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

Abstract

The relationship between leaf photosynthetic capacity (p _{n, max}), net canopy CO₂- and H₂O-exchange rate (NCER and E _t, respectively) and canopy dry-matter production was examined in Lollium 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 E _t measurement was designed and used over an entire growth cycle to calculate a carbon and a water balance. While NCER_max of full-grown canopies was 49% higher at elevated CO₂ level, stimulation of p _n, 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 darkrespiration 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 NCER_max 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.