Kaempferol, a naturally occurring flavonol, inhibited coupled electron transport and both cyclic and noncyclic photophosphorylation in isolated pea (Pisum sativum) chloroplasts. Over a concentration range which gave marked inhibition of ATP synthesis, there was no effect on There have been no reports describing the effects of flavonoids on the reactions of chloroplasts, either in vivo or in vitro. Since kaempferol is a common naturally occurring flavonoid (3, 5), we have studied its effects on reactions of isolated pea chloroplasts. We will demonstrate that kaempferol is a potent inhibitor of photophosphorylation at concentrations which are physiologically significant. MATERIALS AND METHODSChloroplasts were isolated from 10-to 14-day-old pea leaves, Pisum sativum var. Alaska. Ten grams of leaf and stem tissue were homogenized for 5 sec in a Waring Microblender in 20 ml of grinding solution (0.4 M sorbitol; 0.1 M Na-Tricine, pH 7.8; 0.07 M sodium ascorbate; and 2 mg/ml of bovine serum albumin). The brei was filtered through 4 and then 12 layers of cheesecloth and centrifuged at lOOOg for 10 min. The resulting chloroplast pellet was dispersed in 1 ml of resuspension mix (0.4 M sorbitol; 0.05 M Na-Tricine, pH 7.8; 0.01 M KCI; and bovine serum albumin, 5 mg/ml). Chl concentration was determined according to Arnon (1).