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 atypi cally low values. Thus, modern-day plants are adapted to cope with a l ow [CO2]/[O-2] ratio. The present [CO2] does nor saturate C-3 photosyn thesis, so its doubling produces an ''efficiency effect'', but it is n ot always fully realized. Acclimation to high [CO2] during growth can down-regulate photosynthesis, presumably to optimize carbon acquisitio n and utilization. A primary factor in acclimation is a reduction in r ubisco. Two crops, rice and soybean, were used to study this phenomeno n. Rice photosynthesis and growth peaked at 500 mu mol mol(-1), wherea s soybean responded up to 990 mu mol mol(-1) Rubisco concentration dec lined under CO2-enrichment and increasing temperatures, more so in ric e than soybean. The rubisco k(cat) Of rice was unaffected by growth [C O2] or temperature, but that from soybean was increased by both. In ri ce the capacity to handle carbohydrate, as measured by sucrose phospha te synthase activity was up-regulated by CO2-enrichment, but not by te mperature. Leaf carbohydrates were increased by [CO2], but decreased b y higher temperatures, starch more so than sucrose. Even though C-3 sp ecies differ in response to [CO2] and temperature, CO2-enrichment can moderate adverse effects of temperature extremes.