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

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Abstract
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.