| The main objective of this study was to examine the contribution of arbuscular mycorrhizal (AM) colonization on nickel (Ni) uptake and Ni tolerance in sunflower (Helianthus annuus L.) at a vegetative or reproductive stage of development. The combined effect of AM colonization, Ni input, and nitrogen (N) fertilization on N-assimilation in sunflower plants was also investigated. Furthermore, concerns over the transfer of heavy metals (HMs) to higher trophic levels led us to investigate whether the AM colonization and accumulation of Ni within plant tissues would induce synthesis of secondary defense compounds. It was hypothesized that AM colonization increases Ni content and plant Ni tolerance, the activities of N-assimilating enzymes (nitrate reductase, NR; glutamine synthetase, GS; and glutamine dehydrogenase, GDH), and induces the accumulation of sesquiterpene lactones (STLs), in sunflower grown under soil Ni conditions. It was also hypothesized that N-type fertilization affects ammonium assimilation as the activities of GS and GDH would be enhanced in plants supplied with an NH+4 as compared to a NO-3 fertilizer. To verify these hypotheses, three greenhouse experiments were performed with sunflower cv. "Lemon Queen", with or without the AM fungus, Glomus intraradices Schenck & Smith, and treated with (1) 0 or 100 mg Ni kg-1 dry soil (DS) at the reproductive stage, and supplied with NO-3 or NH+4 fertilizer; (2) 0, 100, 200 or 400 mg Ni kg-1, at the reproductive stage and supplied with a complete NH4NO 3 fertilization; and (3) 0, 200 or 400 mg Ni kg-1, at the vegetative stage and supplied with a complete NH4NO 3 fertilizer. The overall results indicated that AM colonization significantly enhances Ni content in sunflower plants, exposed to a moderate soil Ni level of 100 mg Ni kg-1, at the reproductive stage. Furthermore, at 100 mg Ni kg-1, the AM plants had a significantly higher shoot Ni extracted %, suggesting that the AM symbiosis contributed to Ni uptake and its translocation from roots to shoots. The AM contribution to plant Ni content and Ni extracted % were significantly higher in plants supplied with NO-3 than with NH+4 . Moreover, the plant biomass and shoot height were significantly higher in plants supplied with NO-3 than with NH+4 . In late Ni exposed sunflower, the AM colonization significantly increased the Ni extracted % at 400 mg Ni kg-1, yet also resulted in a biomass reduction of 45% as compared to only 14% at 100 mg Ni kg -1. Furthermore, a soil [Ni] of 400 mg Ni kg-1 was toxic to sunflower directly seeded in Ni treated soils, as all seedlings died within four weeks after sowing. The mineral concentrations were enhanced in AM plants, especially at lower soil Ni treatments. It is therefore concluded that the AM contribution to Ni uptake was optimal at 100 mg Ni kg-1 . The AM colonization also contributed to enhance the activities of N-assimilating enzymes, especially under NH+4 fertilization. Moreover, our results showed that the effects of HM stress and N fertilization were linked, as the activities of NR, GS, and GDH were significantly enhanced in plants under NH+4 and at 100 mg Ni kg-1. These results suggest that the combined treatments of soil Ni input and NH+4 nutrition enhance N assimilation via concurrent activities of the GS/GOGAT and GDH pathways. We also observed that both soil Ni input and AM colonization lead to an accumulation of STLs in sunflower leaves. In addition, the combination of AM colonization and soil Ni input would result in a synergistic effect to maximize defense properties while minimizing energy expenditure. These findings support the hypothesis that the AM symbiosis contributes to enhanced Ni uptake and Ni plant tolerance. It is therefore concluded that sunflower, especially in association with AM fungi, shows promise as a "candidate" species in phytoremediation strategies. |
