Plants accumulate flavonoids in response to a myriad of environmental challenges, especially when exposed to ultraviolet (UV) radiation or situations causing oxidative stress. However, the origin and nature of the signal triggering their accumulation remain obscure. In this study, a group of flavonoids belonging to the flavone class was identified in Lemna gibba (duckweed). These flavones accumulated upon exposure to UV radiation, low temperature, copper and the photosynthetic electron transport (PET) inhibitors 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and 1,2-dihydroxyanthraquinone (DHATQ). All of these stressors were also shown to promote PET chain (PETC) reduction; however, in the co-presence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) or a light regime that oxidized the PETC, flavonoid accumulation ceased. Chloroplast-derived reactive oxygen species (ROS) were not associated with all of the stress conditions that promoted both PETC reduction and flavonoid synthesis, indicating that ROS were not a strict requisite for flavonoid accumulation. Transcripts for the flavonoid biosynthetic genes, chalcone synthase (CHS) and chalcone isomerase, were similarly responsive to the PETC redox state, as were a panel of transcripts revealed by differential display PCR. Collectively, these results provide evidence that PETC redox status is one of the factors affecting flavonoid biosynthesis.