Effects of Fe compounds on nutrient uptake by plants grown in sand media with different pH

The effect of soil and foliar application of different iron (Fe) compounds (FeSO₄, Fe‐EDTA, Fe‐EDDS, and Fe‐EDDHA) on nutrient concentrations in lettuce (Lactuca sativa cv. Australian gelber) and ryegrass (Lolium perenne cv. Prego) was investigated in a greenhouse pot experiment using quartz sand as growth medium. Soil application was performed in both the acidic and alkaline pH range, and foliar application to plants grown in the alkaline sand only. Lettuce growth was depressed by Fe deficiency in the alkaline sand, whereas the treatments had no effect on ryegrass growth. Soil‐applied Fe compounds raised the Fe concentrations in lettuce. This was especially true for the Fe chelates, which also increased yields. Soil‐applied Fe compounds had no statistically significant effect on Fe concentrations in ryegrass. Concentrations of manganese (Mn) in lettuce were equally decreased by all soil‐applied chelates. In the alkaline sand, soil application of Fe‐EDDHA elevated copper (Cu) and depressed zinc (Zn) concentrations in lettuce. The chelates increased Zn concentration in ryegrass. Foliar application of Fe‐EDDS increased Fe concentrations in lettuce and in ryegrass most. Fe‐EDDHA depressed Mn and Zn concentrations in lettuce more than other Fe compounds, suggesting the existence of another mechanism, in addition to Fe, that transmits a corresponding signal from shoot to roots with an impact on uptake of micronutrients.     

Effect of commercial amino acids on iron nutrition of tomato plants grown under lime‐induced iron deficiency

The objective of this work was to study the effect of root and foliar application of two commercial products containing amino acids from plant and animal origin on iron (Fe) nutrition of tomato seedlings cultivated in two nutrient media: lime and normal nutrient solutions. In the foliar‐application experiment, each product was sprayed with 0.5 and 0.7 mL L^–1 2, 7, 12, and 17 d after transplanting. In the root application experiment, 0.1 and 0.2 mL L^–1 of amino acids products were added to the nutrient solutions. In both experiments, untreated control plants were included as well. Foliar and root application of the product containing amino acids from animal origin caused severe plant‐growth depression and nonpositive effects on Fe nutrition were found. In contrast, the application of the product from plant origin stimulated plant growth. Furthermore, significantly enhanced root and leaf Fe^III‐chelate reductase activity, chlorophyll concentration, leaf Fe concentration, and Fe^II : Fe ratio were found in tomato seedlings treated with the product from plant origin, especially when the amino acids were directly applied to the roots. These effects were more evident in plants developed under lime‐induced Fe deficiency. The positive results on Fe uptake may be related to the action of glutamic acid, the most abundant amino acid in the formulation of the product from plant origin.     

Uptake of Zn and Fe by wheat (Triticum aestivum var. Greina) and transfer to the grains in the presence of chelating agents (ethylenediaminedisuccinic acid and ethylenediaminetetraacetic acid)

A way to decrease iron and zinc deficiency in humans is to biofortify foods by increasing the bioavailable contents in these elements. The aim of this work was to study if chelating agents could be used to increase the capture of Fe and Zn by wheat grains. Zn and/or Fe in combination with the chelating agents ethylenediaminedisuccinic acid (EDDS) or ethylenediaminetetraacetic acid (EDTA) were added at various times (i.e., at flower head formation, anthesis, and postanthesis) to spring wheat ( Triticum aestivum var. Greina) grown in nutrient solution. Treatments lasted for 2 weeks, and the plants were harvested at grain maturity. The shoots of treated plants accumulated higher Zn and/or Fe concentrations than untreated plants, depending on the treatment. The plants also accumulated significant concentrations of EDDS or EDTA in their shoots. Elevated Zn and Fe concentrations in the shoots did in most cases not lead to significantly higher Zn and Fe concentrations in the grains. The grains of plants treated with EDDS during flower head formation accumulated elevated Fe and Zn concentrations but at the cost of a reduction in yield. The control plants transferred higher percentages of Fe and Zn from the shoot into the grain than the treated plants. This indicates that EDTA and EDDS inhibited in most cases the translocation of Fe and Zn from the shoots into the grains. The amounts of EDDS and EDTA found in the grains of treated plants were very small. This indicates that there was little transfer of the chelates into the symplast and that the apoplastic pathway, which is important for the transport of chelants into the shoots, is efficiently blocked between shoots and seeds.     

