Effect of Claroideoglomus etunicatum inoculation on nickel phytoremediation and uptake of some micronutrients by maize (Zea mays L.)

Nickel (Ni) is a fundamental micronutrient in plants but hampers plant growth and metabolism at elevated levels in the soil. Ni toxicity to plants is manifested mainly by the decrease in germination efficiency, the inhibition of growth and root branching, damage to the photosynthetic apparatus, and the induction of oxidative stress. In recent years, the use of arbuscular mycorrhiza (AM) has gained importance for its role in enabling plants to tolerate Ni toxicity. However, information about their effectiveness in alleviating Ni stress is scanty. The process of element transport in plants may be assumed to be different among heavy metal concentrations in the substrate. Consequently, whether AM fungi enhance the metal transport to shoots (phytoextraction) or immobilize them in the roots (phytostabilization) mainly depends on metal concentration in the substrate. Moreover, Ni has been reported to compete with other micronutrients for absorption sites, which would trigger different changes of elements concentrations. The aim of this study was to investigate the role of AM fungus in alleviating Ni stress and its possible function in plant nutrition.

In this study, the effects of mycorrhizal inoculation of corn plants with Claroideoglomus etunicatum on alleviation of Ni impact on plant were evaluated. Some growth characteristics of the plant, phosphorus content, micronutrients (iron, zinc, and copper), concentration of nickel in shoot and root, and Bradford reactive soil glomalin (BRSG) were assessed. Accordingly, a two-factor experiment (AM inoculation × Ni levels) in completely randomized design was done. The factors included the different concentrations of nickel (control (Ni0), 50 (Ni50), 100 (Ni100) and 250 (Ni250) mg kg-1), and the levels of fungal application (control without inoculation (NM) and inoculated with C. etunicatum (AM)). Plants were grown in the greenhouse for 90 days and then the growth parameters were recorded. The concentration of phosphorus was measured spectrophotometrically and the concentration of iron, zinc, copper, and nickel in digested plant samples was determined by ICP-OES. Bio-concentration factor and translocation factor were also calculated. The colorimetric method was used to quantify Bradford-reactive soil glomalin. The Bradford protein assay was utilized to determine the concentration of easily extractable and total Bradford-reactive soil glomalin using bovine serum albumin (BSA) as a standard.

Increasing the nickel concentration in soil decreased the dry weight of root and shoot, and this decrease was significant in both inoculated and non-inoculated plants at Ni250 treatment (p≤0.05). Plants inoculated with AM fungus showed significantly higher height and dry weight of shoots than plants without inoculation (p≤0.05), but the effect of mycorrhizal inoculation on the dry weight of roots was not statistically significant. The effect of nickel on the colonization percentage of roots and easily extractable Bradford-reactive soil glomalin (EE-BRSG) was significant. EE-BRSG was higher at all levels of nickel in inoculated plants than in non-inoculated ones. Moreover, with the increase of nickel concentration in soil up to 100 mg Kg-1, total Bradford reactive soil glomalin (T-BRSG) increased. The concentration of phosphorus in the shoots and roots of inoculated plants was higher than in non-inoculated plants. Mycorrhizal inoculation significantly increased the concentration of zinc and copper in the aerial part. Moreover, nickel treatment did not show a statistically significant effect on the concentration of copper in the aerial part and iron in the roots. Inoculation with AM fungus showed a significant impact on the nickel concentration of the shoots and roots, and the concentration of nickel in the roots of inoculated plants at Ni250 level was significantly higher than plants without inoculation by 29% (p<0.05). Mycorrhizal plants had lower nickel concentrations in the aerial part at Ni100 and Ni250 by 30% and 33% respectively, compared to the NM plants. The translocation factors in inoculated plants at Ni100 and Ni250 levels were significantly lower than that in non-inoculated plants, which indicates the role of fungi in preventing the transfer of nickel to the aerial parts and its accumulation in the roots. Moreover, inoculated plants in the Ni100 and Ni250 treatments showed a significantly lower bio-concentration factor by 36% and 22%, respectively, compared to non-inoculated plants.

The results showed that AM colonization can help to reduce the toxicity of nickel by increasing plant growth and uptake of phosphorus, zinc and copper. AM colonization had a prominent impact in preventing the nickel transfer to the aerial parts and its accumulation in the roots. It seems that AM fungi can be used for phytostabilization of heavy metals in soils.