The effects of ozone on Mediterranean wheat in a changing environment

Abstract

[eng] The assays were conducted in the La Higueruela/MNCN-CSIC Agricultural Research Station, situated within a climate typical of the Iberian Peninsula, where controlled exposure to ambient and elevated ozone levels was facilitated through open-top chambers. Chapter 1 focused on evaluating the impact of ozone fumigation on different aged cultivars, while Chapters 2 and 3 explored the interactive effect of ozone and nitrogen fertilization. Additionally, Chapter 4 examined the effect of ozone on the fungal pathogen Puccinia striiformis. To assess the impact of ozone on wheat, various factors were studied that are pertinent for crop management and policy-making. Chapter 1 delved into the effects breeding has had on wheat sensitivity to tropospheric ozone pollution, noting that the selection for a higher harvest index in the more modern varieties has led to an increased susceptibility to ozone, though the benefits of breeding mostly offset the effects of the pollutant. However, as landraces have a more extensive genetic base to improve ozone tolerance, they should be considered for future breeding processes. Expanding upon crop management, Chapter 2 investigated the potential mitigative effects of additional nitrogen fertilization on ozone-induced yield and quality reductions. While findings indicated a beneficial impact from the fertilizer, this effect was only observed until the highest levels of ozone concentration, beyond which the fertilizer effect was entirely eliminated, resulting in both yield and growth values resembling those of the lower fertilizer treatment under the same fumigation. This resulted in lower critical levels under the higher nitrogen treatment, prompting a policy-making-oriented consideration of the economic and environmental consequences associated with ineffective fertilizer use at elevated ozone levels. However, as additional nitrogen does provide some compensation at certain elevated ozone levels, the mechanisms through which this could be happening were explored in Chapter 3. It was noted that the additional nitrogen allowed the plant to invest more resources into the photosynthetic machinery, thereby enhancing assimilation to a certain extent. This can be considered for future research aimed at selecting varieties for a higher nitrogen use efficiency, particularly given the distinct nitrogen uptake and storage mechanisms of modern cultivars compared to landraces. Finally, Chapter 4 addressed another factor that could be essential for both crop management and policy-making, pathogens. The effect of ozone on a yellow rust infection was analyzed, noting its mitigation on yellow rust infection. While ozone exhibited beneficial effects at its current levels, higher concentrations – albeit lower than those reducing nitrogen use efficiency – proved detrimental to plant health. In Chapters 1 and 3, isotopic signatures were also investigated. For both, and regardless of nitrogen fertilization treatment, the grain carbon isotope proved to be an excellent indicator of chronic ozone stress, while nitrogen isotopes in both leaf and grain provided valuable insights into nitrogen remobilization in wheat. However, the interpretation of these findings depended on correlations with other measurements due to the intricate nature of nitrogen dynamics within the plant. In conclusion, this thesis underscores the importance of considering cultivar-specific tolerance mechanisms in breeding programs, acknowledging the limitations of crop management practices such as nitrogen fertilization, and recognizing the adaptation of modern wheat cultivars to current ozone levels, which provide them with protection against pathogens such as Puccinia striiformis.