Interaction of iron chelates with several soil materials and with a soil standard

Frequently the effectiveness of iron (Fe) chelates is low because they can be retained or destroyed by soil materials. The high cost of these Fe fertilizers makes it necessary to study soil material reaction with Fe chelates. Commercial Fe chelates with EDTA, EDDHA, and EDDHMA as ligands and their standards, prepared in the laboratory, were shaken for one hour with various soil materials [amorphous Fe(III) oxide, acid peat, calcium (Ca)‐montmorillonite and calcium carbonate (CaCO₃)] and with a soil standard made in the laboratory. After agitation, the chelate‐soil mixtures were filtered and the micronutrients and chelated Fe that remained in solution were determined. Among the soil materials used, amorphous Fe(III) oxide and acid peat had the greatest affect on the amount of chelated Fe remaining in solution. The type of chelating agent was the next major factor that affected the availability of soluble Fe following reaction with the soil materials. Another factor was the commercial formulation of the Fe chelates. The chelates comprised of EDDHA or EDDHMA maintained the highest percentages of chelated Fe in solution after interaction with the solid phases, except for the acid peat. The last soil material, acid peat, retained more chelated Fe for the Fe chelates with EDDHA or EDDHMA than with EDTA as the chelating agent. The commercial Fe‐EDDHA chelates had greater losses of chelated Fe than their standard after interaction with all the solid phases. The commercial Fe‐EDDHA chelate (Sequestrene) and the commercial Fe‐EDDHMA chelate (Hampirón) solubilized the highest amount of copper (Cu) from soil standard. This was attributed to the presence of by‐products in the commercial formulations since the Fe‐EDDHA standard did not have Cu in solution after the interaction. Therefore, the commercial Fe chelate by‐products are able to form Cu‐complexes which could affect chelated Fe and its availability to plants.     

Stability in solution and reactivity with soils and soil components of iron and zinc complexes

Naturally derived complexes with the ability to complex (unidentate) or chelate (polydentate) metals are a cheaper alternative to synthetic chelates to correct micronutrient deficiencies, but despite their widespread use there is a lack of knowledge on their agronomic performance. The aim of this paper was to evaluate the stability of iron (Fe) and zinc (Zn) lignosulfonate, gluconate, amino acid, and humate complexes in solution over time and at different pH values. Also, their stability in a concentrated nutrient solution and their reactivity with soils and soil components was evaluated. In our experimental conditions, all the complexes (except Fe amino acid) remained stable in solution for an extended period of time. All Zn complexes and the Fe lignosulfonate were stable in solution up to pH 7.0–7.5, while Fe gluconate only maintained 20%–40% of the iron in solution in the pH range 5–11 and Fe amino acid and humate complexes barely maintained small concentrations of Fe in solution above pH 3. Most of the complexes maintained Fe and Zn in concentrated nutrient solutions for irrigation systems, but Fe amino acid only maintained around 70% of the iron added. In general, the interactions of complexes with soils and soil components produced a high retention. The interaction of Fe lignosulfonate with peat, illite, and ferrihydrite, and Fe gluconate with peat and illite resulted in significant amounts of Fe to remain in solution, while for the Fe amino acid and humate the Fe remaining in solution was low. All Zn complexes were highly retained in an acidic peat, illite, and montmorillonite clays and soils, while no retention was observed on ferrihydrite. In conclusion, the stability of complexes in different conditions is related to the percentage of complexed element in the products. While complexes can be used to maintain micronutrients in solution in aqueous media (foliar and fertigation), their application to soil should be considered as a measure to increase metal availabilities but not their solubility.     

