Changes in the xanthophyll cycle and fluorescence quenching indicate light-dependent early events in the action of paraquat and the mechanism of resistance to paraquat in Erigeron canadensis (L.) Cronq

Violaxanthin de-epoxidation, chlorophyll fluorescence quenching, and photos ynthetic O-2 evolution in the presence of paraquat (Pq) were studied in int act attached leaves of Pq-susceptible, and Pq-resistant (PqR) biotypes of E rigeron canadensis under different light conditions, Initially, similar cha nges were induced in the two biotypes, but the effects relaxed only in the PqR plants, indicating a Pq elimination process. The penetration of Pq into the chloroplasts of PqR plants proved to be somewhat restricted and highly light-dependent, as revealed by both the light response curves of violaxan thin deepoxidation and fluorescence quenching and the short-term high-light pre-illumination experiments. An irregular downregulation of the non-photo chemical fluorescence quenching processes was observed, reflected by lower steady-state zeaxanthin and non-photochemical fluorescence quenching levels as compared with the corresponding non-treated highlight controls. It is c oncluded that light is essential not only for the initiation of the mechani sm of resistance to Pq, but also for the penetration of Pq into the chlorop lasts in the PqR E. canadensis. Also, the Pq elimination process may cause a modification to the regulation of the non-radiative energy dissipation in PqR plants in the presence of Pq.