Impact of Ozone and Drought Stress on the Cellular Antioxidative Defence System and on Photosynthetic and Protective Pigments in Crop Plants

The present application is based on the results of the European Open Top Chambers Programme (EOTCP) on yield losses due to ozone impacts, which demonstrated a need for studies on combined influences of ozone and environmental factors that are able to modify ozone effects. Therefore a cluster of co-ordinated research projects is intended, each one concentrating on one separate group of ozone/drought stress effects. This project cluster is designed for the analysis of responses of antioxidants and antioxidative enzyme activities to combined ozone and drought stress in leaf tissues of crop plants (wheat and bean). Responses of the antioxidative systems reflect the impact of environmental stress on plant metabolism. For this purpose, the contents and redox status of ascorbic acid and glutathione, the contents of tocopherols and the composition of chloroplast pigments will be measured. Additionally, important enzymic detoxification pathways will be studied, including peroxidases, glutathione reductase, and seperoxide dismutase. The relative importance of antioxidative compounds will be evaluated with respect to cultivars (resistent/sensitive) and injuries. Under different kind of stresses, degradation of photosynthetic pigments is a prime damage symptom. Hence, the contents of chlorophylls and carotenoids will be determined in accordance with the sampling regime for the photosynthesis project to provide basic data for evaluations of photosynthetic performance. Light stress may become a major problem in stressed crop plants and finally leads to destructive alterations in the photosynthetic apparatus. In this connection, the potency and responses of energy dissipating carotenoid systems (xanthophyll cycle) will be investigated.

The effects of combinations of natural and anthropogenic stress impacts were studied in the present project. Drought stress and elevated ozone co-occur frequently and were chosen as treatments. Two wheat cultivars were investigated: Triticum durum cv "Extradur" is more drought resistant (adapted to Mediterranean conditions) than T. aestivum cv. "Nandu". Furthermore, "Extradur" is regarded more ozone resistant than "Nandu". Ozone effects on vegetations are well documented, but combinations with possible modifying factors are less investigated. In the present project the effects of the modifying factor drought on biochemical ozone-stress responses were studied. The hypothesis was that a resistant cultivar is able to counteract oxidative stress (drought induces oxidative stress!) due to antioxidant protection. Ozone as an additional oxidative stress should further stimulate antioxidant defence. Sensitive cultivars should require stronger protection or show degradation of the protective systems. Components of the antioxidant system were used as stress-indicators, accompanied by physiological measurements such as gas exchange and water relations. Experiments (ozone exposure, drought application) were conducted in open-top chambers at the Austrian Research Centers Seibersdorf during two vegetation periods. Biochemical responses to ozone were not significant albeit a reduction in grain yield was found. Drought affected protective systems significantly, e.g. ascorbate, tocopherol, and detoxifying enzymes showed concentration changes in leaf tissues suggesting higher antioxidant protection requirements. Response of sensitive and resistant cultivars were similar, but the resistant cultivar responded faster. Towards senescence atypical changes occurred in antioxidant systems. Strongest effects of drought were observed on the antioxidant system of roots. Enzyme activities and antioxidant concentrations decreased in roots of both cultivars upon water deficiency. Protection of green tissues seems to be more efficient, as shown by increased antioxidant concentrations, whereas roots are dependent on the supply from the shoot. Drought induced shortage obviously leads to a weakening of the biochemical defence in roots. Roots should be included in stress-physiological studies, because yield depressions may be less correlated to changes in above-ground organs (alterations in photosynthesis, protective systems etc) than to root responses. This could also help to explain many contradicting results of field and laboratory studies in literature.