Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2001
Publication Date: N/A
Interpretive Summary: The concentration of carbon dioxide (CO2) in the earth's atmosphere is predicted to double sometime within this century. Some plants acclimate to long-term growth under elevated atmospheric CO2 by adjusting their photosynthesis, the process by which they absorb CO2 from the air. It is not well understood how they do this, but it is known that acclimation to high CO2 is often correlated with an over-abundance of soluble sugars. We grew rice in three outdoor, sunlit, environment-controlled chambers under ambient atmospheric growth CO2 concentration (350 ppm). At 34 days after planting one of the chambers was switched to low (175 ppm) and another to high (700 ppm) CO2. Our objective was to examine how switching growth CO2 concentration effects the activity of key enzymes controlling the distribution of sugars within the plant, to gain a better understanding of what causes acclimation to high CO2. Within a short time after switching rice to high CO2, sucrose production greatly increased in leaves, while it decreased in leaves of plants switched to low CO2. Even though sucrose production significantly increased in the leaves of plants switched to high CO2, most of the sucrose was transported to the stems of plants, rather than remaining in their leaves. The results of this experiment suggest that the control of sugar production in rice under higher and lower CO2 concentrations than we presently experience is closely linked to the speed of photosynthesis plus demand for sugars to fuel growth. This information will help scientists to better predict the type of plants that may acclimate to a doubling of CO2 in the earth's atmosphere, and what traits may have to be altered in future crop selections to fully utilize increasing CO2.
Technical Abstract: Photosynthetic acclimation of C3 plants to elevated atmospheric CO2 is often attributed to the accumulation of soluble carbohydrates. We examined the effects of modifying the carbohydrate source/sink balance on carbohydrate metabolism in mature leaves and partitioning in vegetative tissues of rice (Oryza sativa L.). Plants were grown under ambient atmospheric [CO2] (350 uL L**-1) in outdoor, sunlit, environment-controlle chambers, and during late vegetative development were changed to high (700 uL L**-1) and low (175 uL L**-1) [CO2]. Within 1 d after switching to low CO2, sucrose-phosphate synthase (SPS) activation was significantly reduced in mature leaves, while soluble invertase activity decreased 43%. Plants switched to high CO2 showed a 23 and 24% increase in SPS substrate- saturated and substrate-limited activities, respectively, and invertase activity declined by 20%. 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 dramatically. More than 70% of this increase was due to sucrose accumulation, indicating that excess assimilate was being exported to vegetative sinks. Results indicate that immediately following source/sink modification, regulatory adjustments in key enzymes controlling carbohydrate metabolism were linked to feed-forward rather than feedback processes.