Elevated CO2 and water deficit effects on photosynthesis, ribulose bisphosphate carboxylase-oxygenase, and carbohydrate metabolism in rice

Rice (Oryza sativa [L.] cv. IR-72) was grown for a season in sunlit, controlled-environment chambers at 350 or 700 mu mol CO2 mol(-1) under continuously flooded (unstressed) or drought-imposed periods at panicle initiation (stressed). The midday canopy photosynthetic rates (P-n), measured at the CO2 concentration ([CO2]) used for growth, were enhanced by high [CO2] but reduced by drought. High [CO2] increased P-n by 18 to 34% for the unstressed plants, and 6 to 12% for the stressed plants. In the unstressed plants, CO2 enrichment increased water-use efficiency (WUE) by 26%, and reduced evapotranspiration (ET) by 8 to 14%. Both high [CO2] and severe drought decreased the activity and content of ribulose bisphosphate carboxylase-oxygenase (Rubisco). High-CO2-unstressed plants had 6 to 22% smaller content and 5 to 25% lower activity of Rubisco than ambient-CO2-unstressed plants. Under severe drought, reductions of Rubisco were 53 and 27% in activity and 40 and 12% in content, respectively, for ambient- and high-CO2 treatments. The apparent catalytic turnover rate (K-cat) of midday fully activated Rubisco was not altered by high [CO2], but severe drought reduced K-cat by 17 to 23%. Chloroplasts of the high-CO2 leaves contained more, and larger starch grains than those of the ambient-CO2 leaves. High [CO2] did not affect the leaf sucrose content of unstressed plants. In contrast, severe drought reduced the leaf starch and increased the sucrose content in both CO2 treatments. The activity of leaf sucrose phosphate synthase of unstressed plants was not affected by high [CO2], whereas that of ambient-CO2-grown plants was reduced 45% by severe drought. Reduction in ET and enhancements in both P-n and WUE for rice grown under high [CO2] helped to delay the adverse effects of severe drought and allowed the stressed plants to assimilate CO2 for an extra day. Thus, rice grown in the next century may utilize less water, use water more efficiently, and be able to tolerate drought better under some situations.