Flavonoids Affect the Light Reaction of Photosynthesis in Vitro and in Vivo as Well as the Growth of Plants

Flavonoids retusin (5-hydroxy-3,7,3′,4′-tetramethoxyflavone) (1) and pachypodol (5,4′-dihydroxy-3,7,3′-trimethoxyflavone) (2) were isolated from Croton ciliatoglanduliferus Ort. Pachypodol acts as a Hill reaction inhibitor with its target on the water splitting enzyme located in PSII. In the search for new herbicides from natural compounds, flavonoids 1 and 2 and flavonoid analogues quercetin (3), apigenin (4), genistein (5), and eupatorin (6) were assessed for their effect in vitro on the photosynthetic electron transport chain and in vivo on the germination and growth of the plants Physalis ixocarpa, Trifolium alexandrinum and Lolium perenne. Flavonoid 3 was the most active inhibitor of the photosynthetic uncoupled electron flow (I₅₀ = 114 μM) with a lower log P value (1.37). Results in vivo suggest that 1, 2, 3, and 5 behave as pre- and postemergent herbicides, with 3 and 5 being more active.