Relationship between Photosynthesis and Respiration

The rate of dark CO(2) efflux from mature wheat (Triticum aestivum cv Gabo) leaves at the end of the night is less than that found after a period of photosynthesis. After photosynthesis, the dark CO(2) efflux shows complex dependence on time and temperature. For about 30 minutes after darkening, CO(2) efflux includes a large component which can be abolished by transferring illuminated leaves to 3% O(2) and 330 microbar CO(2) before darkening. After 30 minutes of darkness, a relatively steady rate of CO(2) efflux was obtained. The temperature dependence of steady-state dark CO(2) efflux at the end of the night differs from that after a period of photosynthesis. The higher rate of dark CO(2) efflux following photosynthesis is correlated with accumulated net CO(2) assimilation and with an increase in several carbohydrate fractions in the leaf. It is also correlated with an increase in the CO(2) compensation point in 21% O(2), and an increase in the light compensation point. The interactions between CO(2) efflux from carbohydrate oxidation and photorespiration are discussed. It is concluded that the rate of CO(2) efflux by respiration is comparable in darkened and illuminated wheat leaves.