Spore Thickness of Three Arbuscular Fungi Propagated in Polluted Substrate

Fungi Propagation treatment Spore thickness (|m)

G. mosseae BEG-132a WP-WP 5.4bc

G. caledonium BEG-133a WP-WP 3.9b

G. claroideum Zac-19b WP-WP 5.4a

WP-P 7.4b aFungi isolated from polluted soils and propagated for 2 years in P = polluted substrate and WP = free of pollution.

bFungus isolated from nonpolluted soil propagated for one year in P = polluted substrate and WP = free of pollution.

cMeans from 90 observations, values with the same letter within each fungus represent no significant difference (Tukey a = 0.05).

Source: Modified from Sánchez-Viveros, G. et al., Rev. Int. Cont. Ambiental, 2004, in press. With permission.

diverse terrestrial systems with high adaptation to different environmental and edaphic condi-tions.[4,26,113]. This may be the result of phenotypic plasticity as suggested by Weissenhorn et al. [3] and Meharg and Cairney [104].

To select fungi colonizing roots growing in contaminated soil and at the same time accumulating PTEs, Turnau et al. [35] used a molecular technique and histochemical staining with rhodizoniate simultaneously. They reported that only 5% of the roots showed PTE accumulation when stained with rhodizoniate. Interestingly, G. mosseae was the only fungus present in approximately 75% of these stained roots. Other fungi, such as Glomus intraradices, G. claroideum, and two Glomus species occurred with lower frequency in roots accumulating PTEs (25%). These observations in contaminated soils support the concept of functional biodiversity, which has been observed in natural ecosystems.

Sánchez-Viveros et al. [65] showed that some isolates of AMF may have an inherent ability to tolerate elevated PTE concentrations in soil. These authors observed that spores of G. claroideum (Zac-19), a fungus isolated from agricultural soils, propagated for 1 year in soils contaminated with As/Cu or in noncontaminated soils, were able to tolerate elevated concentrations of these pollutants, even if its spores presented a lower percentage of germination (20 to 30%) than in NP with 90%. In contrast, two AMF (from PS) grown on an As/Cu-contaminated soil presented high germination (50 to 70%), independently of the level of As/Cu present in the soil. This result shows that G. claroideum Zac-19 with low spore germination may be able to colonize plants, thus assuring its survival in polluted soils. For ectomycorrhizal fungi, Blaudez et al. [114] reported a strong interspecific variation in terms of PTE tolerance. In AMF, this variation may also be observed. Adaptive metal tolerance has been reported for a population of the ectomycorrhizal basidiomycete Suillus luteus originating from a metal-contaminated former zinc smelter site [115].

More research is necessary in order to gain knowledge about the stability of fungal tolerance during their cultivation in unpolluted substrates, including fungi isolates from polluted and non-polluted soils. By now, it is still difficult to state the adaptive or constitutive tolerance in AMF. One problem in estimating arbuscular mycorrhizal tolerance to PTE using only the spore germi nation test is that it may not reflect the potential effect of the fungus on plant tolerance, as mentioned before. Even with a low germination percentage, AMF may colonize the plant.

Additionally, often no correlation exists between root rate colonization and fungal effect on the plant. It is possible to find significant effects on plant growth, nutrients, and survival at a low colonization rate. Thus, fungal tolerance within a plant should be investigated too; however, it is very difficult to evaluate. Another important remark is that qualifying a fungus to be "tolerant" to PTE means more tolerant to an element than another fungus, and tolerant to a certain degree in particular conditions ("bioavailability" is often not discussed). Thus, it is rather a relative tolerance.

It is relevant to emphasize that fungal species selection, imposed by any stress, does not always offer the optimal AMF symbionts for the function of the soils [116]. Selection of fungal species may have strong implications in the functioning of contaminated soils. Lokke et al. [117] suggested that a loss of AMF species diversity may increase the susceptibility of the host plant to suffering from environmental stresses.

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