Subcellular distribution and chemical forms of Co2+ in three barley genotypes under different Co2+ levels

The toxicity of many heavy metals in plants is closely associated with its subcellular distribution and chemical forms. The subcellular distribution and chemical forms of cobalt (Co²⁺) were investigated using 3 barley genotypes differing in Co²⁺ toxicity resistance, namely Yan66 (resistant), Ea 52 (sensitive), and Humai 4 (moderate), under two Co²⁺ levels (25 and 100 µM). Higher Co²⁺ level in cultural solution significantly increased Co²⁺ accumulation in all subcellular fractions, with vacuole and cell wall having higher concentration. In comparison with 25 µM Co²⁺, 100 µM Co²⁺ treatment caused significant increase of Co²⁺ concentration in the forms of F-NaCl (extracted with 1 M NaCl), F-Ac (extracted with 2% HAc), F-HCl (extracted by 0.6 M HCl), and F-residue (residue forms) in both shoots and roots. There was a significant difference among genotypes in Co²⁺ subcellular distribution and chemical forms, with Ea52 accumulating more Co²⁺ in organelles and Yan66 accumulating more Co²⁺ in vacuole and cell wall. Moreover, the inorganic form of Co²⁺ extracted with 80% ethanol (F-ethanol) and water-soluble form (F-H₂O) were significantly increased in Ea52, while Yan66 accumulated more Co²⁺ in the forms of low-bioavailable molecules (F-NaCl, F-HAc, and F-HCl). The results suggest that the vacuolar sequestration and cell wall deposition of Co²⁺ is a key resistant mechanism for genotype Yan66.