Effects of carbon dioxide concentration and nutrition on photosynthetic functions of white birch seedlings

Increases in atmospheric carbon dioxide (CO2) can impact photosynthesis and dry mass production of plants. This study investigated the physiological responses of white birch seedlings to elevated carbon dioxide (CO2) at low and high supplies of nitrogen (N), phosphorus (P) and potassium (K). A 2-way factorial experiment was carried out with birch seedlings grown for 4 months in environment-controlled greenhouses. Elevated CO2 enhanced maximal carboxylation rate and photosynthetically active radiation-saturated electron transport rates were measured after 2.5 and 3.5 months of treatment, as well as actual photochemical efficiency and photosynthetic linear electron transport to carboxylation. Net photosynthetic rate increases were observed as well as increases in photosynthetic water use efficiency (WUE); photosynthetic N efficiency and P efficiency. Stomatal conductance, transpiration rate and the fraction of total photosynthetic linear electron transport partitioned to oxygenation were reduced. Low nutrient availability decreased net photosynthetic rates, WUE, and triose phosphate utilization. However, photosynthetic linear electron transport and N use efficiency increased. There were significant interactive effects of CO2 and nutrition over time, with evidence of photosynthetic up-regulation in response to elevated CO2 in seedlings receiving high nutrition. Photosynthetic depression in response to low nutrient availability was attributed to biochemical limitation rather than stomatal limitation. Elevated CO2 reduced leaf N concentration in seedlings receiving low nutrition, but had no significant effect on leaf P or K concentrations. High nutrient availability generally increased area-based leaf N, P and K concentrations but had negligible effects on K after 2.5 months of treatment. Results suggested that increases in electron partitioning to photorespiration in response to low nutrient availability may be related to photoprotective mechanisms. Low carboxylation capacity at low nutrient availability could potentially produce excess amounts of excited electrons that would be detrimental to the photosynthetic systems. 66 refs., 5 figs