No abstract

No abstract

The formation of superoxide ion radicals ([Formula: see text]) by chloroplasts from senescing leaves ofPhaseolus vulgaris L. was determined by electron-spin-resonance measurements of Tiron (1,2-dihydroxybenzene-3,5-disulfonic acid) semiquinone, a radical species formed when Tiron reacts with[Formula: see text]. The Tiron radical signal obtained from chloroplasts is sensitive to superoxide dismutase (EC 1.15.1.1) confirming that it is derived from[Formula: see text], oxygen-dependent and unaffected by 3-(3,4-dichlorophenyl)-1,1-dimethyl urea and hydroxylamine. Further confirmation of the identity of the radical was obtained by using the diagnostic spin trap 5,5'-dimethyl-1-pyrroline-1-oxide. The production of[Formula: see text] by illuminated chloroplasts increases by about fourfold during the early stages of leaf senescence; it declines again as senescence intensifies. A similar pattern of[Formula: see text] production was noted during aging of isolated chloroplasts in buffer. In addition, heat denaturation of freshly isolated chloroplasts greatly increases their ability to form[Formula: see text] upon illumination, indicating that the radical is formed through a photochemical reaction involving chlorophyll, rather than enzymatically. Accordingly, the rise in[Formula: see text] production during senescence may reflect deteriorative molecular rearrangements in the thylakoid membranes, which expose chlorophyll molecules normally inaccessible to oxygen. The propensity of chloroplasts to produce increased levels of[Formula: see text] with advancing senescence is not counterbalanced by an augmented enzymatic radical-scavenging capability. Moreover, the peak in[Formula: see text] production during leaf senescence coincides temporally with the initiation of lipid peroxidation and the formation of gel-phase lipid in chloroplast membranes, phenomena that are known to be induced by this reactive species of oxygen.

Treatment of primary bean leaves with 10 mg/I paraquat induces the formation of gel phase lipid in microsomal and chloroplast membranes and enhances the activity of superoxide dismutase, but only if the leaves are exposed to light. These light‐dependent changes in membrane lipid phase properties show a close temporal correlation with enhanced O27 production by illuminated chloroplasts and the onset of lipid peroxidation. Malondialdehyde and ethane, which are both formed during lipid peroxidation, are produced in large amounts by paraquat‐treated leaves exposed to light, but not by those maintained in the dark. Electron spin resonance measurements indicate that production of O27 by illuminated chloroplasts is more than 2‐fold greater in the presence of paraquat than in its absence. The identity of the radical formed by illuminated chloroplasts in the presence of paraquat was confirmed by using the diagnostic spin trap 5,5′‐dimethyl‐l‐pyrroline‐l‐oxide and by establishing that its formation is sensitive to superoxide dismutase. The observations collectively indicate that paraquat‐mediated membrane deterioration is light‐dependent and attributable to enhanced O27‐production.

No abstract