Uptake and apoplastic retention of EDTA‐ and phytosiderophore‐chelated chromium(III) in maize

Increasing the mobilization and root uptake of chromium (Cr) by synthetic and plant‐borne chelators might be relevant for the design of phytoremediation strategies on Cr‐contaminated sites. Short‐term uptake studies in maize roots supplied with ⁵¹CrCl₃ or ⁵¹Cr(III)‐EDTA led to higher apoplastic Cr contents in plant roots supplied with ⁵¹CrCl₃ and in Fe‐sufficient plants relative to Fe‐deficient plants, indicating that Fe stimulated co‐precipitation of Cr. Concentration‐dependent retention of Cr in a methanol:chloroform‐treated cell‐wall fraction was still saturable and in agreement with the predicted tendency of Cr(III) to precipitate as Cr(OH)₃. To investigate a possible stimulation of Cr(III) uptake by phytosiderophores, Fe‐deficient maize roots were exposed for 6 d to Cr(III)‐EDTA or Cr(III)‐DMA (2'‐deoxymugineic acid). Relative to plants without Cr supply, the supply of both chelated Cr species in a subtoxic concentration of 1 µM resulted in alleviation of Fe deficiency–induced chlorosis and higher Cr accumulation. Long‐term Cr accumulation from Cr(III)‐DMA was similar to that from Cr(III)‐EDTA, and Cr uptake from both chelates was not altered in the maize mutant ys1, which is defective in metal‐phytosiderophore uptake. We therefore conclude that phytosiderophores increase Cr solubility similar to synthetic chelators like EDTA, but do not additionally contribute to Cr(III) uptake from Cr‐contaminated sites.     

Iron‐Enriched Azolla as a slow‐release biofertilizer for cucumber plants grown in a hydroponic system

The unique ability of dried plant residues Azolla to adsorb iron (Fe) was employed to formulate and test an organic Fe biofertilizer. A simplified experimental system was established to examine the effectiveness of Fe‐enriched Azolla as a source of Fe for the remedy of Fe‐deficient plants. The optimal Fe‐enrichment level needed to achieve a complete recovery of starved plant by the Fe‐Azolla complex was tested using a bioassay system of hydroponically grown cucumbers. Dried Azolla plants were mixed a with a solution of ferrous sulfate (FeSO₄) at pH 2.0, rinsed, and dried to form organic, compact material containing 4% (w/w) Fe bound to Azolla. The Fe‐Azolla complex was applied to the nutrient solutions of Fe‐deficient cucumber seedlings. Growth rates and development measurements as well as chlorophyll and the Fe‐containing catalase activity tests have been performed. The effect of the slowly released Fe in correcting Fe deficiency were followed for three weeks and compared with the efficiency of additions of several synthetic Fe chelates. Iron‐starved plants exhibited fast regreening of the chlorotic interveinal tissues after the addition of Fe‐Azolla complex to the nutrient solutions. Iron starvation decreased the activity of catalase. Iron‐treated‐starved plants exhibited recovery of catalase activity compared to the low level activity measured untreated Fe‐starved plants. Iron‐enriched Azolla treatment was found equivalent to Fe‐EDTA and Fe‐EDDHA. This study is the first step in our research program aimed to establish the application of Fe‐enriched Azolla as a bioagent for the benefit of Fe‐deficient crops.     

Strawberry recovers from iron chlorosis after foliar application of a grass‐clipping extract

