Handbook of Plant Nutrition - chapter 11 pps

11 Iron Volker Römheld University of Hohenheim, Stuttgart, Germany Miroslav Nikolic University of Belgrade, Belgrade, Serbia CONTENTS 11.1 Historical Information 329 11.1.1 Determination of Essentiality 329 11.2 Functions in Plants 330 11.3 Forms and Sources of Iron in Soils 330 11.4 Diagnosis of Iron Status in Plants 332 11.4.1 Iron Deficiency 332 11.4.2 Iron Toxicity 332 11.5 Iron Concentration in Crops 335 11.5.1 Plant Part and Growth Stage 335 11.5.2 Iron Requirement of Some Crops 335 11.5.3 Iron Levels in Plants 336 11.5.3.1 Iron Uptake 336 11.5.3.2 Movement of Iron within Plants 338 11.6 Factors Affecting Plant Uptake 339 11.6.1 Soil Factors 339 11.6.2 Plant Factors 343 11.7 Soil Testing for Iron 344 11.8 Fertilizers for Iron 344 References 345 11.1 HISTORICAL INFORMATION 11.1.1 D ETERMINATION OF ESSENTIALITY Julius von Sachs, the founder of modern water culture experiments, included iron in his first nutri- ent cultures in 1860, and Eusèbe Gris, in 1844, showed that iron was essential for curing chlorosis in vines (1,2). Sachs had already shown that iron can be taken up by leaves, and within a few years L. Rissmüller had demonstrated that foliar iron is obviously translocated by phloem out of leaves before leaf fall in European beech (Fagus sylvatica L.). The early developments in the study of iron in plant nutrition were summarized by Molisch in 1892 (3). It was another 100 years before the principal processes of the mobilization of iron in the rhizosphere started to be understood (4–8). 329 CRC_DK2972_Ch011.qxd 7/1/2006 7:15 AM Page 329 330 Handbook of Plant Nutrition 11.2 FUNCTIONS IN PLANTS The ability of iron to undergo a valence change is important in its functions: Fe 2ϩ l Fe 3ϩ ϩ electron It is also the case that iron easily forms complexes with various ligands, and by this modulates its redox potential. Iron is a component of two major groups of proteins. These are the heme proteins and the Fe-S proteins. In these macromolecules, the redox potential of the Fe(III)/Fe(II) couple, nor- mally 770 mV, can vary across most of the range of redox potential in respiratory and photosyn- thetic electron transport (Ϫ340 to ϩ810 mV). When iron is incorporated into these proteins it acquires its essential function (9). The heme proteins contain a characteristic heme iron–porphyrin complex, and this acts as a prosthetic group of the cytochromes. These are electron acceptors–donors in respiratory reactions. Other heme proteins include catalase, peroxidase, and leghemoglobin. Catalase catalyzes the conversion of hydrogen peroxide into water and O 2 (reaction A), whereas peroxidases catalyze the conversion of hydrogen peroxide to water (reaction B): 2H 2 O 2 →2H 2 O ϩ O 2 (A) H 2 O 2 ϩ AH 2 →A ϩ 2H 2 O (B) AH ϩ AH ϩ H 2 O 2 →A Ϫ A ϩ 2H 2 O Catalase has a major role in the photorespiration reactions, as well as in the glycolate pathway, and is involved in the protection of chloroplasts from free radicals produced during the water-splitting reaction of photosynthesis. The reaction sequence of peroxidase shown above includes cell wall peroxidases, which catalyze the polymerization of phenols to form lignin. Peroxidase activity is noticeably depressed in roots of iron-deficient plants, and inhibited cell wall formation and lignification, and accumulation of phenolic compounds have been reported in iron-deficient roots. As well as being a constituent of the heme group, iron is required at two other stages in its manu- facture. It activates the enzymes aminolevulinic acid synthetase and coproporphorinogen oxidase. The protoporphyrin synthesized as a precursor of heme is also a precursor of chlorophyll, and although iron is not a constituent of chlorophyll this requirement, and the fact that it is also required for the conver- sion of Mg protoporphyrin to protochlorophyllide, means that it is essential for chlorophyll biosynthe- sis (10). However, the decreased chloroplast volume and protein content per chloroplast (11) indicate that chlorophyll might not be adequately stabilized as chromoprotein in chloroplasts under iron deficiency conditions, thus resulting in chlorosis. Along with the iron requirement in some heme enzymes and its involvement in the manufac- ture of heme groups in general, iron has a function in Fe-S proteins, which have a strong involve- ment with the light-dependent reactions of photosynthesis. Ferredoxin, the end product of photosystem I, has a high negative redox potential that enables it to transfer electrons to a number of acceptors. As well as being the electron donor for the synthesis of NADPH in photosystem I, it can reduce nitrite in the reaction catalyzed by nitrite reductase and it is an electron donor for sulfite reductase. 11.3 FORMS AND SOURCES OF IRON IN SOILS Iron occurs in concentrations of 7,000 to 500,000 mg kg Ϫ1 in soils (12), where it is present mainly in the insoluble Fe(III) (ferric, Fe 3ϩ ) form. Ferric ions hydrolyze readily to give Fe(OH) 22 ϩ , Fe(OH) 3 , and Fe(OH) 4 Ϫ , with the combination of these three forms and the Fe 3ϩ ions being the total soluble inorganic iron, and the proportions of these forms being determined by the reaction (13): Fe(OH) 3 (soil) ϩ 3H ϩ lFe 3ϩ ϩ 3H 2 O CRC_DK2972_Ch011.qxd 7/1/2006 7:15 AM Page 330 With an increase in soil pH from 4 to 8, the concentration of Fe 3ϩ ions declines from 10 Ϫ8 to 10 Ϫ20 M. As can be seen from Figure 11.1, the minimum solubility of total inorganic iron occurs between pH 7.4 and 8.5 (14). The various Fe(III) oxides are major components of a mineral soil, and they occur either as a gel coating soil particles or as fine amorphous particles in the clay fraction. Similar to the clay col- loids, these oxides have colloidal properties, but no cation-exchange capacity. They can, however, bind some anions, such as phosphate, particularly at low pH, through anion adsorption. For this rea- son, the presence of these oxides interferes with phosphorus acquisition by plants, and in soils of pH above 6, more than 50% of the organically bound forms of phosphate may be present as humic- Fe(Al)-P complexes (15). Although Fe(III) oxides are relatively insoluble in water, they can become mobile in the presence of various organic compounds. As water leaches through decomposing organic matter, it moves the Fe(III) oxide downwards, particularly at acidic pH, so that under such conditions podzols form. The iron is essentially leached from the top layers of soil as iron–fulvic acid complexes and forms an iron pan after precipitation lower down at higher pH. The upper layers are characteristically light in color, as it is the gel coating of Fe(III) oxide that, in conjunction with humus, gives soils their characteris- tic color. However, in soils in general, the intensity of the color is not an indication of iron content. These organic complexes tend to make iron more available than the thermodynamic equilibrium would indicate (16), and in addition to iron-forming complexes with fulvic acid, it forms complexes with microbial siderophores (13), including siderophores released by ectomycorrhizal fungi (17). A water-soluble humic fraction extracted from peat has been shown to be able to form mobile com- plexes with iron, increasing its availability to plants (18). In soils with a high organic matter content the concentration of iron chelates can reach 10 Ϫ4 to 10 Ϫ 3 M (17,18). However, in well-aerated soils low in organic matter, the iron concentration in the soil solution is in the range of 10 Ϫ8 to 10 Ϫ 7 M, lower than is required for adequate growth of most plants (13). Under anaerobic conditions, ferric oxide is reduced to the Fe(II) (ferrous) state. If there are abundant sulfates in the soil, these also become oxygen sources for soil bacteria, and black Fe(II) Iron 331 345678910 16 14 12 10 8 6 4 -log soluble Fe (mol/L) pH