Xanthophyll Cycle Patterns and in vivo Photoinhibition in Herbicide-Resistant Biotypes of Conyza canadensis

Summary The xanthophyll cycle and in vivo photoinhibition were investigated in the herbicide-susceptible (S), paraquat-resistant (PQ-R), atrazine-resistant (Atr-R) and paraquat-atrazine co-resistant (PQAtr-R) biotypes of Conyza canadensis using both low-(LLG) and high-light-grown (HLG) plants. HPLC revealed markedly reduced levels of zeaxanthin under photoinhibitory conditions in atrazine-resistant (Atr-R and PQAtr-R) biotypes as compared with the S and PQ-R biotypes. The variable chlorophyll fluorescence (F v ) decrease and constant fluorescence (F o ) increase revealed an enhanced susceptibility to in vivo photoinhibition in Atr-R and PQAtr-R biotypes. LLG plants were more susceptible than HLG plants to in vivo photoinhibitory treatment. It is suggested that the increased susceptibility to photoinhibition in Atr-R and PQAtr-R biotypes of C. canadensis is a consequence not only of D1 protein mutation, but also possibly of a lower rate of violaxanthin to zeaxanthin de-epoxidation.