Acridone Alkaloids from Swinglea glutinosa (Rutaceae) and Their Effects on Photosynthesis

Continuing our search for herbicide models based on natural products, we investigated the action mechanisms of five alkaloids isolated from Swinglea glutinosa (Rutaceae): Citrusinine‐I (1), glycocitrine‐IV (2), 1,3,5‐trihydroxy‐10‐methyl‐ 2,8‐bis(3‐methylbut‐2‐en‐1‐yl)‐9(10H)‐acridinone (3), (2R)‐2‐tert‐butyl‐3,10‐dihydro‐4,9‐dihydroxy‐11‐methoxy‐10‐methylfuro[3,2‐b]acridin‐5(2H)‐one (4), and (3R)‐2,3,4,7‐tetrahydro‐3,5,8‐trihydroxy‐6‐methoxy‐2,2,7‐trimethyl‐12H‐pyrano[2,3‐a]acridin‐12‐one (5) on several photosynthetic activities in an attempt to find new compounds that affect photosynthesis. Through polarographic techniques, the compounds inhibited the non‐cyclic electron transport in the basal, phosphorylating, and uncoupled conditions from H2O to methylviologen (=MV). Therefore, they act as Hill reaction inhibitors. This approach still suggested that the compounds 4 and 5 had their interaction site located at photosystem I. Studies on fluorescence of chlorophyll a suggested that acridones (1–3) have different modes of interaction and inhibition sites on the photosystem II electron transport chain.