Investigation on photorespiration with a sensitive C-assay.
A leaf disk assay for photorespiration has been developed based on the rate of release of recently fixed (14)CO(2) in light in a rapid stream of CO(2)-free air at 30 degrees to 35 degrees . In tobacco leaves (Havana Seed) photorespiration with this assay is 3 to 5 times greater than the (14)CO(2) output in the dark. In maize, photorespiration is only 2% of that in tobacco.The importance of open leaf stomata, rapid flow rates of CO(2)-free air, elevated temperatures, and oxygen in the atmosphere in order to obtain release into the air of a larger portion of the (14)CO(2) evolved within the tissue in the light was established in tobacco. Photorespiration, but not dark respiration, was inhibited by alpha-hydroxy-2-pyridinemethanesulfonic acid, an inhibitor of glycolate oxidase, and by 3-(4-chlorophenyl)-1,1-dimethylurea (CMU), an inhibitor of photosynthetic electron transport, under conditions which did not affect the stomata. These experiments show that the substrates of photorespiration and dark respiration differ and also provide additional support for the role of glycolate as a major substrate of photorespiration. It was also shown that at 35 degrees the quantity of (14)CO(2) released in the assay may represent only 33% of the gross (14)CO(2) evolved in the light, the remainder being recycled within the tissue.It was concluded that maize does not evolve appreciable quantities of CO(2) in the light and that this largely accounts for the greater efficiency of net photosynthesis exhibited by maize. Hence low rates of photorespiration may be expected to be correlated with a high rate of CO(2) uptake at the normal concentrations of CO(2) found in air and at higher light intensities.
Zelitch
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Department of Biochemistry, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504.