Tropospheric ozone will increase during the nest decades and will reach the critical threshold concentration of 30 nl l(-1) more often in rural regions than in urban areas. Damage inflicted on plants by ozone can be divided into chronic and acute damage, reflecting different defence strategies of the plants, Chronic damage, somewhat similar to a premature senescence, follows disturbances in sugar metabolism inhibition of photosynthesis. disbalances in the redox-status and ROS-production in the chloroplast stroma, The acute damage resembles the hypersensitive response which occurs after a pathogen attack. Either the accumulation of reactive oxygen species (ROS) or the efficiency of scavenging systems in the apoplast may govern the spread of cell death. Probably. different signalling pathways mediate the plant responses, such as ethylene, salicylate and ROS. ROS production and lipid peroxidation take place either on the plasma membrane or in the chloroplastic membrane and lipid hydroperoxides and derivatives, such as jasmonate, can act as signals for subsequent plant reactions. Both binds of plant responses, i.e., acute as well as chronic damage, can affect the plants' abilities to respond to further stress attacks.
