Acclimation of the summer annual species, Lolium temulentum, to CO2 enrichment

Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 mu mol mo l(-1) CO2) and elevated CO2 (700 mu mol mol(-1) CO2) at two irradiances (15 0 and 500 mu mol m(-2) s(-1)) for 35 days at which point the plants were ha rvested. Elevated CO2 did not modify relative growth rate or biomass at eit her irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated CO2 and plants had a greater number of shorter tillers, particularly at th e lower growth irradiance. Both light-limited and light-saturated rates of photosynthesis were stimulated. The amount of ribulose-1,5-bisphosphate car boxylase-oxygenase (Rubisco) protein was increased at elevated CO2, but max imum extractable Rubisco activities were not significantly increased. A pro nounced decrease in the Rubisco activation stale was found with CO2 enrichm ent, particularly at the higher growth irradiance. Elevated-CO2-induced cha nges in leaf carbohydrate composition were small in comparison to those cau sed by changes in irradiance. No CO2-dependent effects on fructan biosynthe sis were observed. Leaf respiration rates were increased by 68% in plants g rown with CO2 enrichment and low light. We conclude that high CO2 will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio. At low irradiances, biomass is more li mited by increased rates of respiration than by CO2-induced enhancement of photosynthesis.