Effects of elevated carbon dioxide concentration on photosynthesis and growth of small birch plants (Betula pendula Roth.) at optimal nutrition

Abstract

ABSTRACTSmall birch plants (Betula pendula Roth.) were grown from seed for periods of up to 70d in a climate chamber at optimal nutrition and at present (350 μmol mol−1) or elevated (700 μmol mol−1) concentrations of atmospheric CO2. Nutrients were sprayed over the roots in Ingestad‐type units. Relative growth rate and net assimilation rate were slightly higher at elevated CO2, whereas leaf area ratio was slightly lower. Smaller leaf area ratio was associated with lower values of specific leaf area. Leaves grown at elevated CO2 had higher starch concentrations (dry weight basis) than leaves grown at present levels of CO2. Biomass allocation showed no change with CO2, and no large effects on stem height, number of side shoots and number of leaves were found. However, the specific root length of fine roots was higher at elevated CO2. No large difference in the response of carbon assimilation to intercellular CO2 concentration (A/Ci curves) were found between CO2 treatments. When measured at the growth environments, the rates of photosynthesis were higher in plants grown at elevated CO2 than in plants grown at present CO2. Water use efficiency of single leaves was higher in the elevated treatment. This was mainly attributable to higher carbon assimilation rate at elevated CO2. The difference in water use efficiency diminished with leaf age. The small treatment difference in relative growth rate was maintained throughout the experiment, which meant that the difference in plant size became progressively greater. Thus, where plant nutrition is sufficient to maintain maximum growth, small birch plants may potentially increase in size more rapidly at elevated CO2.