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

Abstract

Rice ( Oryza sativa [L.] cv. IR‐72) was grown for a season in sunlit, controlled‐environment chambers at 350 or 700 µmol CO 2 mol −1 under continuously flooded (unstressed) or drought‐imposed periods at panicle initiation (stressed). The midday canopy photosynthetic rates (P n ), measured at the CO 2 concentration ([CO 2 ]) used for growth, were enhanced by high [CO 2 ] but reduced by drought. High [CO 2 ] increased P n by 18 to 34% for the unstressed plants, and 6 to 12% for the stressed plants. In the unstressed plants, CO 2 enrichment increased water‐use efficiency (WUE) by 26%, and reduced evapotranspiration (ET) by 8 to 14%. Both high [CO 2 ] and severe drought decreased the activity and content of ribulose bisphosphate carboxylase‐oxygenase (Rubisco). High‐CO 2 ‐unstressed plants had 6 to 22% smaller content and 5 to 25%, lower activity of Rubisco than ambient‐CO 2 ‐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‐CO 2 treatments. The apparent catalytic turnover rate (K cat ) of midday fully activated Rubisco was not altered by high [CO 2 ], but severe drought reduced K cat by 17 to 23%. Chloroplasts of the high‐CO 2 leaves contained more, and larger starch grains than those of the ambient CO 2 leaves. High [CO 2 ] 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 CO 2 treatments. The activity of leaf sucrose phosphate synthase of unstressed plants was not affected by high [CO 2 ], whereas that of ambient‐CO 2 ‐grown plants was reduced 45% by severe drought. Reduction in ET and enhancements in both P n and WUE for rice grown under high [CO 2 ] helped to delay the adverse effects of severe drought and allowed the stressed plants to assimilate CO 2 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.