Dark Respiration during Photosynthesis in Wheat Leaf Slices 1

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The metabolism of [14C]succinate and acetate was examined in leaf slices of winter wheat (Triticum aestivum L. cv Frederick) in the dark and in the light (1000 micromoles per second per square meter photosynthetically active radiation). In the dark [1,4-14C]succinate was rapidly taken up and metabolized into other organic acids, amino acids, and CO2. An accumulation of radioactivity in the tricarboxylic acid cycle intermediates after 14CO2 production became constant indicates that organic acid pools outside of the mitochondria were involved in the buildup of radioactivity. The continuous production of 14CO2 over 2 hours indicates that, in the dark, the tricarboxylic acid cycle was the major route for succinate metabolism with CO2 as the chief end product. In the light, under conditions that supported photorespiration, succinate uptake was 80% of the dark rate and large amounts of the label entered the organic and amino acids. While carbon dioxide contained much less radioactivity than in the dark, other products such as sugars, starch, glycerate, glycine, and serine were much more heavily labeled than in darkness. The fact that the same tricarboxylic acid cycle intermediates became labeled in the light in addition to other products which can acquire label by carboxylation reactions indicates that the tricarboxylic acid cycle operated in the light and that CO2 was being released from the mitochondria and efficiently refixed. The amount of radioactivity accumulating in carboxylation products in the light was about 80% of the 14CO2 release in the dark. This indicates that under these conditions, the tricarboxylic acid cycle in wheat leaf slices operates in the light at 80% of the rate occurring in the dark.

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