Bare‐root transplants of strawberry (Fragaria × ananassa Duch. cv. Selva) were transferred to nutrient solutions with or without iron. After 35 d of growth, plants in the solution without iron became chlorotic and had morphological changes in roots typical of iron‐deficiency chlorosis (IDC). Acidification of the nutrient solution was also observed. We tested a grass‐clipping extract to correct IDC in strawberry plants by foliar application to some chlorotic plants. We also assessed the effects of this product on plant growth, Fe allocation, as well as morphological and physiological parameters related with IDC. After the second spray, leaf chlorophyll increased in the youngest expanded leaves. The total content of iron in plants increased from 1.93 mg to 2.37 mg per plant after three sprays, accounting for 80% of the total iron supplied by the extract. Newly formed roots from sprayed plants had a normal morphology (no subapical swollen zone) but a higher ferric chelate–reductase (FC‐R; EC 1.16.1.17) activity per root apex compared with roots from plants grown with iron or untreated chlorotic plants. Acidification of the nutrient solution continued in sprayed recovered plants. The results suggest an uncoupling of the regulation of morphological and physiological mechanisms related to IDC: FC‐R activity seems to be controlled by roots on their own or together with shoots, while morphological changes in roots are apparently regulated only by the level of iron in shoots.     

Contact between root and iron substrate and the mobilization of iron by soybean roots

The question of whether a direct (uninterrupted) contact between plant roots and solid substrate is a prerequisite for the mobilization of sparingly soluble iron (Fe) by plant roots was investigated. Factorial combinations of two types of contact between the roots and rooting medium [direct or interrupted (roots placed inside a dialysis tube (DT)], two rooting media (nutrient solution or vermiculite), and two Fe supply conditions [without soluble Fe (control) or plus Fe‐EDDHA] were investigated. Placing the roots inside DT, with no visible mechanical impedance for the root, reduced the growth of tops and roots in most cases. Iron mobilization, judged by the total amount of Fe in the tops, strongly decreased if roots were enclosed in DT. This was true in plants grown in vermiculite as well as in nutrient solution containing Fe‐EDDHA. It is concluded that a direct contact between the surface of root cell walls and the Fe substrate is a prerequisite for the Fe mobilization by the plant roots.     

Plant Uptake of Iron Chelated by Humic Acids of Different Molecular Weights

Abstract

Mobilization of iron (Fe) chelated by humic acids (HA) of low (HA_10,000) and high molecular weight (HA_100,000) fractions and its uptake by plants were investigated in growth experiments with sunflower seedlings. The iron chelates (labeled with ⁵⁹Fe) contained in dialysis bags (mw. cutoff=3500) were placed in minus iron Hoagland solutions as the Fe source and at the same time fulvic acid (FA), EDTA, and low and high molecular weight HA fractions were added in the solutions as mobilizators. Characterization of FA, HA_10,000, and HA_100,000 were performed by infrared spectroscopy and chemical analysis, e.g., total acidity, COOH, and phenolic‐OH content. Roots and leaves were harvested, dried, and ground for Fe activity determination. Iron contents and pH in the nutrient solutions were measured before and after treatments. The supply of Fe to the plants was apparently sufficient, because no Fe deficiency has been detected in the test plants but during the whole absorption period, the pH of the nutrient solution was about 4.5. The Fe contents in leaves indicated that part of the Fe was rapidly transported from roots to leaves. Judging from the Fe contents in leaves, it was assumed that the small size HA_10,000 and EDTA were the most efficient in affecting transport of Fe from root to leaf tissue. FA, HA_10,000, and especially HA_100,000 were unable to penetrate the dialysis bags and, hence, were effective in Fe mobilization only after the Fe, dissociated from the Fe‐HA chelate, has passed the dialysis membrane into the nutrient solutions. In contrast, the small size EDTA was expected to have penetrated the dialysis bags, permitting mobilization of chelated Fe by ligand exchange inside the bags, and transporting the Fe to the roots. The results suggested that the humic substances used in this study were able to form with the Fe³⁺ ion complexes that maintained the iron available to the sunflower plants. In the chemical form of Fe.L, where L was FA o HA, the iron within the bags or in solution or in the roots free space, was available for exchange reactions with the natural sunflower plant chelators for its transport to the leaves.     

Impact of liming on utilization of 59Fe‐chelates by lettuce (Lactuca sativa L.)

