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Chlorophyll, Ribulose-1,5-diphosphate Carboxylase, and Hill Reaction Activity in Developing Leaves of Populus deltoides

^a Institute of Forest Genetics, North Central Forest Experiment Station, United States Department of Agriculture Forest Service, Rhinelander, Wisconsin 54501

The synthesis of chlorophyll and ribulose diphosphate carboxylase as well as the development of Hill reaction activity were followed in expanding Populus deltoides leaves and related to photosynthetic patterns. Total chlorophyll, which was not correlated with photosynthetic rate in expanding leaves, decreased slightly with age in very young leaves, due to a decrease in chlorophyll b, but then increased linearly. The ratio of chlorophyll a to b, which rose sharply in young leaves, was highly correlated with the onset of net photosynthesis. Hill reaction activity was very low in young leaves and did not increase significantly until leaves were about half expanded. Ribulose diphosphate carboxylase activity increased in a sigmoid fashion with leaf ontogenesis and closely paralleled development of the photosynthetic system. The study demonstrates the importance of chlorophyll a and Calvin cycle enzyme synthesis to photosynthetic development in expanding leaves.