Reduction of Fe³⁺ as it relates to fe chlorosis

Abstract

Fe exists in soil primarily in the oxidized, and very insoluble Fe³⁺ form. Prior investigations have shown that Fe is absorbed by plant roots primarily, if not entirely, in the Fe²⁺ form. The reduction of Fe³⁺; in the rhizoplane is accordingly crucial.

Certain plants may respond to Fe deficiency by acidification of the ambient medium and by loss of reductants from their roots. The solubilization of Fe minerals and reduction of Fe to the available Fe²⁺ form markedly increases Fe uptake and corrects the chlorotic condition. Plants with the more effective ‘stress‐response mechanism’ are referred to as ‘Efficient’ plants.

The ‘reductant’ produced by a stressed plant consists of several compounds which accumulate in relatively high levels in the periphery of young roots. One of the reductants is Caffeic Acid; its synthesis and oxidation is under enzymatic control in the roots as summarized in the reaction sequence.

Many ions inhibit the reduction of Fe³⁺ by plant roots. Some of the more effective ions include ortho‐phosphate, pyrophosphate, Ni²⁺, Cu²⁺ and, particularly, hydroxide. Other ions, including Mn²⁺, Zn²⁺, and moly‐bdate inhibit to a lesser degree. These findings provide a way of explaining the role of these extraneous ions in inducing or aggravating Fe chlorosis.

During translocation of Fe to growing tissues, Fe²⁺ is re‐oxidized to Fe³⁺. Prior to its utilization in enzyme systems, present evidence suggests that it must be in the Fe²⁺ form. This requirement is apparently met by a light‐induced reduction of Fe3⁺ to Fe²⁺ in those tissues exposed to light in the blue‐ultra violet range. This reduction process, like that induced in the dark by roots, is subject to inhibition by the same extraneous ions mentioned above.

Key words:

• Reductant
• caffeic acid
• coumaric acid
• enzyme systems
• reduction of Fe³⁺ by roots
• light-induced reduction
• inhibition of reduction processes
• stress response mechanism