A pot experiment was conducted to determine the utilization of iron (Fe) by lettuce (Lactuca sativa L. cv. Australian gelber). Iron was applied as ⁵⁹Fe in inorganic and chelated form, particularly biodegradable chelate, ⁵⁹Fe‐EDDS. Two stereoisomeric forms of ethylenediaminedisuccinate: [S,S]‐EDDS and a mixture of EDDS containing 25% [S,S]‐EDDS, 25% [R,R]‐EDDS, and 50% [S/R]/[R/S]‐EDDS, ethylenediaminetetraaceticacid (EDTA) and ethylenediimino bis(2‐hydroxyphenyl)acetic acid (EDDHA) were used as ligands. Lettuce was grown in unlimed and limed quartz sand with nitrate as the sole source of nitrogen. Liming decreased lettuce yields but had no effect on Fe concentrations, indicating that Fe concentrations were a poor indicator of Fe bioavailability within the plant. In unlimed sand, utilization of ⁵⁹Fe from all ⁵⁹Fe‐chelates was on the same level (2.8%–3.6%). In limed sand, only ⁵⁹Fe‐EDDHA maintained the ⁵⁹Fe utilization on a level (3%) comparable to that in unlimed sand. Although the utilization of ⁵⁹Fe from the other chelates decreased to 0.6%–1.1% after liming, Fe concentrations were not affected due to the increased uptake of indigenous Fe. The most biodegradable form of EDDS, namely ⁵⁹Fe‐EDDS(S,S), provided ⁵⁹Fe for lettuce as efficiently as the mixture of ⁵⁹Fe‐EDDS stereoisomers and the ⁵⁹Fe‐EDTA. Utilization of ⁵⁹Fe in inorganic form was 0.5% and 0.03% in unlimed and limed sand, respectively. This study shows that biodegradable ligands are able to serve as chelators to sustain Fe availability in calcareous environments. They may be of use especially in drip irrigation, where ligand accumulations may pose a threat to groundwater quality.     

Interaction of Different Iron Chelates with an Alkaline and Calcareous Soil: A Complementary Methodology to Evaluate the Performance of Iron Compounds in the Correction of Iron Chlorosis

Abstract

A great number of studies have shown that the stability of iron chelates as a function of pH is not the unique parameter that must be considered in order to evaluate the potential effectiveness of Fe‐chelates to correct iron chlorosis in plants cultivated in alkaline and calcareous soils. In fact, other factors, such as soil sorption on soil components or the competition among Fe and other metallic cations for the chelating agent in soil solution, have a considerable influence on the capacity of iron chelates to maintain iron in soil solution available to plants. In this context, the aim of this work is to study the variation in concentration of the main iron chelates employed by farmers under field conditions—Fe‐EDDHA (HA), Fe‐EDDHMA (MA), Fe‐EDDHSA (SA), Fe‐EDDCHA (CA), Fe‐EDTA (EDTA), and Fe‐DTPA (DTPA)—in the soil solution of a calcareous soil over time. To this end, soil incubations were carried out using a soil:Fe solution ratio corresponding to soil field capacity, at a temperature of 23°C. The soil used in the experiments was a calcareous soil with a very low organic matter content. The variation in concentration of Fe and Fe‐chelates in soil solution over time were obtained by measuring the evolution in soil solution of both the concentration of total Fe (measured by AAS), and the concentration of the ortho‐ortho isomers for Fe‐EDDHA and analogs or chelated Fe for Fe‐EDTA and Fe‐DTPA (measured by HPLC). The following chelate samples were used: a HA standard prepared in the laboratory and samples of HA, MA, SA, CA, Fe‐EDTA, and Fe‐DTPA obtained from commercial formulations present in the market. The percentage of iron chelated as ortho‐ortho isomers for HAs was: HA standard (100%); HA (51.78%); MA (60.06%); SA (22.50%); and CA (27.28%). In the case of Fe‐EDTA and Fe‐DTPA the percentages of chelated iron were 96.09 and 99.12, respectively. Results show that it is possible to classify the potential effectiveness of the different types of iron chelates used in our experiments as a function of two practical approaches: (i) considering the variation of total iron in soil solution over time, MA is the best performing product, followed by HA, CA, SA, DTPA, EDTA, and ferrous sulfate in the order listed and (ii) considering the capacity of the different iron chelates to maintain the fraction of chelated iron (ortho‐ortho isomers for HA, MA, SA, and CA and total chelated iron for EDTA and DTPA) in soil solution, the order is: SA > CA > HA > MA > EDTA ≈ DTPA. This result, that is related to the nature of the chelate and does not depend on the degree of chelated Fe in the products, indicates that SA and CA might be very efficient products to correct iron chlorosis. Finally, our results also indicate the suitability of this soil incubation methodology to evaluate the potential efficiency of iron compounds to correct iron chlorosis.     

