Mechanisms of oxygen activation during plant stress: Biochemical effects of air pollutants

Green plants can adapt to drought, temperature, high and low light, infections, air pollution and soil contamination. Dependent on the strength of these impact(s), several symptoms indicate the deviation from normal, steady-state-metabolism. Most of these visible or measurable symptoms are connected with oxygen activation where principally a transition from heterolytic (two electron transitions) to increased homolytic (one electron transitions) reactions is observed. Homolytic reactions create free radicals, which are generally counteracted by a parallel increase of radical scavenging processes or compounds thus warranting metabolic control within certain limits. At advanced states of stress control may be gradually lost and chaotic radical processes dominate. Finally, cellular decompartmentalizations allow lyric and necrotic processes. Every episode during this cascade is characterizable by the balance between pro- and antioxidative capacities. Photosynthetic processes which are under metabolic and oxygen-detoxifying control are converted into photodynamic reactions which are only controlled by light and scavenger- and/or quencher-availabilities. Photoinhibition may represent the threshold between these two light driven events. This (more or less theoretical) sequence of events is not yet fully understood and in most cases can only punctually be characterized and followed by indicator reactions such as ESR. In this communication we shall concentrate on basic redox-mechanisms during oxidative stress situations involving primary air pollutants. Chemical interactions between different gaseous pollutants and generally mechanisms of the finally toxic effects on target molecules will be shown.