Influence of iron efficiency in soybeans on concentration of many trace elements in plant parts and implications on iron‐efficiency mechanisms

Abstract

Hawkeye and PI‐54619–5–1 (also known as T‐203) soybeans (Glycine max [L] Merr. Bragg) are iron‐efficient and iron‐inefficient representatives, respectively, of the species. The iron‐efficient characteristics seem to involve both proton expulsion and production of reductants. Increased uptake of iron by Hawkeye plants compared with the PI‐54619–5–1 plants is not the only result of the efficiency. The Hawkeye usually has slightly lower yields than PI‐54619–5–1 when iron deficiency is not involved. The Hawkeye has larger amounts of zinc, copper, manganese, nickel, cobalt, and lead, whether grown in soil or solution, although occasionally there is an exception. When grown in soil, the Hawkeye usually has more aluminum than the PI‐54619–5–1; silicon is more nearly equal in each. Heavy metal toxicity seems to inactiviate part of the iron‐efficiency mechanisms, but not all of them; because even though the heavy metals decrease plant uptake of iron and zinc and sometimes other trace elements, the ability of Hawkeye to accumulate more metals than PI‐54619–5–1 still remains. The evidence strongly indicates that the iron efficiency in Hawkeye can lead to toxicity stress in the presence of excesses of trace metals, and that multiple element stresses can be severe in an iron‐efficient cultivar or species. One complication is that trace element stress can induce iron deficiency more readily in the iron‐inefficient cultivar than in the efficient species. When iron was supplied to plants, a decrease in the response mechanisms decreased the uptake of some other micronutrients. This could partly explain why iron chelates decrease manganese uptake by plants.

Iron‐efficiency mechanisms make it possible for plants to obtain iron from soil in which iron is too insoluble to be available to plants (Bennett et al., 1982; Cline et al., 1984; Olsen et al., 1981; Romheld et al., 1984; Sijmons et al., 1984). The mechanisms seemed to be varied according to plant species and meet various needs for different conditions (Wallace, 1986). In work in this laboratory in prior years, several different comparative studies have been made between two cultivars of soybean (.Glycine max [L] Merr. Bragg), one of which is iron efficient (Hawkeye) and one of which is iron inefficient (PI‐54619–5–1 or otherwise known as T‐203). These studies have centered on the mineral nutritional aspects of the two cultivars, and the results indicate that the iron efficiency extends to other trace elements as well. Some of the data supporting this idea are reviewed here, followed by a discussion of some possible implications.

Key words:

• Iron efficiency
• iron‐stress interactions
• manganese
• zinc
• copper
• nickel
• cobalt
• lead