The distribution of Mn(II)chemical forms on soybean roots as related to Mn(II) toxicity

Add time:07/25/2019    Source:sciencedirect.com

The manganese(II) [Mn(II)] forms on soybean roots were investigated to determine the main factors affecting the distribution of Mn(II) forms and the relationship with Mn(II) plant toxicity. Fresh roots of soybean obtained from solution culture experiments were reacted with Mn(II) in solutions, and then the Mn(II) adsorbed on soybean roots was differentiated into exchangeable, complexed and precipitated forms through sequentialextraction with KNO3, EDTAand HCl. Results indicated thatthe exchangeable Mn(II) on soybean roots occupied the largest proportion, followed by the complexed Mn(II) and precipitated Mn(II) occupied the least proportion. Mn(II) toxicity to soybean roots was greater at pH5.5 than at pH4.2 due to the larger amount of exchangeable Mn(II) at higher pH. The cations Al3+, La3+, Ca2+, Mg2+ and NH4+competed with Mn2+ for cation exchange sites on root surfaces, thus reduced the exchangeable Mn(II) on rootswith the order of Al3+ and La3+ > Ca2+ and Mg2+ > NH4+. Al3+ and La3+ at 100μmol L−1decreased the exchangeable Mn(II)by 80 and 79%, respectively, and Ca2+ and Mg2+at 1mmol L−1 reduced the exchangeable Mn(II) on soybean roots by 51 and 73%, respectively. Organic anions reduced the concentration of free Mn2+ in solution through formation of complexes with Mn2+, and thus efficiently decreased the exchangeable Mn(II) on plant roots. Oxalate, citrate and malate decreased the exchangeable Mn(II) on soybean roots by 30.9, 19.7 and 10.9 %, respectively, which was consistent with thecomplexing abilityof these organic anions with Mn(II).It can be concluded that exchangeable Mn(II) was the dominant form of Mn(II) on soybean roots and was responsible for Mn(II) toxicity to plants. The coexisting cations and organic anions reduced the exchangeableMn(II).

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