Linear and cyclic electron fluxes through
Photosystem I in 1% CO(2) were quantified in spinach leaf tissue under severe
water stress. Using actinic light with a peak at 697 nm for preferential light absorption by
Photosystem I while also stimulating
Photosystem II to improve redox poising, the cyclic electron flux after 60 s of illumination was a substantial proportion (33-44%) of the total electron flux through PSI at irradiances up to ~1,070 micromol photons m(-2) s(-1). At the maximum irradiance, the cyclic electron flux changed little with the progressive water loss from leaf tissue up to ~60%; by contrast, the linear electron flux was approximately halved. A reason for this differential effect of
water stress on the capacity for cyclic and linear electron flow could be the increased crowding of soluble
proteins in the stroma due to chloroplast shrinkage. Indeed the confinement of soluble
proteins to a smaller chloroplast volume was indicated by cryo-scanning electron microscopy. It is known that the diffusion coefficient of large
proteins is decreased when the background concentration of small
proteins is raised; by contrast, the diffusion coefficient of small
proteins is not affected by increasing the concentration of a large
protein (Muramatsu and Minton in Proc Natl Acad Sci USA 85:2984-2988, 1988). Therefore, we suggest that linear electron flow, being coupled to the Calvin-Benson cycle, is limited by the diffusion of large macromolecules, especially the
ribulose 1, 5-bisphosphate carboxylase/
oxygenase complex. By contrast, cyclic electron flow, involving relatively small macromolecules such as
ferredoxin, is less susceptible to inhibition by crowding in the stroma.