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]/[O-2] ratio. The present [CO2] does nor saturate C-3 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 mu mol mol(-1), whereas soybean responded up to 990 mu mol mol(-1) Rubisco concentration declined under CO2-enrichment and increasing temperatures, more so in rice than soybean. The rubisco k(cat) 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 C-3 species differ in response to [CO2] and temperature, CO2-enrichment can moderate adverse effects of temperature extremes.