Effect of Oxygen on the Contribution of Respiration to the CO2 Compensation Point in Wheat and Bean Leaves

Abstract

The CO2 compensation point at 21% O2 (.GAMMA.21) and at 2% O2 (.GAMMA.2), and the rate of dark CO2 efflux at 21% O2 (Rn) were measured in adult wheat (Triticum aestivum L, cv Gabo) leaves at the end of the night and after a period of photosynthesis of 5 h at 800 .mu.bar CO2. The values of .GAMMA.21 and Rn significantly increased after the light period, due to the stimulation of respiration by carbohydrates. In contrast, .GAMMA.2 did not increase after the same period of photosynthesis, suggesting that the respiratory component of .GAMMA.2 was not stimulated by carbohydrates. In a different experiment, .GAMMA.21, .GAMMA.2, and Rn were studied during the growth period of bean (Phaseolus vulgaris, L, cv Hawkesbury Wonder) leaves. The values of .GAMMA.21 and Rn were high in young leaves, and decreased rapidly in parallel during maturation. However, .GAMMA.2 presented relatively low values in growing bean leaves, and a model predicted that the observed values of .GAMMA.2 should have been considered higher if their respiratory component was considered to be as large as that of .GAMMA.21. The results suggest that the rate of respiration in the light contributing to the CO2 compensation point in wheat and bean leaves is smaller at low O2 levels than at ambient levels.