DIGITALNA ARHIVA ŠUMARSKOG LISTA
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duration of drought episodes will negatively impact plants primarily by reducing plant biomass accumulation but also decreasing shoots height and survival (Vuksanović et al., 2019). Since low water content in soil influences nutrient availability and soil structure, mycorrhizal fungi may play significant positive roles in mitigation of water and nutrient deficiency (Smith et al., 2010). Fungal hyphae are capable of exploring a larger soil volume than plant fine roots and therefore can mine available water from soil. Mycorrhizal plants can obtain nutrients from progressively narrow pores, as soil becomes drier or more compacted. (Smith et al., 2010). Furthermore, ECM fungi influence plant osmotic regulation and increase water-use efficiency because they increase mineral uptake and transfer and provide growth regulators (such as abscisic acid and cytokinins), that result in a reduction of water potential gradient between the soil and plant (Rudawska, 2007). Water stress may influence the growth of root systems and mycorrhizas. Moderate drought stimulates mycorrhiza formation and maintenance through stimulation of newly discovered terpenoid group of plant hormones known as strigolactones (Bahadur et al., 2017), but if drought is severe the mycorrhizal fine roots may die (Rudawska, 2007). Drought might affect mycorrhiza ability to promote the drought tolerance of host plants (Gehring et al., 2017). However, mycorrhizal fungi differ in efficiency of water-regulation mechanisms and tolerance to water deficit, which depends on the properties of the fungal mycelium (Rudawska, 2007). Studies of plant species dually colonized with both ECM and AM fungi (such as members of Salicaceae and Fagaceae) showed that ECM fungi are more sensitive to drought than AM fungi (Gehring et al., 2017). It was recorded that higher soil moisture level favoured colonization of ECM fungi in some members of genus Populus. (Lodge, 1989; Gehring et al., 2006). Furthermore, in multiclonal plantations it was noted that colonization of poplars (Populus spp.) with ECM fungi was significantly affected by site while colonization with AM fungi depended on genotype (Katanić et al., 2014). On the other hand, for some AM fungi it was observed that they are more responsive to temperature. Namely it was observed at the global scale that the intensity of plant root colonization by AM fungi was strongly related to warm-season temperature (Soudzilovskaia et al., 2015). Moreover, AM fungi predominate in more arid areas (Swaty et al., 2016), which indicates that they better tolerate drought. Drought tolerance in plants inoculated with AM fungi may be explained by increased production and accumulation of the sugar trehalose (C12H22O11) in plant cells. Trehalose forms a gel-like substance which stabilizes lipid bilayers in cell organelles. Thus, organelles remain intact during desiccation and can return to life under favourable environmental conditions (French, 2017). Also, drought might affect the activity, abundance and species composition of AM and ECM fungi. Studying the seasonal variation of the ECM community in mature poplar plantation, Katanić (2013) found lower number of ECM types in summer and autumn, when moisture content in soil was lower. Different species of ECM fungi respond differently to changes in soil moisture. For example, it was recorded that Cenococcum geophilum Fr. ectomycorrhizas responded to the drought induced stress better than Lactarius subdulcis (Pers.) Gray ectomycorrhizas (di Pietro, 2007). Moreover, it was observed that both fungal species and genotypes could respond differently to changes in soil moisture even if they are in association with the same species of host plant (Gehring et al., 2017). Although drought tends to decrease plant biomass, mycorrhizal fungi can mitigate that negative effect by improving plant productivity and growth (Kivlin et al., 2013). Effects of increased nitrogen deposition, soil acidification and pollutants on mycorrhizae – Učinci povećanog taloženja dušika, zakiseljavanja tla i zagađivača na mikorize Concentration of nutrients in natural ecosystems is increasing through anthropogenic nutrient deposition via fertilization and pollution, as well as through increased microbial decomposition induced by soil warming. Climate change can affect the nitrogen cycle, and soil warming can impact on the availability of soil nutrients as well (Simard and Austin, 2010). Wet and dry deposition of nitrogenous compounds (NOx and NHx) can become a dominant source of nitrate (NO3- ) in many natural ecosystems (Bellgard and Williams, 2011). Moreover, inputs of atmospheric ions as nitrate (NO3-) and sulphate (SO42-) form acids that lower soil pH and consequently enhance mobility of many metals. According to some studies increased nitrogen deposition shown positive effects on aboveground plant productivity but negative impact on colonization of mycorrhizal fungi and the growth of the extraradical mycelium in both AM and ECM fungi. However, it was observed that generalist ECM fungal species which form a symbiosis with a wide range of plant species are less affected by increased N availability than specialist ECM species which form a symbiosis with a narrow range of plant species (Bellgard and Williams, 2011). If soil nutrient availability increases, plants invest less carbon into roots and development of mycorrhizas for nutrient uptake and allocate more carbon to aboveground parts. The overall negative impact of increased N inputs on the ECM fungi might be also result of increased soil acidity (Simard and Austin, 2010; Bellgard and Williams, 2011). Different species of ECM fungi vary in their response to acidity of growth substrate. Although, most ECM fungi can grow on the pH range from 3.5 to 5.5, many fungal species |