Effect of Increasing Carbon Dioxide on Sorghum (Sorghum bicolor) Plants Under Drought Stressin the Grain Filling Stage

Stressed and non-stressed sorghum plants (Sorghum bicolorcv. Mylon, a local Sudanese cultivar) were subjected to different levelsof carbon dioxide (CO2), ranging from 10 to 2000 ppm at the grainfilling stage. High CO2 levels assisted stressed plants in terms of highphotosynthesis rates (an average of 2.89 µmol m.2 s.1 under elevatedlevels compared to an average of 1.65 µmol m.2 s.1 under ambientlevel), these rates remained constant under a very high CO2 concentration(1800 ppm). In addition, increasing CO2 concentrations decreasedthe stomatal conductance in stressed plants (from 27 µmolm.2 s.1 under ambient level to 15 µmol m.2 s.1 under 1990 ppm CO2concentration) and hence reduction in the transpiration rates (from0.68 mol m.2 s.1 under ambient to 0.38 mol m.2 s.1 under 1990 ppmCO2 concentration). This together with the high photosynthetic rates,increased water use efficiency (WUE), from 2.35 under ambient levelto 5.26 under 1990 ppm CO2 concentration, which are considered asgood factors in increasing sorghum productivity in dry regions