Rubisco activity is associated with photosynthetic thermotolerance in a wild rice (Oryza meridionalis)

Oryza meridionalis is a wild species of rice, endemic to tropical Australia. It shares a significant genome homology with the common domesticated rice Oryza sativa. Exploiting the fact that the two species are highly related but O. meridionalis has superior heat tolerance, experiments were undertaken to identify the impact of temperature on key events in photosynthesis. At an ambient CO2 partial pressure of 38 Pa and irradiance of 1500 mu mol quantam-2 s-1, the temperature optimum of photosynthesis was 33.7 +/- 0.8 degrees C for O. meridionalis, significantly higher than the 30.6 +/- 0.7 degrees C temperature optimum of O. sativa. To understand the basis for this difference, we measured gas exchange and rubisco activation state between 20 and 42 degrees C and modeled the response to determine the rate-limiting steps of photosynthesis. The temperature response of light respiration (Rlight) and the CO2 compensation point in the absence of respiration (G*) were determined and found to be similar for the two species. C3 photosynthesis modeling showed that despite the difference in susceptibility to high temperature, both species had a similar temperature-dependent limitation to photosynthesis. Both rice species were limited by ribulose-1,5-bisphosphate (RuBP) regeneration at temperatures of 25 and 30 degrees C but became RuBP carboxylation limited at 35 and 40 degrees C. The activation state of rubisco in O. meridionalis was more stable at higher temperatures, explaining its greater heat tolerance compared with O. sativa.