An overview of how rubisco and carbohydrate metabolism may be regulated at elevated atmospheric CO2 and temperature

Although atmospheric CO2 concentration (CO2) has been up to 16-fold higher than at present, the past several million years have seen atypically low values. Thus, modern-day plants are adapted to cope with a low CO2/O2 ratio. The present CO2 does not saturate C3 photosynthesis, so its doubling produces an “efficiency effect”, but it is not always fully realized. Acclimation to high CO2 during growth can down-regulate photosynthesis, presumably to optimize carbon acquisition and utilization. A primary factor in acclimation is a reduction in rubisco. Two crops, rice and soybean, were used to study this phenomenon. Rice photosynthesis and growth peaked at 500 μmol mol-1 , whereas soybean responded up to 990 μmol mol-1 . Rubisco concentration declined under CO2-enrichment and increasing temperatures, more so in rice than soybean. The rubisco kcat of rice was unaffected by growth CO2 or temperature, but that from soybean was increased by both. In rice the capacity to handle carbohydrate, as measured by sucrose phosphate synthase activity was up-regulated by CO2 -enrichment, but not by temperature. Leaf carbohydrates were increased by CO2, but decreased by higher temperatures, starch more so than sucrose. Even though C3 species differ in response to CO2 and temperature, CO2 -enrichment can moderate adverse effects of temperature extremes.