PHYSIOLOGICAL-RESPONSES OF REHMANNIA-GLUTINOSA TO PARAQUAT AND ITS TOLERANCE MECHANISMS

Rehmannia glutinosa was extremely tolerant to paraquat at the levels o f leaf disk and chloroplast. The tolerance level of R. glutinosa was m uch greater at the level of leaf disk than at the level of chloroplast , indicating that the tolerance factor(s) are mainly located external to the chloroplast envelope. Possible tolerance mechanisms of R. gluti nosa to paraquat were examined in comparison with paraquat-susceptible corn and soybean. Epicuticular wax and cuticle contents did not corre spond to differential paraquat effects on the three plant species. Cut icular penetration of paraquat was almost the same in the three plant species. Autoradiography experiments with [C-14]paraquat indicated tha t the paraquat absorption and translocation in R. glutinosa leaves wer e not restricted. In addition, the time-and the concentration-dependen t paraquat accumulation into the vacuole of R. glutinosa was found to be very similar to those of corn and soybean. However, paraquat bindin g to the cell wall was greater in R. glutinosa than in corn and soybea n, presumably resulting in the reduction of paraquat movement to its s ite of action in R. glutinosa. The levels of lipid peroxidation in the thylakoid membrane caused by paraquat were found to be almost the sam e in the three plant species, suggesting that altered site of action i s not a mechanism of paraquat tolerance in R. glutinosa. The activitie s of protective enzymes of superoxide dismutase, ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase in the stromal extracts were ail higher in R. glutinosa than in corn and soybean. How ever, the enhanced activities of the protective enzymes are not likely to account for the paraquat tolerance of R. glutinosa, since the acti vities did nor correlate with the differential paraquat effects on the three plant species. Thin-layer chromatographic analysis of leaf extr acts revealed that paraquat metabolism occurred in intact leaves of R. glutinosa, but not in the chloroplast. Our results suggest that paraq uat can be metabolized by unknown factor(s) in the cytosol, nor in the chloroplast, of R. glutinosa leaves. Collectively, it can be conclude d that the tolerance of R. glutinosa to paraquat might mostly be due t o the consequence of paraquat metabolism outside of the chloroplast, a lthough paraquat binding to the cell wall is also likely to confer the tolerance to some extent. (C) 1997 Academic Press.