Clonal Differences of Aspen (Populus spp.) in Responses to Elevated Ozone and Soil Nitrogen

The global background tropospheric ozone concentrations have doubled since the pre-industrial era, and ozone is considered to be one of the most toxic air pollutants to plants. Ozone-induced reductions in the photosynthetic capacity and in the carbon assimilation of plants may lead to increased amounts of carbon dioxide in the atmosphere. Populus species have been reported to be among the most sensitive forest trees to elevated ozone, and in the Nordic and Baltic countries, European aspen (Populus tremula) and hybrid aspen (crossings of the native P. tremula and the American trembling aspen, Populus tremuloides) are used especially in the pulp and paper industry for the production of high quality paper, and for biofuel production. The clones in commercial production have been selected primarily based on high yield and desirable wood physicochemical properties, but it is also important to test the stress tolerance of the clones before they are used in large-scale plantations. We studied the effects of moderately elevated ozone (1.5 x ambient concentration) on the growth, physiology and foliar chemistry of selected aspen and hybrid aspen clones in open-field experiments to find out if there are differences in the ozone sensitivity of the clones, and if the sensitivity is affected by soil nitrogen availability. Both ozone-sensitive and ozone-tolerant clones were found after two years’ growth in elevated ozone based on growth responses to ozone. Differences in the foliar physiology and chemistry among the clones and between the ozone-sensitive and tolerant groups were also found. However, the reason for the significant growth reduction of the ozone-sensitive clones could not be determined by the differences in the leaf characteristics studied. Nitrogen amendment counteracted the adverse ozone effects by increasing the growth of all plant parts. Ozone enters the leaf through stomata and is either detoxified in the apoplast or induces an active production of reactive oxygen species, such as hydrogen peroxide (H2O2). We found accumulation of H2O2 inside the plasma membrane in the cytoplasm and in the chloroplasts of an ozone-sensitive aspen, whereas in the tolerant clones, H2O2 was found only on the outer surface of cell walls, indicating efficient detoxification of ozone in the apoplast. Populus species are also rich in phenolic compounds, and we studied the potential role of leaf phenolics as antioxidants in ozone-induced oxidative stress. The ozonetolerant clones had high concentrations of condensed tannins, which have good radical-scavenging properties, but which are also costly in high concentrations in terms of growth. The ozone-sensitive clones allocated carbon into salicylates, which have no antioxidative capacity but may instead protect the trees from herbivory. When considering the stress tolerance of a species in a changing climate, the interactive effects of many factors have to be taken into account. High yield does not often correlate with stress tolerance, and we found both good growers and slow growers in both the ozone-sensitive and ozone-tolerant groups. The high intraspecific genetic variation in aspen suggests that natural aspen populations may adapt to changes in the environment through selection of tolerant genotypes but for large-scale clonal plantations the stress tolerance or responsiveness to fertilization should be assessed beforehand and weighed against the desirable wood properties or growth potential of the clones. In low deposition areas, such as Scandinavia, the atmospheric N deposition may not counteract the effects of ozone in native aspen stands, but adequate nitrogen fertilization of hybrid aspen plantations could compensate for the decrease in growth caused by elevated ozone. Universal Decimal Classification: 504.5, 546.214, 581.1, 581.2, 581.54, 582.681. 81, 632.151 CAB Thesaurus: climatic change; ozone; trees; Populus; clones; clonal variation; genetic variation; genotypes; hybrids; growth; yields; plant physiology; leaves; nitrogen; soil; hydrogen peroxide; plasma membranes; cytoplasm; chloroplasts; cell walls; detoxification; antioxidants; secondary metabolites; phenolic compounds; tannins; salicylates; stress factors; tolerance