Ozone Stress and Ectomycorrhizal Root–Shoot Signaling

Tropospheric ozone (O3) is a secondary atmospheric pollutant, generated from oxides of nitrogen and volatile organic compounds reacting in the presence of sunlight. It has been recognized as an increasing and damaging agent to plants (Karnosky et al. 2005). The effects of O3 on plant growth and development at different structural and functional levels of the aboveground parts have been studied extensively (Matyssek and Sandermann 2003), while the relative inaccessibility of plant roots and mycorrhiza has hampered efforts to understand effects of O3 below ground (Andersen 2003; Schloter et al. 2005). O3 triggers physiological changes in leaves that affect carbon source strength, i.e., the amount of carbon available for allocation to sink tissues. Decreased carbon assimilation, increased metabolic costs for repair mechanisms, and decreased phloem loading, all lead to decreased carbon allocation below ground, thus affecting roots, root symbionts, rhizodeposition, litter quality and quantity, and consequently the whole soil food web (Andersen 2003).