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Investigation of the lichen Parmotrema tinctorum led to the isolation of several known compounds. Among them, lecanorin (1), methyl-beta-orcinol carboxylate, methyl orsellinate, orcinol, and methyl haematommate (3) caused significant inhibition of the radicle growth and germination of seedlings of Amaranthus hypochondriacus and Echinochloa crusgalli. In addition, lecanorin (1) and gyrophoric acid (2) significantly inhibited the light-dependent synthesis of ATP and uncoupled electron transfer on the reducing side of photosystem II in freshly lysed, illuminated spinach chloroplasts. The targets of 1 and 2 were located at the water-splitting enzyme level and in one of the redox enzymes in the range of electron transport from P(680) to Q(A), respectively.

Xanthyletin (1), 3-(1',1'-dimethylallyl)xanthyletin (2), and chalepensin (3), the major coumarins isolated from Stauranthus perforatus, inhibit ATP synthesis from water to methylviologen in spinach thylakoids in a concentration-dependent manner. At low concentration chalepensin (3) inhibits basal and phosphorylating electron flow from water to K(3)[Fe(CN)(6)] without affecting uncoupled electron flow but accelerating Mg(2+)-ATPase activity. Thus, at low concentration the compound behaves as an energy transfer inhibitor. However, at higher concentrations this coumarin acts as an uncoupler because it enhances basal and phosphorylating electron transfer. On the other hand, coumarins 1 and 2 act as Hill reaction inhibitors, although 2 exhibited also uncoupler properties because it induces stimulation of basal and phosphorylating electron flow from water to ferricyanide. The site of interference of xanthyletin was located at the b(6)f-PC level of the electron transport chain.

Phytotoxic Compounds from Esenbeckia yaxhoob. -During studies on plant-growth inhibiting agents from Mexican plants the new compound (I) is identified. (I) is a potent radicle growth inhibitor with L. esculentum being the most sensitive species. -(MATA, R.; MACIAS, M. L.; ROJAS, I. S.; LOTINA-HENNSEN, B.; TOSCANO, R. A.; ANAYA, A. L.; Phytochemistry 49 (1998) 2, 441-449; Dep. Farm., Fac. Quim., Unidad Invest. Plantas Med. Inst. Quim., Coyoacan, 04510 Mexico, Mex.; EN)

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