Inhibition of CO₂ Assimilation by Supraoptimal CO₂: Effect of Light and Temperature

Abstract

In cotton the rate of CO₂ assimilation, at O₂ partial pressures up to 200 mbar, increased to a maximum and then declined as the intercellular partial pressure of CO₂ was increased. The specific intercellular partial pressure of CO₂ at which rate of assimilation began to decline depended on the environmental conditions. At 19 mbar partial pressure of O₂ the decline occurred at CO₂ partial pressure >390 µbar. At 200 mbar partial pressure of O₂ it occurred at CO₂ partial pressure > 534 µbar. O₂ increased the CO₂ partial pressure required for inhibition but it did not appear to affect the steepness of the decline of rate of assimilation with further increase in partial pressure of CO₂ once the decline became apparent. The decline was more readily observed at low temperature and low O₂ partial pressure, and in plants grown at low light and NO₃^- levels. It was also observed in cowpea and sunflower. Changes in quantum efficiency in cotton at high and low CO₂ concentrations were observed. At ambient CO₂ concentration (300 µbar), the quantum yields measured at 19 and 200 mbar partial pressure of O₂ were 0.072 ± 0.0003 and 0.053 ± 0.0060 mol CO₂ per mol absorbed quanta, respectively. In contrast, at 900 µbar CO₂ partial pressure the respective values were 0.050 ± 0.0023 and 0.070 ± 0.0006 mol CO₂ per mol absorbed quanta. The nature of the inhibition of CO₂ assimilation by high partial pressure of CO₂ is discussed.