The Regulation of Rubisco Activity in Response to Variation in Temperature and Atmospheric CO2 Partial Pressure in Sweet Potato

Abstract

The temperature response of net CO2 assimilation rate (A), the rate of whole-chain electron transport, the activity and activation state of Rubisco, and the pool sizes of ribulose-1,5-bisphosphate (RuBP) and 3-phosphoglyceric acid (PGA) were assessed in sweet potato (Ipomoea batatas) grown under greenhouse conditions. Above the thermal optimum of photosynthesis, the activation state of Rubisco declined with increasing temperature. Doubling CO2 above 370 μbar further reduced the activation state, while reducing CO2 by one-half increased it. At cool temperature (<16°C), the activation state of Rubisco declined at CO2 levels where photosynthesis was unaffected by a 90% reduction in O2 content. Reduction of the partial pressure of CO2 at cool temperature also enhanced the activation state of Rubisco. The rate of electron transport showed a pronounced temperature response with the same temperature optimum as A at elevated CO2. RuBP pool size and the RuBP-to-PGA ratio declined with increasing temperature. Increasing CO2 also reduced the RuBP pool size. These results are consistent with the hypothesis that the reduction in the activation state of Rubisco at high and low temperature is a regulated response to a limitation in one of the processes contributing to the rate of RuBP regeneration. To further evaluate this possibility, we used measured estimates of Rubisco capacity, electron transport capacity, and the inorganic phosphate regeneration capacity to model the response of A to temperature. At elevated CO2, the activation state of Rubisco declined at high temperatures where electron transport capacity was predicted to be limiting, and at cooler temperatures where the inorganic phosphate regeneration capacity was limiting. At low CO2, where Rubisco capacity was predicted to limit photosynthesis, full activation of Rubisco was observed at all measurement temperatures.