| Popis: |
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. |