Fe 2+ Fe(OH) 4 − Fe(OH) 2 + Fe 3+ Fe(OH) 3 Total soluble inorganic Fe FIGURE 11.1 Solubility of inorganic Fe in equilibrium with Fe oxides in a well-aerated soil. The shaded zone represents the concentration range required by plants for adequate Fe nutrition. (Redrawn from Römheld, V., Marschner, H., in Advances in Plant Nutrition, Vol. 2, Praeger, New York, 1986, pp. 155–204 and Lindsay, W.L., Schwab, A.P., J. Plant Nutr., 5:821–840, 1982.) CRC_DK2972_Ch011.qxd 7/1/2006 7:15 AM Page 331 sulfide is formed. Such reactions occur when a soil becomes waterlogged, but on subsequent drainage the Fe(II) iron is oxidized back to Fe(III) compounds. Alternate bouts of reduction and oxi- dation as the water table changes in depth give rise to rust-colored patches of soil characteristic of gleys. Ferrous iron, Fe 2ϩ , and its hydrolysis species contribute toward total soluble iron in a soil only if the sum of the negative log of ion activity and pH together fall below 12 (equivalent to Eh of ϩ260 mV and ϩ320 mV at pH 7.5 and 6.5, respectively) (13,14). It is likely that the presence of microorganisms around growing roots causes the redox potential in the rhizosphere to drop because of the microbial oxygen demand, and this would serve to increase concentrations of Fe 2ϩ ions for plant uptake (21). Because the solubility of Fe 3ϩ and Fe 2ϩ ions decreases with increase in pH, growing plants on calcareous soils, and on soils that have been overlimed, gives rise to lime-induced chlorosis. The equilibrium concentration of Fe 3ϩ in calcareous soil solution at pH 8.3 is 10 Ϫ19 mM (22), which gives noticeable iron deficiency in plants not adapted to these conditions. It has been estimated that up to 30% of the world’s arable land is too calcareous for optimum crop production (23,24). Iron deficiency can also arise from excess of manganese and copper. Most elements can serve as oxidizing agents that convert Fe 2ϩ ions into the less soluble Fe 3ϩ ions (25), and excess man- ganese in acid soils can give rise to deficiencies of iron although it would otherwise be present in adequate amounts (26). Corn (Zea mays L.) and sugarcane (Saccharum officinarum L.) may show iron deficiency symp- toms when deficient in potassium. It seems that under these circumstances iron is immobilized in the stem nodes, a process that is accentuated by good phosphorus supply (27). Iron can bind a significant proportion of phosphate in well-weathered soil (as the mineral strengite), and as this sub- stance is poorly soluble at pH values below 5, iron contributes to the poor availability of phospho- rus in acid soils (25). 11.4 DIAGNOSIS OF IRON STATUS IN PLANTS 11.4.1 I RON DEFICIENCY The typical symptoms of iron deficiency in plants are chlorotic leaves. Often the veins remain green whereas the laminae are yellow, and a fine reticulate pattern develops with the darker green veins contrasting markedly with a lighter green or yellow background (Figure 11.2, see also Figure 1.1 in Chapter 1). In cereals, this shows up as alternate yellow and green stripes (Figure 11.3). Iron deficiency causes marked changes in the ultrastructure of chloroplasts, with thylakoid grana being absent under extreme deficiency and the chloroplasts being smaller (27,28). As iron in older leaves, mainly located in chloroplasts, is not easily retranslocated as long as the leaves are not senescent, the younger leaves tend to be more affected than the older leaves (Figure 11.4). In extreme cases the leaves may become almost white. Plant species that can modify the rhizosphere to make iron more available can be classified as iron-efficient and those that cannot as iron-inefficient. It is among the iron-inefficient species that chlorosis is most commonly observed. 