Effect of phosphate on regulation of Fe‐stress response in soybean and peanut

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Abstract
A re‐evaluation of the literature on fe‐type Fe‐inefficiency in soybean indicated that fe‐types may not be able to express the Fe‐stress‐response in the presence of elevated phosphate. Experiments were conducted to measure Fe‐uptake‐translocation to shoots, Fe‐uptake rate, and Fe‐chelate reduction rate in Fe and fe soybeans grown under Fe‐stress at varied levels of P and Ca. Increased P significantly and substantially inhibited the increase in Fe uptake of fe‐types, and slightly affected that of Fe‐types. In contrast, peanut had a curvilinear response to increasing P (somewhat lower at intermediate P than at low or high P). The basis for fe‐type Fe‐inefficiency is failure of derepression of Fe‐stress‐response at levels of P found in the environment. This understanding was used to design a method to screen fe‐types, and tested experimentally. Fe_ and ‘Wayne'‐type soybean cultivars were green, and fe‐types severely chlorotic in 1/5 Johnson nutrient solutions containing 200 μM P, 5 μM Fe, 10 μM EDDHA, and CaC03 to maintain pH at 7.5; Fe‐availability is controlled by a chelate buffer system. Higher chelator:Fe ratios, lower Fe, or higher P only Increased severity of chlorosis of fe types, having no effect on Fe‐ and ‘Wayne'‐type cultivars. Chlorotic leaves contained less than 40 mg Fe/kg dry weight, while other microelements were at normal sufficient levels. The proposed method selectively screens fe‐types from other Fe‐efficiency types in soybean based on the inability of fe‐types to adapt to the availability of Fe in their environment in the presence of phosphate.