Relationship between Photosynthesis and Respiration

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Abstract
The rate of dark CO2 efflux from mature wheat (T. aestivum cv. Gabo) leaves at the end of the night is less than that found after a period of photosynthesis. After photosynthesis, the dark CO2 efflux shows complex dependence on time and temperature. For .apprx. 30 min after darkening, CO2 efflux includes a large component which can be abolished by transferring illuminated leaves to 3% O2 and 33 .mu.bar CO2 before darkening. After 30 min of darkness, a relatively steady rate of CO2 efflux was obtained. The temperature dependence of steady-stage dark CO2 efflux at the end of the night differs from that after a period of photosynthesis. The higher rate of dark CO2 efflux following photosynthesis is correlated with accumulated net CO2 assimilation and with an increase in several carbohydrate fractions in the leaf. It is also correlated with an increase in the CO2 compensation point in 21% O2 and an increase in the light compensation point. The interactions between CO2 efflux from carbohydrate oxidation and photorespiration are discussed. Evidently, the rate of CO2 efflux by respiration is comparable in darkened and illuminated wheat leaves.