Elodea canadensis under N and CO2 limitation:: Adaptive changes in Rubisco and PEPCase activity in a bicarbonate user

Diffusion of CO2 in water is 10,000 times slower than in air. Because of this photosynthesis in submerged aquatic macrophytes is often limited by CO2 availability. Elodea canadensis shows HCO3- utilization under conditions of CO2 limitation. A closely related species, Hydrilla verticillata, which also belongs to the Hydrocharitaceae, shows a C-4 like mechanism when grown under so-called summer conditions where low availability of CO2 is expected. Here it is shown that a similar C-4-like mechanism is possibly also present in E. canadensis. To study the possible interference between the two processes, E. canadensis was grown at low and high CO2 and NO3-, conditions expected to affect HCO3- utilization. Moreover the effect was studied of these growth conditions on the occurrence of the C-4 state. A low G(CO2) was observed in E. canadensis when it was grown under low CO2 conditions indicative for plants in the C-4-state. The low G(CO2) was related to a change in the ratio Rubisco and PEPCarboxylase indicating a central role of these enzymes. The very same conditions induced HCO3- utilization in this species. Both mechanisms thus operate in the same plant simultaneously. Growing E. canadensis at high NO3-, 0.75 mM, had a strong stimulatory effect on O-2 production in the light both in low and high CO2 grown plants, suggesting that it has no direct relation with HCO3- utilization. The actual cause for this stimulatory effect of high N on photosynthesis is unclear.