Characterization of Paraquat Resistance inSolanum americanumMill. II. Evidence for a Chloroplast Mechanism

Abstract

Paraquat treatment caused a net decline in total ascorbate in a paraquat-sensitive (S) biotype ofSolanum americanumMill. over a 3-h period. A resistant biotype (R) responded with a transient decrease in total ascorbate after 1 h. By 2 h after treatment total ascorbate levels had recovered and the ratio of ascorbate to dehydroascorbate had increased three-fold. Electrolyte leakage from paraquat-treated tissue occurred at a faster rate in the S biotype, and the difference was more pronounced in light than in darkness. Studies of concentration-dependent paraquat-induced electrolyte leakage emphasized the disparity between light and dark responses of the R and S biotypes, suggesting a light-dependent mechanism of resistance. Based on variable fluorescence (Fv), there was no biotypic difference in the rate of penetration of diquat or paraquat to the site-of-action in PSI. Although both biotypes responded to increasing levels of paraquat by suppressingFv, theFvvalues of the R biotype exceeded those of the S biotype at all paraquat concentrations. TheFvof the resistant biotype was inherently higher than that of the sensitive biotype in the absence of paraquat. A chloroplast mechanism-of-resistance involving the electron transport chain is implicated.