Chemical properties and reactivity of manganese chelates and complexes in solution and soils

Commercial fertilizers containing synthetic manganese (Mn) chelates and complexes are currently used to alleviate Mn deficiency in crops. However, studies conducted on Mn sources in order to evaluate their behavior maintaining Mn soluble in nutrient solution and soil have not been done. In this work, representative commercial Mn fertilizers based on chelates and complexes were characterized and their chemical stability in solution and interaction with soils has been evaluated. Fertilizers studied were two ethylene diamine tetraacetic acid (EDTA) Mn chelates, one N‐(1,2‐dicarboxyethyl)‐D,L‐aspartic acid (IDHA) Mn chelate, two lignosulfonates, one carboxylate, one fulvate, one gluconate, and one heptagluconate‐based Mn complex. Characterization consisted of the determination of the soluble and chelated or complexed Mn, and the ligand identification by nuclear‐magnetic resonance (NMR). Stability study included batch experiments in Ca solution at different pH and three batch experiments with soil comparing with MnSO₄. Results indicate that most of the Mn fertilizers comply with the declared “soluble and chelated or complexed” metal content. At a usual pH range of calcareous soils (7.5–8.5), both chelates and complexes maintain more Mn in solution than MnSO₄ in the presence of Ca. Several factors affect the Mn remaining in solution after the interaction with the soil, especially, the soil‐to‐solution ratio. All chelates and complexes are better alternatives to the use of MnSO₄ in agronomical practices such as fertigation and soil application. Mn‐IDHA as chelate and Mn‐HGl or Mn‐Carb as complexes can be efficient, economical, and environmental friendly fertilizers for foliar application and hydroponic cultures. In soil application, Mn‐EDTA or Mn‐LS would be the best options. In this case, lignosulfonic acid represents a sustainable and low‐cost solution.     

Response of five citrus rootstocks to iron deficiency

Citrus established in calcareous soils can be affected by iron (Fe)‐deficiency chlorosis which limits yield and the farmers' income. The degree of deficiency depends on the rootstock, but the resistance to Fe chlorosis still requires further investigation. To study physiological parameters of citrus rootstocks that could be used to evaluate resistance to Fe deficiency, plants of Troyer citrange (Citrus sinensis L. Osb. × Poncitrus trifoliata L. Raf.), Carrizo citrange, Volkamer lemon (Citrus volkameriana Ten. & Pasq.), alemow (Citrus macrophylla Wester), and sour orange (Citrus aurantium L.) were grown in nutrient solutions with 0, 5, 10, 15, or 20 μM Fe. For each rootstock, plant height, root and shoot dry weights, and concentration of Fe in the shoots and roots were measured at the end of the experiment. Chlorophyll (CHL) concentration was estimated throughout the experimental period using a portable CHL meter (SPAD‐502) calibrated for each rootstock. At the end of the experiment, CHL fluorescence parameters were measured in each rootstock with a portable fluorimeter. Maximal and variable fluorescence values indicated that the photochemistry of Troyer was more affected by a low concentration of Fe in the nutrient solution than that of other rootstocks. To compare rootstocks, the absolute CHL concentration was converted into relative yield by employing a scaling divisor based on the maximum value of total CHL in plants without Fe‐deficiency symptoms. Exponential models were developed to determine the minimum Fe concentration in nutrient solution required to maintain leaf CHL at 50% of the maximum CHL concentration (IC50). Models were also developed to assess the period of time the rootstocks were able to grow under Fe‐stress conditions before they reached IC50. Volkamer lemon and sour orange needed the lowest Fe concentration (between 4 and 5 μM Fe) to maintain IC50, and Troyer citrange had the highest Fe requirement (14 μM Fe). Citrus macrophylla and Carrizo citrange required 7 and 9 μM of Fe, respectively. Similarly, Volkamer lemon and sour orange rootstocks withstood more days under total Fe depletion or with a low concentration of Fe (5 μM Fe in nutrient solution) until they reached IC50, compared to the other rootstocks. The approach used led to a classification of the rootstocks into three categories, regarding their internal tolerance to Fe chlorosis: resistance (sour orange and Volkamer lemon), intermediate resistance (C. macrophylla and Carrizo citrange), and reduced resistance (Troyer citrange).     

