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.