CHARACTERIZATION OF PARAQUAT RESISTANCE IN SOLANUM-AMERICANUM MILL - II - EVIDENCE FOR A CHLOROPLAST MECHANISM

Paraquat treatment caused a net decline in total ascorbate in a paraqu at-sensitive (S) biotype of Solanum americanum Mill. over a 3-h period . A resistant biotype (R) responded with a Irans-ent decrease in total ascorbate after 1 h. By 2 h after treatment total ascorbate levels ha d recovered and the ratio of ascorbate to dehydroascorbate had increas ed three-fold. Electrolyte leakage from paraquat-treated tissue occurr ed at a faster rate in the S biotype, and the difference was more pron ounced in light than in darkness. Studies of concentration-dependent p araquat-induced electrolyte leakage emphasized the disparity between l ight and dark responses of the R and S biotypes, suggesting a light-de pendent mechanism of resistance. Based on variable fluorescence (F-v), there was no biotypic difference in the rare of penetration of diquat or paraquat to the site-of-action in PSI. Although both biotypes resp onded to increasing levels of paraquat by suppressing F-v, the F-v val ues of the R biotype exceeded those of the S biotype at all paraquat c oncentrations. The F-v of the resistant biotype was inherently higher than that of the sensitive biotype in the absence of paraquat. A chlor oplast mechanism-of-resistance involving the electron transport chain is implicated. (C) 1998 Academic Press.