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 foun d in agricultural soils and industrial sites. Phytoremediation, which relie s on phytoextraction of Cr with biological organisms, is an important alter native to costly physical and chemical methods of treating contaminated sit es. The ability of the arbuscular mycorrhizal fungus (AM), Glomus intraradi ces, 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 spe cies [trivalent cation (Cr3+) {CI(III)}, and divalent dichromate anion (Cr2 O7-) (Cr(VI)}]. Both Cr species depressed plant growth, decreased net photo synthesis (A) and increased the vapor pressure difference; however, Cr(VI) was more toxic. Chromium accumulation was greatest in roots, intermediate i n 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 t olerate and hyperaccumulate Cr. At higher Cr levels greater mycorrhizal dep endency occurred, as indicated by proportionally greater growth, higher A a nd reduced visual symptoms of stress, compared Co Non-AM plants. AM plants had greater Cr-accumulating ability than Non-AM plants at the highest conce ntrations of Cr(lll) and Cr(VI), as indicated by the greater Cr phytoextrac tion 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. Arbus cules, which play an important role in mineral ion exchange in root cortica l cells, had the greatest sensitivity to Cr toxicity. Higher levels of both Cr species reduced leaf tissue phosphorus (P). While tissue P was higher i n AM plants at the highest Cr(lll) level, tissue P did not account for myco rrhizal benefits observed with Cr(VI) plants.