Tannins in mangrove tree roots and their role in the root environment

Mangrove trees grow under reductive and sometimes acidic conditions, both of which are injurious to the root growth. In this report, the role of tannins in mangrove tree roots was studied in order to alleviate their detrimental effect on the soil environment. The mangrove tree roots contain a large amount of tannins and it combines with ferric ion in the soil solution resulting in the blackening of the roots due to the formation of a tannin-ferric iron complex. The black complex is decolorized by the reduction treatment with sodium hydrosulfite. Hydrogen sulfide is also able to decolorize the black complex. The decolorized roots can become reversibly black with the addition of ferric iron. Based on these phenomena, it is considered that the tannins in mangrove tree roots combine with iron in the soil solution, to alleviate the iron excess damage. The tannin-ferric iron comlex thus formed counteracts the hydrogen sulfide toxicity to roots by oxidation. Both of these harmful substances are usually present in the root environment of mangrove trees.