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BIOENERGETICS AND PHOTOSYNTHESIS

The Regulation of Rubisco Activity in Response to Variation in Temperature and Atmospheric CO₂ Partial Pressure in Sweet Potato^1,[w]

Department of Botany, University of Toronto, Toronto, Ontario, Canada M5S 3B2

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.

² Present address: Biology Department, Queens University, Kingston, Ontario, Canada K7L 3N6.

^[w] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.066233.

^* Corresponding author; e-mail rsage{at}botany.utoronto.ca; fax 1–416–978–5878.

Received May 26, 2005; returned for revision August 6, 2005; accepted August 9, 2005.

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