue-N. In contrast, PNUE increased with increasing tissue-N when measured at sub-saturating inorganic carbon concentrations. 6. The total Rubisco activity was positively correlated with tissue-N in Elodea and was substantially higher than the photosynthetic capacity indicating that Rubisco may not play a primary role in regulating the photosynthetic capacity of Elodea. Rather, it may have a role in regulating photosynthesis at low inorganic carbon concentrations, as suggested by the observed linear relationship between CO 2 conductance and Rubisco