Growth, photosynthesis and photorespiration of Lemna gibba: response to variations in CO2 and O 2 concentrations and photon flux density.

Dry weight and Relative Growth Rate of Lemna gibba were significantly increased by CO2 enrichment up to 6000 μl CO2 l(-1). This high CO2 optimum for growth is probably due to the presence of nonfunctional stomata. The response to high CO2 was less or absent following four days growth in 2% O2. The Leaf Area Ratio decreased in response to CO2 enrichment as a result of an increase in dry weight per frond. Photosynthetic rate was increased by CO2 enrichment up to 1500 μl CO2 l(-1) during measurement, showing only small increases with further CO2 enrichment up to 5000 μl CO2 l(-1) at a photon flux density of 210 μmol m(-2) s(-1) and small decreases at 2000 μmol m(-1) s(-1). The actual rate of photosynthesis of those plants cultivated at high CO2 levels, however, was less than the air grown plants. The response of photosynthesis to O2 indicated that the enhancement of growth and photosynthesis by CO2 enrichment was a result of decreased photorespiration. Plants cultivated in low O2 produced abnormal morphological features and after a short time showed a reduction in growth.