The major phytotoxic compounds (encecalin, euparin and demethylencecalin) isolated from Helianthella quinquenervis (Hook) A Gray (Asteraceae) were evaluated on di †erent photosynthetic activities in chloroplasts isolated from spinach leaves. ATP synthesis, proton uptake and electron Ñow (basal, phosphorylating and uncoupled) were inhibited by encecalin and demethylencecalin in a concentration dependent manner, therefore acting as Hill reaction inhibitors. Encecalin and demethylencecalin did not a †ect photosystem I (electron transport from diaminodurene to methylviologen), but they inhibited photosystem II (from water to 2,5-dibromo-3-methyl-6-isopropyl-1,4-p-benzoquinone). Since these compounds inhibited electron Ñow in the photosystem II partial reactions from water to silicomolybdate and from diphenylcarbazide to dichlorophenol-indophenol, the site of inhibition was located in the span from to of the electron transport chain. Euparin, inhibited ATP synthesis, P 680 Q A proton uptake and basal and phosphorylating electron transports, but it has not e †ect on uncoupled electron Ñow from water to methylviologen. Mg2`-ATPase activity from bound membrane thylakoid chloroplasts was also inhibited by this compound. These results suggested that euparin inhibited phosphorylation in chloroplasts, acting as an energy-transfer inhibitor.
1998 Society of Chemical ( Industry.