Apoplastic reactive oxygen intermediates, which are formed during the exposure of a higher plant to ozone (O 3 ), have been proposed to be detoxified by apoplastic ascorbate (ASC). An investigation to determine whether the differential sensitivity of two white clover clones ( Trifolium repens L. cv Regal) to O 3 was related with their levels of ASC, glutathione derivatives or with the total antioxidative capacity. In contrast to what might be expected, the sensitive clone of white clover (NC-S) constitutively showed a 72% higher concentration of apoplastic ASC compared to the O 3 -tolerant clone (NC-R). Furthermore, NC-S also showed a higher redox status of apoplastic ASC. These results indicate that higher ASC levels in the apoplast of NC-S are not sufficient to induce a higher O 3 tolerance. The redox status, but not the absolute concentration of homoglutathione in the symplast was found to be constitutively higher in NC-R than in NC-S. It is not clear, however, whether homoglutathione is a direct cause of the differential O 3 detoxification capacity of both clones. Total antioxidative capacity measurements ruled out the contribution of other low-molecular antioxidants to the relative tolerance of NC-R. It was concluded that elevated apoplastic ASC levels can not always be sufficient to render a plant O 3 tolerant.

Brassicaceae are characterised by glucosinolates (GS), which appear to be involved not only in biotic but also in abiotic stress responses of plants. We investigated the effect of O (3) stress on leaf GS concentrations in two lines of BRASSICA NAPUS L., differing in GS content. Ozone fumigation decreased GS concentrations in leaves of B. NAPUS of one line. In control conditions, chlorophyll content, rates of saturating photosynthesis, and quantum yield of photosystem 2 differed between the two BRASSICA lines, but differences were smaller in O (3)-stress conditions, suggesting that the relationship between leaf GS concentration and sensitivity to abiotic stress merits further research. In agreement with other ecophysiological measurements, chlorophyll fluorescence imaging clearly distinguished both lines and in some cases also treatments. A method for analysis of fluorescence images accounting for the two-dimensional leaf heterogeneity is presented.

To better understand the response of a plant to O3 stress, an integrated microarray analysis was performed on Arabidopsis plants exposed during 2 days to purified air or 150 nl l−1 O3, 8 h day−1. Agilent Arabidopsis 2 Oligo Microarrays were used of which the reliability was confirmed by quantitative real‐time PCR of nine randomly selected genes. We confirmed the O3 responsiveness of heat shock proteins (HSPs), glutathione‐S‐tranferases and genes involved in cell wall stiffening and microbial defence. Whereas, a previous study revealed that during an early stage of the O3 stress response, gene expression was strongly dependent on jasmonic acid and ethylene, we report that at a later stage (48 h) synthesis of jasmonic acid and ethylene was downregulated. In addition, we observed the simultaneous induction of salicylic acid synthesis and genes involved in programmed cell death and senescence. Also typically, the later stage of the response to O3 appeared to be the induction of the complete pathway leading to the biosynthesis of anthocyanin diglucosides and the induction of thioredoxin‐based redox control. Surprisingly absent in the list of induced genes were genes involved in ASC‐dependent antioxidation, few of which were found to be induced after 12 h of O3 exposure in another study. We discuss these and other particular results of the microarray analysis and provide a map depicting significantly affected genes and their pathways highlighting their interrelationships and subcellular localization.