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Functional and Structural Changes in Senescing Populus deltoides (Bartr.) Chloroplasts ¹

^a Department of Forest Botany and Pathology, State University of New York College of Environmental Science and Forestry, Syracuse, New York 13210

Chloroplasts isolated from Populus deltoides leaves were used to study age-dependent changes in the rate of cyclic photophosphorylation. Single leaves were used to measure CO₂ fixation by leaf discs, chlorophyll concentration, and ATP synthesis. The ability of chloroplasts to synthesize ATP diminished steadily from the time of full leaf expansion, regardless whether the results are expressed on a leaf area or chlorophyll basis. This decline in the rates of ATP synthesis was paralleled by the decline in the rate of CO₂ fixation. The results suggest that the efficiency of the membrane-bound ATP synthesizing system declines with age.

¹ This study is drawn from the Ph.D. thesis research of R. Hernández-Gil.