Tropospheric ozone is a secondary air pollutant which is formed during chemical reactions involving nitrogen oxides (NOx)and volatile organic compounds (VOC) in the presence of solar radiation. Ozone enters the plant leaf mainly through the stomata. Inside the leaf, the oxidizing potential of ozone and the resulting formation of radicals and reactive oxygen species (ROS) make ozone highly phytotoxic. Ozone exposure thus can cause visible foliar injury, biomass reduction and increased leaf senescence, as well as alteration in the assimilate partitioning.The background for this thesis is that there are indications of that plants growing under Nordic conditions seem less tolerantto ozone than plants which grow further south in Europe. Thus, the main aim of this thesis was to compare the reaction toozone exposure of plants growing under a long daylength and plants growing under a short daylength. Three experimentswere performed where long day plants of the genus Trifolium were grown under controlled conditions in a phytotron. Inexperiment I, 52 d old plants of T. subterraneum were investigated, in experiment II 21 d old plants of T.subterraneumwere investigated whereas in experiment III, 31 d old plants of T. subterraneum, T .pratense var. Bjursele and T. repensvar. Norstar were investigated. To study the effect of daylength in relation to ozone exposure, the plants were subjected tothe same daily ozone dose (0 ppb or 70 ppb for 3d for 6h day-1) and grown under different photoperiods where the onlyfactor varying was the light during the night period (long day: 10 h PAR/14 h dim light, short day: 10h PAR/14 dark).The different daylength treatments resulted in different degree of visible ozone-induced foliar injury; LD inducedsignificantly more visible ozone- induced foliar injury in 21 and 52 days old plants of Trifolium subterraneum and 31 daysold plants of Trifolium pratense compared to SD. The LD promoted visible ozone-injury was mostly accompanied by anincrease in the above ground biomass. The LD treated plants thus might have been using a greater proportion of theirmetabolic gain in order to grow at the expense of the repair of the ozone-induced damage. Other explanations involveincreased ethylene formation under LD conditions, or that a photoreceptor (e.g. phytochrome) activates the defense systemto a higher degree under SD - than under LD conditions.In experiment III, two new methods to detect ozone stress were investigated by using an infrared camera. By using passivethermography, parallel digital and infrared imaging of the leaves of T. subterraneum and T. pratense revealed an ozone induced change in the temperature. In the fourth trifoliate leaf of T.subterraneum, there was a significant increase in theleaf temperature after three days of ozone exposure, and an increase in the temperature of the fifth trifoliate leaf after twodays of ozone exposure. In Trifolium pratense there was a temperature decrease in the third trifoliate leaf after 3 days ofozone exposure. These temperature changes were not dependent on visible ozone-induced foliar injury. As the leaftemperature might be an indirect measure of the transpiration, the different responses can possibly be related to stomatalbehaviour. Other additional or alternative explanations might involve changes in the internal structure of the leaves, orchanged biochemical events which might affect the leaf temperature.Leaves of all the Trifolium species were also subjected to active thermography where the leaf temperature responses wererecorded before, during and after a 25 s light pulse. The time constant (ô) was calculated and used as an indicator of howquickly the temperature cools down to the leaf s temperature before the light pulse. In addition, the maximum temperaturedifferences due to the light pulse were compared. The results showed an indication towards a decrease in the maximumtemperature difference, followed by an increase in the time for cooling in ozone-exposed leaves of T. subterraneum and T.pratense. T.repens seemed to respond in an opposite manner, and showed a less stable temperature response.The use of thermography seems to be a supplementary method to investigate ozone stress as it is non-destructive and hasthe potential to detect ozone injury before it becomes apparent.