OZONE EFFECTS ON DRY-MATTER PARTITIONING AND CHLOROPHYLL FLUORESCENCEDURING PLANT DEVELOPMENT OF WHEAT

In closed-chamber fumigation experiments dry matter partitioning and c hlorophyll fluorescence of wheat were studied, analysing the effects o f ozone during different stages of plant development. Ozone causes enh anced leaf senescence, leading to a loss of green leaf area and, conse quently to a decreased supply of assimilates, affecting (in increasing order of severeness) stem, ear and grain productivity because of redu ced storage pools for translocation. Leaves of plants before shooting stage were most sensitive but the lack of green leaf area after ear em ergence had the most pronounced effects on grain yield. Measurements o f photochemical capacity showed that evidence for negative ozone effec ts could be found in changes of chlorophyll fluorescence parameters in leaf sections not yet showing visible ozone injury. Negative effects on photosynthesis were more distinct with increasing accumulated ozone dose, with increasing age of leaf tissue and with increasing ozone se nsitivity of the cultivar. The changes in chlorophyll fluorescence are most likely to be explained by a decreased pool size of plastoquinone s caused by ozone.