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ŠUMARSKI LIST 5-6/2021 str. 81     <-- 81 -->        PDF

and strains are tolerant to high acidity and even some of them prefer lower pH (Rudawska, 2007). Thus, soil acidification can favour growth of ECM fungal species which are more tolerant to low pH in soil. If the individual species reproductive fitness is affected, a permanent shift in the diversity of ECM community will happen (Bellgard and Williams, 2011). At the end of 20th century in different regions of Europe with increased soil acidification was observed a decline of sporocarps production and ECM diversity in forest communities (Arnolds, 1991).
It is also known that ECM and AM fungi prefer soils with different pH values. ECM fungi are associated with the dominant, mostly tree species, and occupy soils that have high accumulations of organic surface litter and thus often lower pH values. Furthermore, these mycorrhizal fungi do not invade soils of high pH value and high buffering capacity. Where such soils are converted to intensive crop agriculture, AM associations will become dominant and ECM will be excluded. On the other hand, AM associations dominate in soils with neutral to relatively high pH, with high buffering capacity, which are immune to changes in the pH value. If these systems are disturbed, ECM fungi do not invade these soils, but AM fungi continue to dominate (Bellgard and Williams, 2011). Comparing the colonization level of roots with ECM, AM and dark septated endophytic fungi in poplars growing under different environmental conditions, at the site that was contaminated with pyrite tailings (FeS2), heavy metals and had a low pH, Katanić et al. (2013) did not find structures of AM fungi while other fungal groups were present. These results are in acordance with previous results that AM fungi prefer soils with neutral or higher pH values.
Elevated concentrations of heavy metals in soil may have toxic effects on soil microorganisms and mycorrhizas of forest trees (Smith and Read, 2008). Heavy metals are accumulated in the organic layer of forest soils and can inhibit numerous soil processes which decrease decomposition and nutrient availability (Rudawska, 2007). Different species of mycorrhizal fungi and their strains are highly variable in the response to heavy metals in the growth medium (Rudawska, 2007). Heavy metals can detrimentally affect the formation and maintenance of ectomycorrhizas as well as its diversity on different tree species such as Populus spp. (Katanić et al., 2011; Katanić et al., 2015), Salix spp. (Regvar, 2010) and Picea spp. (Rudawska, 2007).
On the other hand, fungal partner in mycorrhizal symbiosis can prevent heavy metal transport from soil to the plant shoots. Response of mycorrhizas to heavy metals may be explained by various mechanisms of metal detoxification (Bellion et al., 2006; French, 2017). Metal ions may bind to compounds such as chitin, glomalin and melanin found in cell walls of fungal hyphae. The latter one is particularly well-known for its ability to protect fungi from a variety of unfavourable environmental conditions. Further, mycorrhizal fungi can stimulate the biosynthesis of chelating agents such as phytochelatins and metallothioneins to bind the metal ions and decrease their toxicity. Also, metal ions can be deposited throughout the wall, cytoplasm, and vacuole of fungal cell. However, mechanisms of detoxification of heavy metals inside the fungal mycelium require carbon. Mycorrhizae may also change metabolism of host plants in order to respond to metal toxicity (Bellion et al., 2006; Rudawska, 2007; French, 2017). It was observed that fungi which produce the largest quantity of extraradical mycelium are most efficient in accumulation of heavy metals and provide the best protection for host plants (Smith and Read, 2008; Bojarczuk and Kieliszewska-Rokicka, 2010).
Elevated concentrations of ozone (O3) may have detrimental effects on mycorrhizal colonization and diversity. Tropospheric ozone has been recognized as a damaging agent to plants. It triggers numerous physiological changes in plant organism which lead to decreased carbon allocation below-ground, thus affecting roots and indirectly root symbionts such as mycorrhizal fungi (Cudlin et al. 2007). However, the sensitivity to ozone differs between tree species and clones, experimental growth conditions, and between the age-related physiological differences within the same species. Root growth reductions induced by ozone might make rhizosphere organisms more susceptible to drought or nutrient deficiency, as well (Cudlin et al. 2007).
The decreased growth of roots and mycorrhizas might be an early indicator of the damaging impacts of ozone in some tree species, occurring prior to visible responses of aboveground parts (Cudlin et al., 2007; Rudawska, 2007; Katanić et al., 2014).
Effects of ecosystem fragmentation and habitat loss on mycorhizae – Učinci fragmentacije ekosustava i gubitka staništa na mikorize
Climate change has a significant influence on the distribution of species as well. Fragmentation of natural land ecosystems is a result of colonization by humans and their domesticated animals. Habitat loss induced by the conversion of wildlands and forest ecosystems to agricultural lands threatens biodiversity and contributes to increasing of atmospheric CO2. Ecosystem fragmentation directly impacts dispersal of mycorrhizal fungi therefore affecting their community structure. Changes in the abundance and distribution of host plants significantly impact viability, productivity, and efficiency of fungal partner. This is particularly important for these mycorrhizal fungi that are obligatory dependent on their partner such as AM fungi (Bellgard and Williams, 2011).