11.4.2 IRON TOXICITY Iron toxicity is not a common problem in the field, except in rice crops in Asia (29). It can also occur in pot experiments, and in cases of oversupply of iron salts to ornamental plants such as azaleas. The symptoms in rice, known as ‘Akagare I’ or ‘bronzing’ in Asia, include small reddish-brown spots on the leaves, which gradually extend to the older leaves. The whole leaf may turn brown, and the older leaves may die prematurely (29). In other species, leaves may become darker in color and roots may turn brown (29). In rice, iron toxicity seems to occur above 500mg Fe kg Ϫ1 leaf dry weight (30) (Figure 11.5). 332 Handbook of Plant Nutrition CRC_DK2972_Ch011.qxd 7/1/2006 7:15 AM Page 332 Iron 333 FIGURE 11.2 Iron-deficient cucumber (Cucumis sativus L.) plant. (Photograph by Allen V. Barker.) (For a color presentation of this figure, see the accompanying compact disc.) FIGURE 11.3 Iron-deficient corn (Zea mays L.) plant. (Photograph by Allen V. Barker.) (For a color pres- entation of this figure, see the accompanying compact disc.) CRC_DK2972_Ch011.qxd 7/1/2006 7:15 AM Page 333 334 Handbook of Plant Nutrition FIGURE 11.5 Symptoms of iron toxicity in lowland rice (Oryza sativa L.) in Sri Lanka as a consequence of decreased redox potential under submergence. (Photograph by Volker Römheld.) (For a color presentation of this figure, see the accompanying compact disc.) FIGURE 11.4 Iron-deficient pepper (Capsicum annuum L.) plant. The young leaves are yellow, and the older leaves are more green. (Photograph by Allen V. Barker.) (For a color presentation of this figure, see the accom- panying compact disc.) CRC_DK2972_Ch011.qxd 7/1/2006 7:15 AM Page 334 11.5 IRON CONCENTRATION IN CROPS 11.5.1 P LANT PART AND GROWTH STAGE Most of the iron in plants is in the Fe(III) form (11). The Fe(II) form is normally below the detec- tion level in plants (31). A high proportion of iron is localized within the chloroplasts of rapidly growing leaves (10). One of the forms in which iron occurs in plastids is as phytoferritin, a protein in which iron occurs as a hydrous Fe(III) oxide phosphate micelle (9), but phytoferritin is also found in the xylem and phloem (32). It also occurs in seeds, where it is an iron source that is degraded during germination (33). However, in general, concentrations of iron in seeds are lower than in the vegetative organs. A large part of the iron in plants is in the apoplast, particularly the root apoplast. Most of this root apoplastic pool is in the basal roots and older parts of the root system (34). There is also a noticeable apoplastic pool of iron in the shoots. In the iron hyperaccumulator Japanese blood grass (Imperata cylindrica Raeuschel), iron accu- mulates in rhizomes and leaves in mineral form, in the rhizomes in particular as jarosite, KFe 3 (OH) 6 (SO 4 ) 2 , and in the leaves probably as phytoferritin (35). In the rhizome this accumula- tion is in the epidermis and the xylem, and in the leaves it is in the epidermis. 11.5.2 IRON REQUIREMENT OF SOME CROPS Iron deficiency can be easily identified by visible symptoms, so this observation has made quanti- tative information on adequate concentrations of iron in plants more scarce (Table 11.1) (29). Iron 335 TABLE 11.1 Fe Deficiency Chlorosis-Inducing Factors That Are Often Observed, and Synonyms for These Chlorosis Symptoms Chlorosis-Inducing Factor Synonym Weather factors High precipitation Bad-weather chlorosis High soil water content Low soil temperature Soil factors High lime content Lime-induced chlorosis High bicarbonate concentration Bicarbonate-induced chlorosis Low O 2 concentration High ethylene concentration Ethylene-induced chlorosis High soil compaction High heavy metal content Management factors Soil compaction ‘Tractor’ chlorosis High P fertilization Phosphorus-induced chlorosis High application of Cu-containing fungicides Copper chlorosis Inadequate assimilate delivery and late vintage (harvest) Weakness chlorosis, stress chlorosis Plant factors Low root growth High shoot:root dry matter ratio Low Fe efficiency Source: From Kirkby, E.A., Römheld, V. Micronutrients