Iron stress response in tomato affected by potassium and renewing nutrient solutions

Iron‐efficient T3238FER tomatoes (Lycopersicon esculentum Mill.) did not respond to Fe‐deficiency stress by releasing hydrogen ions and reductants from their roots when the plants were grown in a K‐deficient nutrient solution with or without sodium. When increments of K were added to the nutrient solution, the plants responded proportionally to Fe‐deficiency stress, Fe was transported to plant tops and the chlorophyll concentration in plant tops increased. As the leaf Fe concentration was increasing, root K concentration was increasing and root Mn concentration was decreasing. The K and Mn in tops did not show the marked differences observed in roots.

In the presence of adequate K, renewing the solutions each time the pH was lowered to near 4 (days 7 and 11) caused an increased concentration of most elements in the plant, especially Mn in both tops and roots. These plants had the same Fe concentration as plants grown in unchanged solutions but they contained much less chlorophyll. Balance of nutrient elements to some degree seems required in order for iron to be made available to function properly in the plant.     

Uptake of Iron (59Fe) Complexed to Water‐Extractable Humic Substances by Sunflower Leaves

Abstract

A research was carried out to evaluate the leaves' ability to utilize Fe supplied as a complex with water‐extractable humic substances (WEHS) and the long‐distance transport of ⁵⁹Fe applied to sections of fully expanded leaves of intact sunflower (Helianthus annuus L.) plants. Plants were grown in a nutrient solution containing 10 µM Fe(III)‐EDDHA (Fe‐sufficient plants), with the addition of 10 mM NaHCO₃ to induce iron chlorosis (Fe‐deficient plants). Fe(III)‐WEHS could be reduced by sunflower leaf discs at levels comparable to those observed using Fe(III)‐EDTA, regardless of the Fe status. On the other hand, ⁵⁹Fe uptake rate by leaf discs of green and chlorotic plants was significantly lower in Fe‐WEHS‐treated plants, possibly suggesting the effect of light on photochemical reduction of Fe‐EDTA. In the experiments with intact plants, ⁵⁹Fe‐labeled Fe‐WEHS or Fe‐EDTA were applied onto a section of fully expanded leaves. Irrespective of Fe nutritional status, ⁵⁹Fe uptake was significantly higher when the treatment was carried out with Fe‐EDTA. A significant difference was found in the amount of ⁵⁹Fe translocated from treated leaf area between green and chlorotic plants. However, irrespective of the Fe nutritional status, no significant difference was observed in the absolute amount of ⁵⁹Fe translocated to other plant parts when the micronutrient was supplied either as Fe‐EDTA or Fe‐WEHS. Results show that the utilization of Fe complexed to WEHS by sunflower leaves involves an Fe(III) reduction step in the apoplast prior to its uptake by the symplast of leaf cells and that Fe taken up from the Fe‐WEHS complexes can be translocated from fully expanded leaves towards the roots and other parts of the shoot.     

