Increase in lipid-dependent carotene destruction as compared to ethylene formation and chlorophyllase activity following mixed infection of sugar beet (Beta vulgaris L.) with beet yellows virus and beet mild yellowing virus

Mechanisms of oxygen activation during plant stress

Green plants, within certain limitations, can adapt to a wide variety of unfavourable conditions such as drought, temperature changes, light variations, infectious attacks, air pollution and soil contamination. Depending on the strength of the individual impact(s), fluent or abrupt changes in visible or measurable stress symptoms indicate the deviation from normal metabolic conditions. Most of the visible or measurable symptoms are connected with altered oxygen metabolism principally concerning the transition from mostly heterolytic (two-electron transition) to increased homolytic (one-electron transition) processes. Homolytic reactions within metabolic sequences create, however, free radicals and have to be counteracted by the increase in radical-scavenging processes or compounds, thus warranting reaction sequences under metabolic control. At later states of stress episodes, the above control is gradually lost and more or less chaotic radical processes take over. Finally, cellular decompartmentalisations induce lytic and necrotic processes which are visible as the collapse of darkening cells or tissues. Every episode during this process is governed by a more or less denned balance between pro- and antioxidative capacities, including photosynthetic (strongly under metabolic and oxygen-detoxifying control) and photodynamic (only controlled by scavenger- and/or quencher-availability) reactions. This (theoretical) sequence of events in most cases can only be characterised punctually by strongly defined (analytical) indicator reactions (ESR) and is certainly species- and organ-specific.

Antioxidant functions of carotenoids

Carotenoid pigments, including hydrocarbons such as beta-carotene or xanthophylls such as lutein and zeaxanthin, are very widely distributed in nature, where they play an important role in protecting cells and organisms against the harmful effects of light, air, and sensitizer pigments. This process has been demonstrated in bacteria, algae, plants, animals, and even in humans in the light-sensitive disease, erythropoietic protoporphyria. The primary mechanism of action of this phenomenon appears to be the ability of carotenoids to quench excited sensitizer molecules as well as quench 1O2. In addition to this protection, and potentially of even greater biological importance, is the fact that carotenoids can also serve as antioxidants under conditions other than photosensitization. This review presents the data available indicating the extent of this important function. Antioxidant action can be documented in both enzymic and nonenzymic systems, and has been reported in subcellular, cellular, and animal studies. In fact, the many reports indicating that carotenoids may possess some anticarcinogenic properties may well be related to their ability to interact with and quench various radical species that can be generated within cells.

Radical intermediates involved in the bleaching of the carotenoid crocin. Hydroxyl radicals, superoxide anions and hydrated electrons

The participation of the primary radicals in the bleaching of aqueous solutions of the carotenoid crocin by ionizing radiation was investigated, employing both X-radiolysis and pulse radiolysis. The pulse-radiolytic data demonstrated a very rapid diffusion-controlled attack by both hydroxyl radicals (.OH) and hydrated electrons (eaq-), while superoxide anions (O2-) did not react at all. The site of the initial reaction of these radicals was not limited to the polyene chromophore. Slower secondary reactions involving crocin alkyl or peroxy radicals contribute mainly to the overall bleaching, in particular during steady-state irradiation.