Interference by flavone and flavonols with chloroplast-mediated electron transport and phosphorylation

The effects of flavone and seven flavonols on the light-induced electron transport and phosphorylation of isolated spinach ( Spinacia oleracea L.) chloroplasts were investigated. With the exception of flavonol (3-hydroxyflavone), all of the compounds interacted with components of both the ATP-generating and electron transport pathways. Flavonol only interacted with the phosphorylation pathway. Interference with the phosphorylation pathway was evidenced by the greater sensitivity of the phosphorylation reaction than coupled whole-chain electron transport, inhibition of cyclic phosphorylation, inhibition of the light-activated Mg 2+-ATPase, and inhibition of the heat-activated Ca 2+ -ATPase associated with CF 1. The overall decreasing order of effectiveness for inhibition of cyclic phosphorylation was: galangin > quercetin = kaempferol = myricetin = flavonol > fisetin > flavone > morin. On the electron transport pathway, all of the compounds, except flavonol, interacted with the Q B-protein complex as, evidenced by inhibition of uncoupled electron transport, alteration of chlorophyll fluorescence transients, and competitive displacement of previously bound radiolabeled atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)- s-triazine]. The decreasing order of effectiveness for inhibition of uncoupled electron transport was: fisetin > quercetir = galangin > kaempferol > flavone > morin.