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Photosynthetic Responses of ‘Bragg’ Soybean Leaves Adapted to Different CO₂ Environments¹

Carbon dioxide exchange rates (CER) were measured during seed fill on leaflets of soybean [Glycine max (L.) Merr. ‘Bragg’] plants grown at 330 and 660 µmol mol^–1 CO₂ environments in outdoor growth chambers to investigate their photosynthetic response to light. During midday periods at the R5 growth stage, the plants were submitted to short-term CO₂ levels ranging from 90 to 990 µmol mo1^–1 to determine differences in leaf photosynthetic response caused by CO₂ acclimation. The data were fitted to a rectangular hyperbola of the Michaelis-Menten form in which the independent variable was radiation or CO₂. Light and CO₂ compensation points were calculated from the parameters of the model. Light response curves showed that leaflets grown and measured at 660 µmol mol^–1 CO₂ had higher CER asymptotes, higher apparent Michaelis-Menten constants for light, higher apparent quantum yield, and lower light compensation points than leaflets adapted to and measured at 330 µmol mol^–1 CO₂. The CER-CO₂ curve parameters also indicated higher asymptotic ceilings of CER at high CO₂ and higher apparent Michaelis- Menten constants for CO₂ for the higher CO₂ adapted leaflets. The CO₂ compensation point was significantly lower in leaflets adapted to high CO₂ levels than those adapted to ambient CO₂, indicating a decrease in photorespiration. It was concluded that, during seed fill, leaflets adapted to high CO₂ environments exhibited a capability to utilize CO₂ and radiation more efficiently at elevated CO₂ and throughout all light levels than leaflets grown at low CO₂.

Key Words: CO₂-enrichment • Light-response-curves • Photosynthesis • Carbon-dioxide-compensation-point

² Post-doctoral research associate, Agronomy Dep.; professor, Agric. Engineering Dep.; graduate research associate, Agronomy Dep.;professor, Agric. Engineering Dep., Univ. of Florida; Gainesville, FL 32611, USA; and soil scientist, USDA, Gainesville, FL.

Received for publication April 3, 1984.

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