Effect of Oxidizing Power of Roots on Iodine Uptake by Rice Plants

Abstract

Although iodine is harmful to plants, rice plants (Oryza sativa L.) absorbed iodine more selectively than bromine. To explain this selective absorption, the authors proposed the following hypothesis based on the fact that the standard redox potential for (I₂+ 2e = 2I⁻) is lower than that for (Br₂+ 2e = 2Br⁻) and (Fe³⁺+ e = Fe²⁺), and the roots of rice plants are able to oxidize ferrous ion (Fe²⁺) into ferric ion (Fe³⁺), namely rice plants oxidize iodide ion (I⁻) to form molecular iodine (I₂) via the oxidizing power of their roots, and absorb the molecular iodine formed more selectively than iodide ion. Bromine, by contrast, is absorbed by rice plants only in the form of ion (Br⁻). According to this hypothesis, there should be a significant correlation between the oxidizing power of the rice roots and the amount of iodine absorbed. Therefore, the relationship between the oxidizing power of the roots and the concentration of iodine absorbed was studied in a water culture using 8 varieties of rice plants. Rice seedlings, 14 d after germination, were cultured in a solution containing 1 mg L⁻¹ each of iodide and bromide ions for 3 d. The oxidizing power of the rice roots was evaluated based on the amount of 1-naphthylamine oxidized by the roots. A significant correlation (0.78, n= 16, 0.1% significant level) was found between the oxidizing power and the concentration of iodine absorbed by the roots. However, no relationship was found between the oxidizing power of the roots and the amount of bromine absorbed.

Key words:

• Akagare disease
• iodine
• molecular iodine
• oxidizing power of rice root
• rice plant