Mycorrhizal fungi increase chromium uptake by sunflower plants: Influence on tissue mineral concentration, growth, and gas exchange

As a potential phytoremediation system for phytoextraction of chromium (Cr), we evaluated the influence of the arbuscular mycorrhizal fungus Glomus intraradices on leaf tissue elemental composition, growth and gas exchange of sunflower (Helianthus annuus L.). Sunflower seedlings were either inoculated with mycorrhizal fungi (AM) or non-inoculated (Non-AM) and then exposed to two Cr species: {12mmol of trivalent cation (Cr+3) [Cr(III)] or 0.1 mmol of divalent dichromate anion (Cr2O7-) [Cr(VI)]}. Both Cr species depressed plant growth, decreased stomatal conductance (g(s)) and net photosynthesis (A). However, Cr(VI) was more toxic than Cr(III) as indicated by early reduction in g(s), and greater reduction in plant growth and arbuscule formation. Mycorrhizal fungi helped to partially alleviate Cr toxicity as indicated by greater growth in plants treated with either Cr species, and increased gas exchange of Cr(III) plants. With Cr(III), Cr concentrations were the same in tissue of AM and Non-AM plants, while Non-AM Cr(VI) treatments had higher Cr concentration than AM plants; however with both Cr species, the larger-sized AM plants had greater total Cr accumulation on a whole-plant basis. Chromium reduced leaf elemental nitrogen (N), phosphorus (P), and potassium (K), and increased aluminum (Al), iron (Fe), and zinc (Zn) uptake. Chromium-treated AM plants had higher K and decreased levels of Fe and Al than Non-AM plants. Except for the depression of arbuscular formation in Cr(VI) treated plants, high colonization occurred with either Cr species. While higher P may have contributed to mycorrhizal benefits with Cr(III) plants, it does not account for any AM benefits observed with Cr(VI) plants.