Factors related to iron uptake by dicotyledonous and monocotyledonous plants. II. The reduction of Fe3+ as influenced by roots and inhibitors

In a companion paper (10), varieties of four plant species [two monocotyledons (oats and corn) and two dicotyledons (soybeans and tomato)] were shown to differ widely in their ability to respond to Fe‐stress. The ability of the more Fe‐efficient varieties was manifested by a lowering of the pH of the ambient medium of the root and/or by loss of reductants from the root. Both effects can enhance uptake of Fe by the roots, since Fe is taken up primarily, if not entirely, as Fe²⁺ ions. Thus, a given stressed plant has a means, under some degree of metabolic control, for modifying the root environment and, thereby, alleviating its chlorotic condition.

The present investigation deals with environmental factors, particularly chemical inhibitors, modifying the effectiveness of the stress response. Without inhibitors, excised root samples of the four species exhibited a wide range of abilities to reduce Fe³⁺ to Fe²⁺. Roots of the dicotyledonous species reduced about twice as much Fe³⁺ as did equal weights of the monocotyledonous species. Iron‐efficient tomato, soybean, and oat roots reduced more Fe³⁺ than did roots of the Fe‐inefficient varieties. The two corn varieties were about equal in their effectiveness.

Comparable samples of roots were also exposed to chemicals that induce or aggravate Fe chlorosis. Those found to be very effective inhibitors of Fe³⁺ reduction by the roots included: hydroxide, orthophosphate, pyrophosphate, Cu²⁺ and Ni²⁺. Other ions (includ ing Mn²⁺, Zn²⁺ and molybdate) and ethyl ammonium phosphate also inhibited Fe³⁺ reduction but to a lesser degree. Citrate, however, enhanced Fe³⁺ reduction. The degree of inhibition or enhancement differed for each of the varieties. In general, the Fe‐efficient plants were best able to reduce Fe³⁺ in spite of the inhibitory influence of the imposed treatments. Thus, our findings indicated that inhibition of the Fe³⁺ ‐reduction process at, or near, the periphery of the root is an apparent cause of Fe chlorosis.