Differential Response of Two Soybean Cultivars to Paraquat

Abstract

The soybean cultivars “ Kwangkyo” and “ Hood ” are differentially sensitive to the bipyridylium herbicide paraquat (1,1′-dimethyl-4 ,4′-bipyridiniumion). This was confirmed by visible injury observations, measurements of desiccation levels and chlorophyll content, and tracings of chlorophyll fluorescence induction of fully expanded first trifoliate leaves of these two cultivars after exposure to a wide range of paraquat concentrations. The margin of this intraspecific differential tolerance to paraquat was narrow an d the ratio of the paraquat concentrations causing 50% injury to the tolerant Kwangkyo and to the susceptible Hood soybean (approximate tolerance factor) was found to be 10. Paraquat at 1 μᴍ or higher inhibited rapidly the CO2 fixation capacity of leaf mesophyll cells, isolated enzymatically from both cultivars. Thus, the tolerance of Kwangkyo soybean to paraquat does not appear to result from any differences at the site of paraquat action in chloroplast membranes. At early time periods (30 min to 2 h) after treatment with 100 μᴍ of paraquat, chlorophyll fluorescence induction was completely suppressed in first trifoliate leaves of Hood, but not in those of Kwangkyo soybean. At longer time periods (≧ 3 h), paraquat suppressed chlorophyll fluorescence induction similarly in leaves of both soybean cultivars. These results suggest that reduced mobility or a delayed release of paraquat in the mesophyll cells of Kwangkyo may be involved in the observed tolerance of this soybean cultivar to this herbicide.