Effect of Oxygen on the Contribution of Respiration to the CO(2) Compensation Point in Wheat and Bean Leaves.

The CO(2) compensation point at 21% O(2) (Gamma(21)) and at 2% O(2) (Gamma(2)), and the rate of dark CO(2) efflux at 21% O(2) (R(n)) 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 mubar CO(2). The values of Gamma(21) and R(n) 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 R(n) were studied during the growth period of bean (Phaseolus vulgaris L, cv Hawkesbury Wonder) leaves. The values of Gamma(21) and R(n) 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 considerably 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 CO(2) compensation point in wheat and bean leaves is smaller at low O(2) levels than at ambient levels.