Photosynthetic traits of Australian wild rice (Oryza australiensis) confer tolerance to extreme daytime temperatures

Above similar to 32 degrees C, impaired photosynthesis compromises the productivity of rice. We compared leaf tissues from heat-tolerant wild rice (Oryza australiensis) with temperate-adapted O. sativa after sustained exposure to heat, as well as diurnal heat shock. Leaf elongation and shoot biomass in O. australiensis were unimpaired at 45 degrees C, and soluble sugar concentrations trebled during 10 h of a 45 degrees C shock treatment. By contrast, 45 degrees C slowed growth strongly in O. sativa. Chloroplastic CO2 concentrations eliminated CO2 supply to chloroplasts as the basis of differential heat tolerance. This directed our attention to carboxylation and the abundance of the heat-sensitive chaperone Rubisco activase (Rca) in each species. Surprisingly, O. australiensis leaves at 45 degrees C had 50% less Rca per unit Rubisco, even though CO2 assimilation was faster than at 30 degrees C. By contrast, Rca per unit Rubisco doubled in O. sativa at 45 degrees C while CO2 assimilation was slower, reflecting its inferior Rca thermostability. Plants grown at 45 degrees C were simultaneously exposed to 700 ppm CO2 to enhance the CO2 supply to Rubisco. Growth at 45 degrees C responded to CO2 enrichment in O. australiensis but not O. sativa, reflecting more robust carboxylation capacity and thermal tolerance in the wild rice relative.