Photosynthesis and Rubisco kinetics in spying wheat and meadow fescue under conditions of simulated climate change with elevated CO2 and increased temperatures

Spring wheat (Triticum aestivum L. cv. Polkka) and meadow fescue (Festuca p ratensis Hudson cv. Kalevi) were grown in ambient and elevated (700 mu l 1( -1)) carbon dioxide concentration both at present ambient temperatures and at temperatures 3 degrees C higher than at present simulating a future clim ate. The CO2 concentrations were elevated in large (3 m in diameter) open t op chambers and the temperatures in a greenhouse built over the experimenta l field. The photosynthetic rate of both wheat and meadow fescue was 31-37% higher in elevated carbon dioxide (eCO(2)) than in ambient CO2(aCO(2)) thr oughout the growing season. The enhancement in wheat photosynthesis in eCO( 2) declined 10-13 days before yellow ripeness, at which point the rate of p hotosynthesis in both CO2 treatments declined. The stomatal conductance of wheat and meadow fescue was 23-36% lower in eCO(2) than in aCO(2). The amou nt and activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco ) in wheat were lower under conditions of eCO(2), except at elevated temper atures in 1993 when there was a clear yield increase. There was no clear ch ange in the amount and activity of Rubisco in meadow fescue under eCO(2) at either elevated or ambient temperature. This suggests that adaptation to e levated CO2 at biochemical level occurs only when there is insufficient sin k for photosynthetic products. While the sink size of wheat can be increase d only by introducing new, more productive genotypes, the sink size of mead ow fescue can be regulated by fitting the cutting schedule to growth.