Influence of micronutrient and phosphorous levels and chelator to iron ratio on growth,chlorosis, and nutrition of bluecrop highbush blueberries

‘Bluecrop’ blueberry plants were exposed to 2×2×2 factorial treatments of high and low phosphate and micronutrients (Cu, Zn, Mn, and B) and excess or stoichiometric concentrations of a chelator (EDDHA) added to Fe(NO₃), in pH 6.8 solution cultures. The effects on growth, Fe chlorosis and overall nutrition were accessed. Phosphorous was applied at 20 (low) or 400 (high) μM levels, micronutrients at low or high levels and either a 1:1 or 10:1 ratio of chelator to Fe concentration. Plants grown in excess chelator and low micronutrient supply grew normally. Plants grown in all high micronutrient solutions were chlorotic after 34 days. Plants in low micronutrient and stochiometric chelator to Fe ratio solutions were also chlorotic.

Foliar Fe was not a good indicator of Fe chlorosis development. High Cu levels in roots grown in high micronutrients without excess chelator may have been a causative factor in Fe chlorosis development, but not in reducing foliar Fe levels. There was no indication that excess chelator facilitated Fe transport as has been reported for other crops. However, this work demonstrates that highbush blueberry has an effective ability to absorb Fe from strong Fe chelates and remain non‐chlorotic even at high solution pH levels under an all nitrate‐nitrogen regime. Chemical name used: EDDHA‐ethylenediaminedi‐o‐hydroxyphenlyacetic acid.     

Efficacy of commercial Fe(III)‐EDDHA and Fe(III)‐EDDHMA chelates to supply iron to sunflower and corn seedlings

Use of synthetic iron (Fe) chelates is the most common and effective way to treat Fe chlorosis in plants. Most commercial products contain Fe‐EDDHA or Fe‐EDDHMA but their efficacy can be quite different. Commercial products with EDDHA or EDDHMA as active components were chosen based on the data obtained by Lucena et al. (1992) in their chemical test. The chelates present extreme differences in behavior in the mentioned chemical tests. The analysis of the products revealed that the total Fe concentration is greater than the one indicated by the manufacturer in spite of a lesser amount of FeY^‐ present. The plant response to these commercial products was tested using short‐term greenhouse hydroponic cultures. Sunflower and corn were chosen because of their different behavior under Fe‐stress conditions. No significant difference between plants treated with Fe‐EDDHA or Fe‐EDDHMA chelates were observed. Since the purity index indicates there are too many differences between commercial formulations of the same type of chelate, the differentiation between groups cannot be determined with commercial products. Index I3, described by Lucena et al. (1992), does not correlate with the plant response because it did not consider the purity percentage of the products.     

Root reductant release as a measure of sorghum cultivar iron efficiency

Measurement of root reductant levels developed during plant Fe stress was tested as a possible assay for sorghum cultivar Fe‐efficiency screening. Iron‐stressed sorghum was shown to release reductants into CaCO₃ buffered nutrient solution; however, considerably more plants could be tested by extracting reductants from excised roots of Fe‐stressed sorghum in 35 ml of pH 3 nutrient solution and 1 mM glucose. An Fe‐efficient cultivar, RT×2536, and an Fe‐inefficient cultivar, BT×378, could be separated by measurement of reductants released into CaCO₃ buffered nutrient solution and by an excised root extraction method; however, neither method was as effective as visual rating methods.     

Effectiveness of iron chelates and FeSO4 for correcting iron chlorosis of peanut on calcareous soils

The effectivenness of different Fe chelates to correct lime induced chlorosis of peanut (Arachis hypogaea) was tested on calcareous soils in Cyprus. Among the chelates tested, Fe‐DTPA and Fe‐EDTA were less effective than Fe‐EDDHA. In one experiment. Fe‐EDDHMA Was less effective while in another experiment was equally effective compared to Fe‐EDDHA . Three different commercial chelates of the form Fe‐EDDHA were equally effective in correcting iron chlorosis. The stability of the Fe‐EDDHA chelate in the soil does not affect its efficiency in curing chlorosis of peanut due to the short growing season for this crop. Ferrous sulfate applied without an acidifying soil amendment was not effective to correct iron chlorosis of peanut.