Comparative thermoluminescence study of triazine-resistant and -susceptible biotypes of Erigeron canadensis L

Abstract

By means of thermoluminescence measurements, the midpoint oxidation-reduction potentials of the primary (Q A) and secondary (QP B) quinone electron acceptors of Photosystem II, as well as the equilibrium constant of the equilibrium Q − A · Q B ⇌ K Q A · Q − B , were compared in chloroplasts of triazine-susceptible and triazine-resistant biotypes of Erigeron canadensis L. It was observed that at pH 7.5 in DCMU-treated chloroplasts the main thermoluminescence band attributed to radiative charge recombination of the S 2Q − A redox state appeared at about the same temperature (approx. 10°C) in both biotypes, indicating that the triazine resistance is not accompanied by an alteration in the midpoint potentials of the S 2 S 3 and Q A Q − A redox couples. On the other hand, the peak position of the thermoluminescence band associated with the S 2Q − B state was shifted from 32 to 15°C in the glow curve of untreated resistant chloroplasts as compared to the sensitive ones, suggesting a decrease in the midpoint potential of the Q B Q − B redox couple in resistant chloroplasts. Computer-assisted analysis of the thermoluminescence measurements showed that the redox distance between Q A and Q B was 71 mV in sensitive chloroplasts, and about 32 mV in resistant ones. This corresponded approximately to a change in the value of the equilibrium constant between Q − A· QB and Q A·Q − B from 16 to 3.5 in sensitive and resistant thylakoids, respectively. The semiquinone equilibrium between Q A and Q B was found to be greatly dependent upon pH. Decrease of the pH from 7.5 to 6.0 resulted in a 2.8-fold increase in the value of the equilibrium constant, from 16 to 44, in sensitive chloroplasts. The shift in the peak position of the 32°C thermoluminescence band (and consequently in the redox potential of the Q B Q − B couple) was also observed in three other triazine-susceptible/resistant biotype pairs. However, the extent of the shift varied with the plant species investigated. From the results of thermoluminescence measurements it is concluded, in agreement with the hypothesis of other authors, that the lowering of the redox potential of the Q B Q − B couple compared to that of the Q A Q − A couple is probably a trait characteristic of the triazine-resistant biotypes.