Down-regulation of Rubisco activity under combined increases of CO2 and temperature minimized by changes in Rubisco kcat in wheat

Increases in growth temperature have been observed to affect photosynthesis differently under long-term exposure to ambient-and twice ambient-air CO2 concentrations. This study investigates the causes of this interaction in wheat (Triticum aestivum L.) grown in the field over two consecutive years under temperature gradient chambers in ambient (370 mu mol mol(-1)) or elevated (700 mu mol mol(-1)) atmospheric CO2 concentrations and at ambient or ambient +4 degrees C temperatures, with either a low or a high nitrogen supply. The photosynthesis-internal CO2 response curves and the activity, activation state, k(cat) and amount of Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) were measured, as well as the soluble protein concentration in flag leaves at ear emergence and 8-15 days after anthesis. A high nitrogen supply increased V-cmax, the Rubisco amount and activity and soluble protein contents, but did not significantly change the Rubisco kcat. Both elevated CO2 and above ambient temperatures had negative effects on V-cmax and Rubisco activity, but at elevated CO2, an increase in temperature did not decrease V-cmax or Rubisco activity in relation to ambient temperature. The amounts of Rubisco and soluble protein decreased with elevated CO2 and temperature. The negative impact of elevated CO2 on Rubisco properties was somewhat counteracted at elevated temperatures by an increase in kcat. This effect can diminish the detrimental effects on photosynthesis of combined increases of CO2 and temperature.