Antioxidant responses to drought and heatwave as markers for climate stress and adaptation

Abstract

© 2015 Dr. Agnieszka Wujeska-KlauseAccording to predictions, extreme events such as droughts and heatwaves will increase in frequency and intensity in the near future. Droughts and heatwaves can have negative effect on forests leading to dieback of more sensitive tree species with negative effect on ecosystem stability. The response of trees to stress conditions can vary between species or within one species, and such variation may depend on specific adaptations of trees to their preferred habitat. This study focuses on Australian trees from two common genera – Acacia and Eucalyptus, which have many species distributed across different habitats. Adaptation to various habitats of congeneric species provides a good model for studying responses of defence system to drought and heatwave stresses of ecologically different, but closely related species. Responses of the defence systems under stress conditions can help to understand which species will cope more efficiently under future climate change predictions. This thesis consists of one meta-analytical review on antioxidative and photoprotective defence system responses to drought stress, three experimental studies conducted under controlled conditions and one field study. In the controlled experiments, two Acacia and two Eucalyptus species adapted to contrasting habitats were exposed to drought stress or heatwaves conditions and to a combination of both stress factors. Response of seedlings to the treatments was examined with measurements of antioxidative defence systems, specifically two ubiquitous and abundant low-molecular weight antioxidants glutathione and ascorbic acid, along with gas-exchange, chlorophyll fluorescence and gene-expressions measurements. In the field study, responses of antioxidative defence systems were measured on multiple provenances within one species in a common garden experiment. Provenances were from different sites of origin representing a range of habitats with different annual rainfall. It was hypothesised that responses of antioxidative defence systems will depend on stress intensities, the stress factors in question, seasonal changes (for the field study) and on species adaptation to habitat conditions. In conclusion, antioxidative defence systems responded differently depending on stress intensities, different stress factors and adaptation to habitat conditions. Concentration of antioxidants increased in response to initial stages of stress, but more severe stress conditions led to decreased concentration of investigated antioxidants in conjunction with more oxidised redox states of those antioxidants. Antioxidants showed stronger response to heatwaves than to drought stress alone, and a more pronounced response to a combination of both stress factors than to each factor separately. Responses of antioxidative defence systems varied between closely genetically related trees species adapted to contrasting habitats. Tree species from arid or low – moderate rainfall habitats showed delayed stomatal closure, more efficient photoprotection and no or weak antioxidative defence system responses to stress conditions. In contrast, tree species from humid or high rainfall habitats closed stomata early, had less efficient photoprotection and stronger antioxidative defence systems responses. Antioxidant responses within the same species but different provenances were highly dependent on environmental conditions, and to some extent dependent on the provenance, but there was no clear relationship to environmental factors at the site of provenance origin