Sucrose-phosphate synthase activity in mature rice leaves following changes in growth CO

• Photosynthetic acclimation of C3 plants to elevated atmospheric [CO 2 ] 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 [CO 2 ] in outdoor, sunlit, environmentcontrolled chambers. During late vegetative development treatments were changed to high or low [CO 2 ]. • Within 1 d of changing to low [CO 2 ], sucrose-phosphate synthase (SPS) activation was significantly reduced in mature leaves, while soluble invertase activity decreased. Plants switched to high [CO 2 ] 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 [CO 2 ], 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.