in Plant Physiology: Functions, Uptake and Mobility. Proceedings No. 543, International Fertiliser Society, Cambridge, U.K., December 9, 2004, pp. 1–54. CRC_DK2972_Ch011.qxd 7/1/2006 7:15 AM Page 335 Furthermore, the so-called chlorosis paradox gives confusing results when critical levels are being determined. This confusion seems to be brought about by restricted leaf expansion due to shortage of iron, giving rise to similar concentrations of iron in the smaller, chlorotic leaves as in healthy green leaves (36). This paradox has been described in grapevine (Vitis vinifera L.) (37,38) and peach (Prunus persica Batsch) (39). In general, the deficiency range is about 50 to 100mg kg Ϫ1 depending on the plant species and cultivars (Table 11.2) (28). This range is somewhat complex to determine, as iron-efficient plant species are able to react to low availability of iron by employing mechanisms for its enhanced acqui- sition (see below), whereas iron-inefficient species are more dependent on adequate supplies of iron being readily available. In fact, it is apparent from simple calculations that plants must employ root- induced mobilization of iron to obtain enough element for normal growth (28). Calculations based on the iron concentration of crops at harvest compared with the concentration of iron in soil water indi- cate an apparent shortfall in availability of a factor of approximately 2000, and calculations based on the iron concentration of crops at harvest and their water requirements indicate a shortfall of a factor of approximately 36,000. Both are very crude calculations, but they clearly indicate that the presence of plants, at least iron-efficient plants, makes iron more available in the soil than would be expected. The data indicate a requirement of iron for an annual crop of 1 kg ha Ϫ1 year Ϫ1 , but even for tree species the requirement is considerable. It has been estimated that for a peach tree in northeastern Spain, the amount of iron in the prunings in particular, but also lost in the harvested fruit, in leaf and flower abscission and immobilized in the wood, is between 1 and 2 g per tree per year (40). 11.5.3 IRON LEVELS IN PLANTS 11.5.3.1 Iron Uptake Transport of iron to plants roots is limited largely by diffusion in the soil solution (41,42), and thus the absorption is highly dependent on root activity and growth, and root length density. The overall processes of iron acquisition by roots have been described in terms of different strategies to cope with iron deficiency (Figure 11.6) (10,43). Strategy 1 plants, such as dicots and other nongraminaceous species, reduce Fe(III) in chelates by a rhizodermis-bound Fe(III)-chelate reductase and take up released Fe 2ϩ ions into the cytoplasm of root cells by a Fe 2ϩ transporter. Strategy 2 plants, mostly grasses, release phytosiderophores that chelate Fe(III) ions and take up the phytosiderophore–Fe(III) complex by a transporter (44,45). A more recently postulated Strategy 3 may involve the uptake of microbial siderophores by higher plants (46), although this could be an indirect use of microbial siderophores through exchange chelation with phytosiderophores in Strategy 2 plants or through Fe III chelate reductase in Strategy 1 plants (47,48). In Strategy 1 plants, one of the major responses to iron deficiency is the acidification of the rhi- zosphere, brought about by differential cation–anion uptake (49), the release of dissociable reduc- tants (8,50) and particularly by the action of an iron-deficiency-induced proton pump in the plasmalemma of rhizodermis cells of apical root zones (51). This acidification of the rhizosphere serves to make iron more available and to facilitate the required Fe(III)-chelate reductase activity (52). There is also an enhanced growth of root hairs (53) and the development of structures like transfer cells in the rhizodermis (10) as a response to iron deficiency. In chickpea (Cicer arietinum L.) subjected to iron deficiency, anion and cation uptake were shown to be depressed, but anion uptake was depressed more than cation uptake (54). This effect gives rise to excess cation uptake, with consequent release of H ϩ ions in a direct relationship to the extent of the cation–anion imbalance. The origin of the H ϩ release in such circumstances could be through enhanced PEP carboxylase activity (55). The release of reductants increases the reduction of Fe 3ϩ to Fe 2ϩ in the apoplast, and has been linked to compounds such as caffeic acid (56,57). These may reduce Fe 3ϩ to Fe 2ϩ ions, and also chelate the ions either for uptake or for reduction on the plasmalemma. Such reduction of Fe 3ϩ 336 Handbook of Plant Nutrition CRC_DK2972_Ch011.qxd 7/1/2006 7:15 AM Page 336 on the plasma membrane involves an iron-chelate reductase. It was thought at one time that there are two forms of such reductases, a constitutive form that works at a low capacity and is contin- uously present, and an inducible form that works with high capacity and is induced under iron deficiency (10). However, in tomato (Lycopersicon esculentum Mill.), iron deficiency gives rise to increased expression of constitutive Fe III -chelate reductase isoforms in the root plas- malemma (58). Action of the Fe III -chelate reductase is the rate-limiting step of iron acquisition of Strategy 1 plants under deficiency conditions (59–61). Genes encoding for proteins in Fe III - chelate reductase and involved with the uptake of Fe 2ϩ in Fe-deficient plants have been identified in the Strategy 1 plant Arabidopsis thaliana, and have been named AtFRO2 and AtIRT1, respec- tively (62,63). In Strategy 2 plants the phytosiderophores, nonprotein amino acids such as mugineic acid (64), are released in a diurnal rhythm following onset of iron deficiency (43,52). This release occurs par- ticularly in the apical regions of the seminal and lateral roots (65). The phytosiderophores form sta- ble complexes with Fe 3ϩ ions, and these complexes are taken up by a constitutive transporter in the plasmalemma of root cells (66). Activity of this transporter also increases during iron deficiency. Mutants such as corn (Zea mays L.) ys1/ys1 are very susceptible to iron chlorosis (44). In the Strategy 1 species cucumber (Cucumis sativus L.), Fe 3ϩ attached to the water-soluble humic fraction is apparently reduced by the plasmalemma reductase, allowing uptake to occur (67,68), whereas in Strategy 2 barley (Hordeum vulgare L.), there is an indirect method for uptake of this Fe 3ϩ component that involves ligand exchange between the humic fraction and phytosiderophores released in response to iron deficiency (68). Uptake of iron then occurs as a Fe(III)–phytosiderophore complex. In Strategy II plants, iron deficiency also leads to a small increase in the capacity to take up Fe 2ϩ , uptake previously thought only to occur in Strategy 1 plants (69). It has been suggested in the past that the large root apoplastic pool of iron could be a source of iron for uptake into plants under iron deficiency. However, the apoplastic pool occurs largely in the older roots (34), yet the mobilization of rhizosphere iron and the uptake mechanisms that are induced under iron deficiency stress occur in the apical zones of the roots, so this seems unlikely (70). The Strategy 1 and Strategy 2 mechanisms are switched on by mild iron deficit stress, although under severe deficiency they become less effective. They are switched off within a day of resump- tion of iron supply to the plant. The various iron transporters in plant cells have been well characterized. They include Nramp3 transporters on the tonoplast, and IRT1, IRT2 and Nramp transporters on the plas- malemma (71). Nramp (natural resistance associated macrophage proteins) transporters are involved in metal ion transport in many different organisms, and in Arabidopsis roots, three different Nramps