In order to explore the physiological mechanism of paraquat resistance in tall fleabane, a widespread weed in Taiwan where resistance to this herbicide has been observed since 1980, the role of the antioxidative system was assessed. The susceptible (S) and resistant (R) biotypes of tall fleabane were distinguished clearly by the relative distribution frequency of injury index caused by 78 µM paraquat. Although malondialdehyde, an indicator for peroxidation damage to the plant, in the the R-biotype was not changed, in the S-biotype malondialdehyde increased within 2 h after treatment of 50 µM paraquat. Analysis of several antioxidants and pertinent enzymes revealed that ascorbate peroxidase activity was decreased by paraquat treatment in the S-biotype; and a lower basal level of ascorbate was present in the S-biotype as well. The maintenance of a high ratio of reduced glutathione to total glutathione, coupled with a pronounced and rapid increase of glutathione reductase (GR) activity in the the R-biotype, suggests that an active reduced glutathione/oxidized glutathione (GSH/GSSG) cycle is critical to paraquat resistance of tall fleabane. The decisive contribution of a functional GSH/GSSG cycle to paraquat resistance through an enhancement of GR activity in this weed was further confirmed by an experiment of exogenous application of ascorbate.

Mass spectrometry, the science and technology of gaseous ions (1), has as its basis the measurement of mass-to-charge ratios (m/z) of ions. All atomic and molecular ions are, in principle, accessible by mass spectrometry, making it a universal method for chemical analysis. Its implementation requires suitable methods of ion generation, ion analysis, and ion detection. We treat each of these processes in turn and show below that there are multiple methods of accomplishing each.The first step in recording a mass spectrum is to convert analyte molecules (or atoms) into gas phase ions. In biological applications, the most common ionization techniques are electrospray ionization (ES) (2), atmospheric pressure chemical ionization (APCI) (3,4), and matrix-assisted laser desorption ionization (MALDI) (5). These are soft ionization methods in the sense that at least some analyte molecules are converted, intact, into corresponding ions. Solution phase samples are examined with ES and APCI while MALDI is particularly appropriate for solid phase samples. Having successfully generated gas phase ions, they must then be mass analyzed. There are several different types of mass analyzers, all based on the interactions of charged particles with electric and/ or magnetic fields. T1 summarizes the most common mass analyzers and lists some analytical performance characteristics by which they can be compared (6). As with the various ionization methods, there is no single right choice -the nature of the problem and the resources of the laboratory will dictate which mass analyzer is most appropriate.Most uses of mass spectrometry are made in combination with chromatographic separation, principally in the form of the GC/MS or LC/MS technique. These combinations have been used, for example, in organic analysis in the environmental sciences and in characterization of biological compounds, including molecular weight (MW) determinations, and sequence analyses of biopolymers (7). Increasingly important applications have been found in drug metabolism and protein sequencing due to the high sensitivity and chemical specificity of mass spectrometry. These advantages apply even when the samples are presented to the mass spectrometer as mixtures since the two-stage tandem mass spectrometry (MS/MS) experiment serves as a method of separation as well as characterization of the separated components. Quadrupoles and Ion TrapsWith the above background on ionization and mass analysis, we can now introduce a family of mass analyzers whose operation is based on ion motion in rf electric fields. The quadrupole mass filter (8), or linear quadrupole, consists of a linear array of four symmetrically arranged rods (F1) to which rf and dc voltages are supplied. Forces are exerted in a plane normal to the direction (z-direction) in which the ions drift through the array in their journey from the ion source to the detector. The rf potential gives rise to a field which alternatively reinforces and then dominates the dc field, also applied by coupling opposite sets...