Photosynthesis, photorespiration, and ribulose bisphosphate carboxylase/oxygenase activities were measured in intact flag leaves of wheat during ageing. Photorespiration declined very little as the leaves aged, and much less than photosynthesis. These changes could not be explained by changes in the ribulose bisphosphate carboxylase to oxygenase ratio of fraction 1 protein. As the leaves grew older the enzyme activities in extracts of leaves declined in parallel so the ratio remained constant.
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Thomas et al. (1978) studied this question.