are upregulated under iron deficiency. A model of iron transport in Arabidopsis has been shown elsewhere (72). The transporter used by Strategy 1 plants is an AtIRT1 transporter, whereas Strategy 2 plants take up the phytosiderophore–Fe(III) complex by ZmYS1 transporters (44,45). Uptake of zinc, and possibly manganese and copper also, may increase in Strategy 2 plants under iron deficiency, because although the iron-phytosiderophore transporter is specific to iron complexes, the presence of the phytosiderophores in the rhizosphere may increase the availability of these other ions both in the rhizosphere itself and in the apoplast (73). As well as uptake through roots, iron is able to penetrate plant cuticles, at least at 100% humid- ity. Chelates of Fe 3ϩ were shown to penetrate cuticular membranes from grey poplar (Populus x Canescens Moench.) leaves without stomata with a half-time of 20 to 30 h (74), although at 90% humidity Fe 3ϩ chelated with lignosulfonic acid was the only chelate tested that still penetrated the membrane. Sachs himself showed that iron is taken up by plants after application to the foliage, and iron chelates have been applied to foliage to correct iron deficiencies because inorganic iron salts are unstable and phytotoxic (see (3)). Fe(III) citrate and iron-dimerum have been found to penetrate the leaves of chlorotic tobacco (Nicotiana tabacum L.) plants, and to be utilized by the cells (75), but it is the chelated forms of iron that enter most effectively. Iron 337 CRC_DK2972_Ch011.qxd 7/1/2006 7:15 AM Page 337 11.5.3.2 Movement of Iron within Plants Once taken up by root cells, iron moves within cells and between cells. The understanding of iron homeostasis at the subcellular level is incomplete, and the role of the vacuole is uncertain. A car- rier called AtCCC1 may transport iron into vacuoles, and AtNRAMP3 and AtNRAMP4 are candi- dates for transporting it out (72). Of the cellular organelles, mitochondria and chloroplasts have a high requirement for iron, and the chloroplasts may be sites of storage of iron (76). Transport into chloroplasts is stimulated by light (77), and it occurs in the Fe(II) form (78). Knowledge of the movement of iron between cells is also incomplete. Experiments in which 59 Fe-labelled iron-phytosiderophores were fed to roots of intact corn plants for periods of up to 2 h demonstrated intensive accumulation of iron in the rhizodermis and the endodermis (72,79). This accumulation was higher with iron deficiency stress, and probably reflected the role of increased number of root hairs and increased expression of the ZmYS1 iron-phytosiderophore transporter. From the endodermis, the iron is loaded into the pericycle and from there into the xylem. Very little is known about these processes. Once in the shoots, much of the iron is present in the apoplast, from where it is loaded into the cytoplasm and into the organelles where it is required. It was 338 Handbook of Plant Nutrition Strategy 1: Dicotyledons and nongraminaceous plant species Strategy 2: Graminaceous plant species Fe(OH) 3 Fe(III)-PS Phytosiderophore (PS) ZmYS1 Chelator Fe 3+ Fe(OH) 3 Chelate AtFRO2 AtIRT ATP ADP H + -ATPas e H + Fe 2+ Fe(III)-Chelate Rhizosphere Apoplast Plasma Cytoplasm membrane Chelators, reductants FIGURE 11.6 Strategies for acquisition of Fe in response to Fe deficiency in Strategy 1 and Strategy 2 plants. (Redrawn from Römheld, V., Schaaf, G., Soil Sci. Plant Nutr., 50:1003–1012, 2004.) CRC_DK2972_Ch011.qxd 7/1/2006 7:15 AM Page 338 [...]