The relationship between leaf photosynthetic capacity (pn, max), net canopy CO2- and H2O-exchange rate (NCER andEt, respectively) and canopy dry-matter production was examined inLollium perenneL. cv. Vigor in ambient (363±30 μl· l-1) and elevated (631±43 μl·l-1) CO2concentrations. An open system for continuous and simultaneous regulation of atmospheric CO2concentration and NCER andEtmeasurement was designed and used over an entire growth cycle to calculate a carbon and a water balance. While NCERmaxof full-grown canopies was 49 higher at elevated CO2level, stimulation ofpn, 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-CO2effect was therefore a disadvantage at low irradiance, but a far greater benefit at high irradiance. Higher canopy darkrespiration rates under elevated CO2level 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 NCERmaxdifference. While total water use was the same under high and low CO2levels, 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 CO2concentr
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