Response of rice ribulose-1,5-bisphosphate carboxylase/oxygenase activity to elevated CO2 concentration and temperature

Elevated CO2 increases photosynthetic rates, but many plants, including ric e (Oryza sativa L.), lose photosynthetic capacity via decreases in amount a nd activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO). O ur objective was to determine the effect of both elevated CO2 and temperatu re on rice RuBisCO activity and carbamylation state. Rice (cv. IR-30) was g own at 330 ("ambient") and 660 (elevated) mu mol CO2 mol(-1) air, and at 40 /33/37, 34/27/31, and 28/21/25 degreesC daytime/nighttime/paddy temperature s, respectively. At 23 d after planting (DAP), leaves were sampled in pre-d awn darkness and in midday sunlight. At 59 DAP, leaves were sampled eight t imes from predawn to sunset. In high sunlight at 23 DAP, carbamylation stat e, manifested by initial (in vivo) RuBisCO activity, remained constant at a ll temperatures in ambient CO2, but decreased with increasing daytime tempe rature from 28 to 40 degreesC in elevated CO2. At 59 DAP, carbamylation sta te response to elevated CO2 and temperature at midday was confounded by var iable sunlight. The total (Mg2+/CO3.-activated) RuBisCO activity at midday was generally not affected by elevated CO2 at either DAP. Thus the endogeno us inhibitor of RuBisCO, 2-carboxyarabinitol-1-phosphate (CA1P), was not re sponsible for changes of initial RuBisCO activity in response to elevated C O2 in high sunlight. In darkness at 23 DAP, under elevated CO2, percentage of potential maximum activity of RuBisCO was similar across all temperature s. Under ambient CO2, percentage of potential maximum activity of RuBisCO w as similar to elevated CO2 at 34 degreesC, but increased at 28 degreesC and decreased at 40 degreesC, due to changes in inhibition of RuBisCO by CA1P. RuBisCO protein per unit total soluble protein was generally not affected by CO2. Photosynthetic rates were always greater in elevated CO2. Thus, hig h CO2 overcame lack of complete carbamylation of RuBisCO in maintaining hig h rates of CO2 uptake at elevated temperatures.