R. A. Olsen

Chemical aspects of the Fe stress response mechanism in tomatoes

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 ac...

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...

Chemical identification of iron reductants exuded by plant roots

Abstract Different varieties of plants exhibit varying abilities to mobilize iron from the soil and to some extent this is dependent upon the reduction of the iron from the ferric form to the ferrous form. Tomato, barley, soybean, and sunflower were studied to identify the iron reductants exuded by roots during periods of iron stress. The tomato reductants were fractionated into one major and at least three minor components. The major component has been identified as chlorogenic acid and considerable data has been collected on the other components. The reductant extract from sunflower has chlorogenic acid as a primary component. The reductant fractions from soybean and barley are similar but different from those of tomato and sunflower. The barley reductants contain glucose and the soybean reductants contain glucose and galactose as well as a ultraviolet light absorbing aglycone.

Photochemical reduction of iron. II. Plant related factors

Abstract Photochemical reduction of ferric iron induced by ultraviolet (UV) and blue radiation is enhanced by certain di‐ and tri‐carboxylic acids. Iron photoreduction proceeds according to the following relative rates in Fe3+‐organic acid solutions containing the major plant acids listed: tartaric >oxalic>citric> malic>aconitic > fumaric ≥succinic≥FeCl3 (control). Any sensitized ferric to ferrous photoreduction occurring in plant foliage exposed to sunlight or artificial light would make iron more available to the tissues for metabolism. Iron is translocated within plants primarily complexed with citric acid (Tiffin, 1972). Citric acid is decarboxylated during Fe‐citrate photoreduction‐oxidation. Ferric iron photoreduction is thus accompanied by citrate degradation. In plant foliage, the fate of ferric citrate taken up the stem depends upon many plant‐related factors. Chelated iron is translocated predominately to actively growing regions where enzymatic reactions largely determine the immediate fate. In...

Role of chelahon by ortho dihydroxy phenols in iron absorption by plant roots

Abstract Under conditions of iron stress, certain higher plants are able to exude ortho‐dihydroxyphenols (including caffeic acid) into the root medium. These compounds can reduce ferric iron to the more soluble ferrous form. By the use of radioactive 14C we have shown that caffeic acid (or a degradation product) can be reabsorbed by plant roots (Hordeum vulgare) and that absorption of ferrous iron can be enhanced several‐fold by the presence of the caffeic acid. The finding is attributed to the chelation of ferrous iron and movement of the chelated species into the roots. An analagous mechanism in stressed microorganisms for enhancing availability of ferric iron by means of chelation has been reported previously.

Photochemical mobilization of ferritin iron

The release of Fe from horse spleen ferritin through photochemical reduction of Fe3+ to Fe2+ was studied in vitro. Spectrophotometric measurement of the Fe(Ferrozine)34− complex (specific for Fe2+) was used to quantify rates of Fe2+ mobilization. Light radiation from cool white fluorescent plus incandescent bulbs effectively promoted the rate of Fe2+ release. Compounds known to be present in plants provided further regulation of photorelease. Reductive removal from ferritin was inhibited by phosphate, and hydroxide, whereas citrate, oxalate, tartrate, and caffeate enhanced the release. Of the organic acids studied, caffeate was the only compound which induced detectable Fe2+ mobilization in the absence of irradiation. Rate constants for photorelease ranged from 2.7×10−3 sec−1 (pH=4.6) to 2.1×10−3 sec−1 (pH=7.1) at 26.5°C. These findings provide one possible explanation for the low level of ferritin-Fe in healthy, illuminated plant tissue.

Interactions between iron nutrition and Verticillium wilt resistance in tomato

The relationship between Fe nutritional status and Verticillium wilt disease in tomatoes possessing single gene resistance to Race 1 of Verticillium dahliae was investigated using hydroponic culture media. Iron limiting conditions increased the sensitivity of resistant tomatoes to the pathogen as expressed by wilting and chlorosis. Distance of fungal vascular invasion was approximately the same in both Fe replete and Fe limited treatments. Comparison of near-isolines revealed that the magnitude of disease expressed in Fe deficient Pixie II (resistant) was considerably less than that expressed by the susceptible Pixie variety. Infection of tomato did not enhance the severity of low-Fe stress as quantified by root peroxidase activity and chlorophyll content of young leaves.

Changes in the roots of sunflower under iron stress

Abstract Growth and physiological changes in sunflower (Helianthus annus L. cv. Northrup King 265) were observed when plants were grown at selected concentrations of iron in solution. Plant roots were tested for reduction capacity with tetrazolium (2,3,5‐triphenyl‐tetrazolium chloride). Iron stressed roots indicated very high capacities for reduction. The amount of reduction was quantified and expressed as amount of tetrazolium reduced per gram of root fresh weight. Root respiration levels were monitored using an oxygen electrode. Iron stressed roots consumed more oxygen than nonstressed roots.

Absorption of ferric iron by plants

Abstract In an earlier publication the conclusion was reached that ferric iron is not absorbed by plant roots and that the reduction of Fe(III) to Fe(II) is an obligatory step preceding absorption of Fe(III). Inasmuch as Fe(III) is the predominant form in well‐aerated soils and its reduction is an energy‐requiring process, this conclusion appears to necessitate the continuous functioning of a special mechanism in roots for inducing Fe absorption. Findings of the present investigation cast doubt upon the validity of the earlier conclusion and support the belief that both Fe(III) and Fe(II) ions can be absorbed. Demands upon the plant are correspondingly less and, during non‐stress periods, might be negligibly small. During non‐stress periods, rate of absorption of Fe(III) might be adequate to meet plant requirements. Reduction is viewed, not as an obligatory process, but as a supplementary process for enhancing rate of absorption of Fe particularly during stress periods. Thus the stress response mechanism,...

An inert prototype of an ion pump

Abstract A simple inert prototype system has been devised which exhibits the principal characteristics required of an active transport mechanism. The system has been shown to exhibit an ability to accumulate both anions and cations against very significant concentration gradients. It has been shown to be able to induce efflux of one cation in preference to another and thereby exhibit specificity. The system has been shown to be able to transport H+ ions against a concentration gradient as high as 3‐thousand‐fold extending ‘outward’ through the membrane. The anion used for inducing efflux transport was caprylate; in the model it represents an anion being produced in the cell as an end‐product of metabolism. The model accordingly provides a simple way to link efflux transport to metabolism. Unlike traditional carrier models, which require a very large fraction of the energy made available from metabolism, the present model requires no energy for its operation inasmuch as the driving force is the diffusion o...