Photosynthetic activity and molecular composition of thylakoid membranes of sensitive and atrazine/paraquat coresistant biotypes of Conyza canadensis (L.) Cronq was compared before and after 5 × 10-4 molar paraquat spraying. In contrast to the irreversibly damaged sensitive plants the Rfd values (vitality indices) calculated from parameters of fluorescence induction curves, and the in vivo CO2 fixation in resistant plants showed - after a transitory inhibition - a recovery.
Resistant thylakoids contained higher amount of PS II and LHC II, (i.e. lower relative amount of CP I) which can be the result of an adaptation process. Fatty acid composition of total leaf extract was almost the same in both biotype even after spraying. The amount of the oligomeric form of LHCP II decreased in paraquat treated sensitive thylakoids, which was in correlation with the reduction of Δ3-transhexadecanoic acid content. Decrease in oligomeric LHCP II and/or special lipids (PG) in sensitive thylakoids may change the association of LHCP II and photosystem II core as it is evidenced from lowering of the Mg2+ induced changes of the short wavelength fluorescence intensity and increase in the relative quantum requirement values. The results are discussed in connection with a possible effect of paraquat on PS II.