Contrasting ozone × pathogen interaction as mediated through competition between juvenile European beech (Fagus sylvatica) and Norway spruce (Picea abies)

Based on the growth-differentiation balance theory (GDB) and the influence of tropospheric ozone (O3) on plants, we hypothesized that pre-conditioning with elevated O3 reduces adverse effects of the root rot pathogen Phytophthora citricola Sawada. To this end a 2-year phytotron study with juvenile European beech (Fagus sylvatica L.) and (Picea abies [L.] Karst.) grown in mixture was performed. The hypothesis was tested on phenological, leaf and root morphological as well as physiological aspects of plant performance. Contrasting with spruce, elevated O3 limited leaf and root biomass development, photosynthetic performance and N uptake of beech. The growth limitation by O3 conveyed increased resistance in beech against the pathogen. Conversely, spruce displayed enhanced susceptibility in the combined O3/P. citricola treatment. The hypothesis was supported in the case of beech rather than spruce. Nevertheless, conclusions support GDB regarding the trade-off between growth and stress defense, although compliance appears to be species-specific.