Mechanisms of oxygen activation during plant stress

Synopsis 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.