Different sensitivity of photosynthetic electron transport of plant species to lichen acids

Abstract

Lichen accumulates large amounts of phenolics like lichen acids, whereas their physiological functions are not clear. We investigated the effects of lichen acids on photosynthesis. In vitro assay with spinach thylakoid membranes showed that lichen acids inhibit PSII at both oxidizing (probably at Yz) side and reducing side (QB) of PSII. Then, we examined the effects of lichen acids on different plant species. When cells were treated with lichen acids, these cells showed different inhibition.; i.e., oxidzing side of PSII was strongly inhibited in a free-living green algae (Chlamydomonas reinhardtii), whereas reducing side was strongly inhibited in tobacco cells. Neither oxidizing side nor reducing side of PSII was inhibited in a symbiotic green algae (Trebouxia sp.). Isolated-thylakoid membranes from these materials, however, showed identical sensitivity to lichen acids. These findings suggested that Trebouxia would have the mechanism to exclude lichen acids from cell or chloroplast, or to modify the inhibitory activity. The difference in sensitivity to lichen acids suggested that lichen acids may function as allelopathic agents. Further characterization of lichen acids on atrazine-resistant tobacco cells indicated that atrazine-resistant cells were more sensitive to lichen acids than wild type, especially at reducing side. This phenomenon is similar to the action of phenol type herbicide on atrazine-resistant PSII membrane, suggesting that lichen acids may function as phenol type herbicide.