Rising atmospheric carbon dioxide (CO2) concentration and temperature will influence photosynthesis, growth and yield of agronomic crops. To investigate effects of elevated CO2 and high temperature on leaf gas exchanges, activities of Rubisco and phosphoenolpyruvate carboxylase (PEPC) and growth of grain sorghum (Sorghum bicolor L. Moench), plants were grown in controlled environments at day-time maximum/night-time minimum temperatures of 30/20°C or 36/26°C at ambient (350 µmol mol-1) or

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... d (700 µmol mol-1) CO2. Gas-exchange rates, activities of Rubisco and PEPC and growth parameters (leaf, stem and total dry weights) were determined at different stages of leaf development. Between 6 and 25 days after leaf tip emergence, leaf carbon exchange rate (CER) of elevated CO2 plants was greater at 30/20°C and 36/26°C than that of ambient CO2 plants at the same temperatures. The positive response of CER to elevated CO2 was greater in young leaves than in old leaves. In young leaves, elevated CO2 enhanced Rubisco activity at 30/20°C and 36/26°C, whereas PEPC activity was not affected by elevated CO2 at 30/20°C but was marginally enhanced at 36/26°C. At 30/20°C, growth parameters were not affected by elevated CO2 until 50 days after sowing (DAS); at 36/26°C, they were progressively enhanced by elevated CO2 to as high as 49 to 62% by 50 DAS. Leaf CER and Rubisco activity were enhanced by elevated CO2 at early stages of leaf ontogeny for the C4 grain sorghum. Such enhancement should have a significant role in dry matter production under elevated CO2.