Mycorrhizal fungi enhance accumulation and tolerance of chromium in sunflower (Helianthus annuus)

Chromium (Cr) is a heavy metal risk to human health, and a contaminant found in agricultural soils and industrial sites. Phytoremediation, which relies on phytoextraction of Cr with biological organisms, is an important alternative to costly physical and chemical methods of treating contaminated sites. The ability of the arbuscular mycorrhizal fungus (AM), Glomus intraradices, to enhance Cr uptake and plant tolerance was tested on the growth and gas exchange of sunflower (Helianthus annuus L.). Mycorrhizal-colonized (AM) and non-inoculated (Non-AM) sunflower plants were subjected to two Cr species [trivalent cation (Cr3+) {CI(III)}, and divalent dichromate anion (Cr2O7-) (Cr(VI)}]. Both Cr species depressed plant growth, decreased net photosynthesis (A) and increased the vapor pressure difference; however, Cr(VI) was more toxic. Chromium accumulation was greatest in roots, intermediate in stems and leaves, and lowest in flowers. Greater Cr accumulation occurred with Cr(VI) than Cr(lll). AM enhanced the ability of sunflower plants to tolerate and hyperaccumulate Cr. At higher Cr levels greater mycorrhizal dependency occurred, as indicated by proportionally greater growth, higher A and reduced visual symptoms of stress, compared Co Non-AM plants. AM plants had greater Cr-accumulating ability than Non-AM plants at the highest concentrations of Cr(lll) and Cr(VI), as indicated by the greater Cr phytoextraction coefficient. Mycorrhizal colonization (arbuscule, vesicle, and hyphae formation) was more adversely affected by Cr(VI) than Cr(lll), however high levels of colonization still occurred at even the most toxic levels. Arbuscules, which play an important role in mineral ion exchange in root cortical cells, had the greatest sensitivity to Cr toxicity. Higher levels of both Cr species reduced leaf tissue phosphorus (P). While tissue P was higher in AM plants at the highest Cr(lll) level, tissue P did not account for mycorrhizal benefits observed with Cr(VI) plants.