Photoacoustic Measurements in Vivo of Energy Storage by Cyclic Electron Flow in Algae and Higher Plants

Abstract

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

Dephosphorylation of photosystem II core proteins is light-regulated in vivo.

“…These concentrations are in accordance with those shown to be effective at inhibiting PSI cyclic electron transport in vivo (Herbert et al, 1990). …”

“…However, in the presence of this inhibitor, cyclic PSI electron transport still occurs in Spirodela plants under far-red light (Jansen et al, 1992). The hypothesis that PSI excitation is involved in regulating D1 dephosphorylation was tested by introducing DBMIB, a plastoquinone antagonist (Trebst, 1980) and in vivo inhibitor of PSI-driven cyclic electron transport (Herbert et al, 1990). DBMIB inhibited white-light-stimulated Dl dephosphorylation in a concentration-dependent manner with an I50 of -5-10 /AM and complete inhibition at 50-100 AtM ( Figure 6A and B).…”

Concerning a Dual Function of Coupled Cyclic Electron Transport in Leaves

“…Using a photoacoustic method, Herbert et al (10) recently provided evidence for the occurrence of energy storage by coupled cyclic electron flow in various algae and higher plants, with the possible exception of C3 plants. It remains unclear whether in their experiments, which involved infiltration of leaves, redox poising was really achieved for the latter group of plants.…”

Cyclic electron transport through photosystem I

“…Several techniques, including P700 re-reduction measurement and photoacoustic spectroscopy, have been applied to detect cyclic activity in the leaf (Herbert et al 1990;Joët et al 2002). The re-reduction kinetics of P700 in the dark accelerates when electrons coming from the stroma are donated to PSI.…”