Leaf and canopy responses of Lolium perenne to long-term elevated atmospheric carbon-dioxide concentration

The relationship between leaf photosynthetic capacity (pn,max), net canopy CO2- and H2O-exchange rate (NCER and Et, respectively) and canopy dry-matter production was examined in Lolium perenne L. cv. Vigor in ambient (363±30 μl·l-1) and elevated (631±43 μl·l-1) CO2 concentrations. An open system for continuous and simultaneous regulation of atmospheric CO2 concentration and NCER and Et measurement was designed and used over an entire growth cycle to calculate a carbon and a water balance. While NCERmax of full-grown canopies was 49% higher at elevated CO2 level, stimulation of pn,max was only 46% (in spite of a 50% rise in one-sided stomatal resistance for water-vapour diffusion), clearly indicating the effect of a higher leaf-area index under high CO2 (approx. 10% in one growing period examined). A larger amount of CO2-deficient leaves resulted in higher canopy dark-respiration rates and higher canopy light compensation points. The structural component of the high-CO2 effect was therefore a disadvantage at low irradiance, but a far greater benefit at high irradiance. Higher canopy dark-respiration rates under elevated CO2 level and low irradiance during the growing period are the primary causes for the increase in dry-matter production (19%) being much lower than expected merely based on the NCERmax difference. While total water use was the same under high and low CO2 levels, water-use efficiency increased 25% on the canopy level and 87% on a leaf basis. In the course of canopy development, allocation towards the root system became greater, while stimulation of shoot dry-matter accumulation was inversely affected. Over an entire growing season the root/shoot production ratio was 22% higher under high CO2 concentration.