Chemical aspects of the Fe stress response mechanism in tomatoes

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Abstract
Chemical properties of Fe3+ reducing substances released by Fe stressed tomato (Lycopersicon esculentum Mill.) roots were studied along with metabolic mechanisms for generating and oxidizing the principal reductant thought to be produced—i.e., caffeic acid or derivatives. The general findings can be summarized by the reactions: In the presence of tomato roots, p‐coumaric acid (a nonreductant) was a precursor for the formation of chemicals capable of reducing Fe3+to Fe2+. Iron‐deficient, chlorotic plants treated with 10‐4 M p‐coumaric acid in the nutrient solutions recovered from their Fe stress. The conversion reaction was specific for p‐coumaric acid. Neither o‐ nor m‐coumaric acid was converted to reductant(s); nor did either alleviate Fe‐deficiency chlorosis. The presence of p‐coumarate hydroxylase in tomato roots was confirmed. Hydroxylase activity was approximately twice as high in roots of the Fe‐efficient cultivar (T3238FER) as in roots of the Fe‐inefficient genotype (T3238fer). Caffeic acid was confirmed as a principal component of the ‘reductant’ fraction in the exudate of Fe‐stressed tomato roots. At least three other possible derivative compounds capable of reducing Fe3+ were present. Caffeic acid behaved chemically and physiologically like the reductant exuded from tomato roots. Its transient appearance in the ambient solution of Fe‐stressed plants was at least partly due to the polyphenol oxidase in the roots. The activity of this enzyme was nearly twice as high in roots of the Fe‐inefficient genotype (T3238fer) as in those of the Fe‐efficient genotype (T3238FER). Caffeic acid concentrations were lower in roots of the Fe‐inefficient tomato plants than in roots of the Fe‐efficient plants.