Energy storage by cyclic electron flow through photosystem I (PSI) was measured in vivo using the photoacoustic technique. A wide variety of photosynthetic organisms were considered and all showed measurable energy storage by PSI-cyclic electron flow except for higher plants using the C-3 carbon fixation pathway. The capacity for energy storage by PSI-cyclic electron flow alone was found to be small in comparison to that of linear and cyclic electron flows combined but may be significant, nonetheless, under conditions when photosystem 11 is damaged, particularly in cyanobacteria. Light-induced dynamics of energy storage by PSI-cyclic electron flow were evident, demonstrating regulation under changing environmental conditions.The oxygen-evolving photosynthetic apparatus contains two reaction center complexes, designated PSI electron flow through PSI in C3 plants has been proposed to generate ATP over and above that produced by linear electron flow, adjusting the ratio of ATP to NADPH generated by the light reactions of photosynthesis in accordance with the needs of the plant (1). As an example, cyclic electron flow through PSI has been proposed as a source of ATP for repair of PSII units damaged by environmental stress, since PSI is typically much less susceptible to stress than PSII (8, 9). Much of the ambiguity concerning the function and significance of cyclic electron flow through PSI is due to the difficulty of measuring it in whole cells and tissues. Most previous studies ofPSI-cyclic electron flow and accompanying phosphorylation have necessarily used either in vitro measurements of thylakoid fragments or somewhat ambiguous light-induced absorbance changes in whole cells or tissues (12,17). The photoacoustic method for measurement of photosynthetic energy storage is well suited for study of PSI-cyclic electron flow, however, because it is capable of simple, direct, and quantitative measurement of energy storage by cyclic electron flow in intact leaves and algae as well as in thylakoid preparations (9,10,16,18). Presumably, the bulk of such energy storage by PSI-cyclic electron flow represents photophosphorylation of ADP.In the present report, the occurrence, capacity, and regulation of energy storage by PSI-cyclic electron flow in whole tissues of a variety of photosynthetic organisms are characterized using the photoacoustic method.
MATERIALS AND METHODS
Plant Material