... Padilla, J.D Etchevers, K Grafton, J.A Acosta-Gallegos Iron accumulation in seed of common bean Plant Soil 246:175–183, 2002 115 J.T Moraghan Accumulation and within-seed distribution of iron in common bean and soybean Plant Soil 264:287–297, 2004 CRC_DK2972_Ch 011. qxd 350 7/1/2006 7:15 AM Page 350 Handbook of Plant Nutrition 116 J.V Wiersma High rates of Fe-EDDHA and seed iron concentration suggest... of iron phytosiderophores Plant Physiol 106:71–77, 1994 45 N von Wirén, H Marschner, V Römheld Root of iron-efficient maize also absorb phytosiderophorechelated zinc Plant Physiol 111 :111 9 112 5, 1996 46 H Bienfait Prevention of stress in iron metabolism of plants Acta Bot Neerl 38:105–129, 1989 47 M Shenker, R Ghirlando, I Oliver, M Helman, Y Hadar, Y Chen Chemical structure and biological activity of. .. Organic matter content of the soil CRC_DK2972_Ch 011. qxd 7/1/2006 7:15 AM Page 340 340 Handbook of Plant Nutrition TABLE 11. 2 Deficient and Adequate Concentrations of Iron in Leaves and Shoots of Various Plant Species Concentration in Dry Matter (mg kgϪ1) Plant Species Deficient Adequate Allium sativum L (onion) Plant Part Type of Culture Upper shoot Sterile nutrient culture 24 224 117 Avena sativa L (oats)... 185–268 V Römheld, H Marschner Mobilization of iron in the rhizosphere of different plant species In: B Tinker, A Läuchli, eds Advances in Plant Nutrition Vol 2, New York: Praeger, 1986, pp 155–204 CRC_DK2972_Ch 011. qxd 346 7/1/2006 7:15 AM Page 346 Handbook of Plant Nutrition 14 W.L Lindsay, A.P Schwab The chemistry of iron in soils and its availability to plants J Plant Nutr 5:821–840, 1982 15 J Gerke Orthophosphate... Some of the effects of lime-induced chlorosis on the early stages of plant growth can be overcome by planting seeds that are high in iron In the case of common bean (Phaseolus vulgaris L.), seeds from plants grown on acid soils are higher in iron than seeds from plants grown on calcareous soils, but the seed iron content can be increased by supply of iron to the soil at planting or after flowering (114 )... Transition metal transporters in plants J Exp Bot 54:2601–2613, 2003 72 V Römheld, G Schaaf Iron transport in plants: a future research in view of a plant nutritionist and a molecular biologist Soil Sci Plant Nutr 50:1003–1012, 2004 73 F.-S Zhang, V Römheld, H Marschner Diurnal rhythm of release of phytosiderophores and uptake rate of zinc in iron-deficient wheat Soil Sci Plant Nutr 37:671–678, 1991 74... secondary effects of EDDHA in some vegetable species Soil Sci Plant Nutr 50 :110 3 111 0, 2004 107 V Römheld, H Marschner Mechanism of iron uptake by peanut plants I FeIII reduction, chelate splitting, and release of phenolics Plant Physiol 71:949–954, 1983 108 R Rosado, M.C del Campillo, M.A Martinez, V Barrón, J Tarrent Long-term effectiveness of vivianite in reducing iron chlorosis in olive trees Plant Soil... Marangoni Prevention of iron-deficiency induced chlorosis in kiwifruit (Actinidia deliciosa) through soil application of synthetic vivianite in a calcareous soil J Plant Nutr 26:2031–2041, 2003 110 K Mengel, E.A Kirkby Principles of Plant Nutrition 5th ed Dordrecht: Kluwer, 2001, p 569 111 V Fernandez, G Ebert, G Winkelmann The use of microbial siderophores for foliar iron application studies Plant Soil 272:245–252,... transport of Fe(II)-nicotianamine in phloem loading and translocation of metals into the grain (91) Expression of a nicotianamine synthase gene from Arabidopsis thaliana in Nicotiana tabacum gave increased levels of nicotianamine, more iron in the leaves of adult plants, and improvement in the iron use efficiency of plants grown under iron deficiency stress (92) 11. 6 FACTORS AFFECTING PLANT UPTAKE 11. 6.1... deficiency in soybeans Agron J 97:924–934, 2005 117 J.A Manthey, B Tisserat, D.E Crowley Root response of sterile-grown onion plants to iron deficiency J Plant Nutr 19:145–161, 1996 118 J.C Brown Differential use of Fe3ϩ and Fe2ϩ by oats Agron J 71:897–902, 1979 119 U.C Gupta Levels of micronutrient cations in different plant parts of various crop species Commun Soil Sci Plant Anal 21:1767–1778, 1990 120 R.E Worley, . 335 11. 5.1 Plant Part and Growth Stage 335 11. 5.2 Iron Requirement of Some Crops 335 11. 5.3 Iron Levels in Plants 336 11. 5.3.1 Iron Uptake 336 11. 5.3.2 Movement of Iron within Plants 338 11. 6. