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Plant Physiology 48:712-719 (1971) © 1971 American Society of Plant Biologists The Rate of Photorespiration during Photosynthesis and the Relationship of the Substrate of Light Respiration to the Products of Photosynthesis in Sunflower Leaves 1a Department of Biology, Queen's University, Kingston, Ontario, Canada
Single attached leaves of sunflower (Helianthus annus L. "Mennonite") were supplied 14CO2 of constant specific radioactivity in gas mixtures containing various CO2 and O2 concentrations. The 14CO2 and CO2 fluxes were measured concurrently in an open system using an ionization chamber and infrared gas analyzer. The rate of photorespiration (5.7 ± 0.3 mg CO2·dm–2·–1) during photosynthesis in 21% O2 at 25 C and 3,500 footcandles was over three times the rate of dark respiration and was independent of CO2 concentrations from 0 to 300 µl/l. The steady rate of CO2 evolution into CO2-free air was about 30% lower. Low oxygen (1%) inhibited both 14CO2 and CO2 evolution, both during photosynthesis and in CO2-free air in the light. At 300 µl/l CO2 apparent photosynthesis was inhibited 41% by 21% O2. Two-thirds of the inhibition was due to the inhibition of true photosynthesis by oxygen and one-third due to the stimulation of photorespiration. At 50 µl/l CO2, where the percentage inhibition of apparent photosynthesis by 21% oxygen was 92%, photorespiration accounted for two-thirds of the total inhibition. The rate of 14CO2 uptake by the leaf decreased about 30 seconds after the introduction of 14CO2, indicating that 14CO2 was rapidly evolved from the leaf. The rate of 14CO2 evolution increased rapidly with time, the kinetics depending on the CO2 concentration. The high specific radioactivity of the 14CO2 evolved during photosynthesis or in the early period of flushing in CO2-free air showed that the substrate for light respiration was an early product of photosynthesis. From the measurement of 14CO2 and CO2 evolution into CO2-free air over a longer time period it was apparent that at least three compounds, each of decreased 14C content, could supply the substrate for light respiration. Based on a consideration of the specific radioactivity of 14CO2 evolved under a variety of conditions, it is suggested that total CO2 evolution in the light or photorespiration is composed of two processes, dark respiration and light respiration. Light respiration is a process that only occurs in the light, persists for some time on darkening, and metabolizes substrates that are quite different from those of dark respiration.
2 Present address: Glasshouse Crops Research Institute, Worthing Road, Rustington, Littlehampton, Sussex, England. 1 Supported in part by the National Research Council of Canada. This article has been cited by other articles:
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