Sucrose-Phosphate Synthase Activity in Mature Rice Leaves Following Changes in Growth CO2 Is Unrelated to Sucrose Pool Size

• Photosynthetic acclimation of C3 plants to elevated atmospheric [ CO2] is often attributed to soluble carbohydrate accumulation. We report the effects of modifying the carbohydrate source-sink balance on carbohydrate metabolism in mature leaves and partitioning in vegetative tissues of rice (Oryza sativa). • Plants were grown under ambient atmospheric [ CO2] in outdoor, sunlit, environment-controlled chambers. During late vegetative development treatments were changed to high or low [ CO2]. • Within 1 d of changing to low [ CO2], sucrose-phosphate synthase (SPS) activation was significantly reduced in mature leaves, while soluble invertase activity decreased. Plants switched to high [ CO2] showed increases in SPS substrate-saturated and substrate-limited activities and a decline in invertase activity. The changes in SPS activity did not correlate with leaf sucrose pool size. By 9 d after the change from ambient to high [ CO2], nonstructural carbohydrates in stems and leaf sheaths increased significantly; > 70% of this increase was due to sucrose accumulation, indicating that excess assimilate was being rapidly exported to vegetative sinks. • Results indicate that immediately following source-sink modification, regulatory adjustments in key enzymes controlling carbohydrate metabolism were linked to feedforward, rather than feedback, processes.