Response of Photosynthesis and Cellular Antioxidants to Ozone in Populus Leaves

Abstract

Atmospheric ozone causes formation of various highly reactive intermediates (e.g. peroxyl and superoxide radicals, H202, etc.) in plant tissues. A plant's productivity in environments with ozone may be related to its ability to scavenge the free radicals formed. The effects of ozone on photosynthesis and some free radical scavengers were measured in the fifth emergent leaf of poplars. Clonal poplars (Populus deltoides x Populus cv caudina) were fumigated with 180 parts per billion ozone for 3 hours. Photosynthesis was measured before, during, and after fumigation. During the first 90 minutes of ozone exposure, photosynthetic rates were unaffected but glutathione levels and superoxide dismutase activity increased. After 90 minutes of ozone exposure, photosynthetic rates began to decline while glutathione and superoxide dismutase continued to increase. Total glutathione (reduced plus oxidized) increased in fumigated leaves throughout the exposure period. The ratio of GSH/GSSG also decreased from 12.8 to 1.2 in ozone exposed trees. Superoxide dismutase levels increased twofold in fumigated plants. After 4 hours of ozone exposure, the photosynthetic rate was approximately half that of controls while glutathione levels and superoxide dismutase activity remained above that of the controls. The elevated antioxidant levels were maintained 21 hours after ozone exposure while photosynthetic rates recovered to about 75% of that of controls. Electron transport and NADPH levels remained unaffected by the treatment. Hence, elevated antioxidant metabolism may protect the photosynthetic apparatus during exposure to ozone.All organisms that have evolved in aerobic environments have a variety of enzymatic and nonenzymatic mechanisms to prevent oxidation of cellular components. It is quite probable that existing mechanisms for detoxifying oxyradicals are invoked in response to ozone since it causes the formation of some highly reactive oxyradicals in aqueous solutions, a major product being the superoxide anion (I1). The superoxide anion can be metabolized by several isozymes of superoxide dismutase found in plants (14). The hydrogen peroxide formed by this reaction is toxic. It can inactivate -SH containing enzymes (5, 13) or react with superoxide to form the hydroxyl radical, which can attack many macromolecular

Oxidative Stimulation of Glutathione Synthesis in Arabidopsis thaliana Suspension Cultures

“…Complications to this model arise through demonstrations that large variations in the level of GSH accumulated depend upon the plant species, leaf age, the basal level of GSH, and the ratio of reduced GSH:GSSG accumulated (Smith, 1985). Other studies have shown that in response to gaseous pollutants in some (Guri, 1983;Madamanchi and Alscher, 1991) but not a11 (Sen Gupta et al, 1991) cases, the proportion in GSSG remains small while total GSH levels rise. The mildness of the oxidative stimulus in our system distinguishes our approach from previous studies, and we feel that differences between observations made in our system and those of Smith and colleagues (Smith et al, 1984Smith, 1985) simply reflect differences in the severity of the stress applied.…”

“…Correlative studies have indicated that this response is an important aspect of stress tolerance. GSH is likely to play an important role in stress tolerance and has been implicated in the adaptation of plants to environmental stresses such as drought (Dhindsa and Matowe, 1981), atmospheric pollution (Lee and Bennet, 1982;Guri, 1983;Sen Gupta et al, 1991), and extremes of temperature; furthermore, its synthesis is induced in response to these stresses (Alscher, 1989). However, as yet, no studies have demonstrated an absolute dependence on GSH for stress tolerance, nor have studies demonstrating an induction of GSH synthesis in response to an oxidative stimulus been extended to demonstrate that this elevation has any physiological significance.…”

“…GSH is an important antioxidant in a11 aerobic organisms and plays a crucial role in the defense against activated oxygen species arising as by-products of metabolism (Meister, 1983;Alscher, 1989;Sen Gupta et al, 1991). In plants, the potential for the production of activated oxygen species is greatly enhanced by a wide range of environmental stresses, and it is thought that the ensuing damage results from the accumulation of these species to levels exceeding the antioxidant capacities of the cell (Hamilton, 1991).…”

The response of sensitive and tolerant clones of Populus tremuloides to dynamic ozone exposure under controlled environmental conditions

“…According to Gupta et al (1991), total GSH increased in O $ -fumigated hybrid poplar leaves during exposure. On the other hand, reduction of GSH due to O $ has been reported (Guri, 1983), the reduced GSH levels significantly lower in the sensitive cultivar of Phaseolus vulgaris than in the tolerant, where reductions were slight.…”

Physiological disturbances promoted by ozone in five cultivars of Phaseolus vulgaris L.

“…Besides, gluthatione, which is acknowledged as the central component of antioxidant defense in most aerobic organisms (Foyer et al, 1997), requires NADPH both for its synthesis and for its reduction. Thus again, the photosynthesis decline observed may be the result of less NADPH available for carbon reduction (Gupta et al, 1991). Calatayud et al (2002) mention decreased electron transportation as one of the mechanisms responsible for reducing the photosynthesis process in Lactuca sativa L.…”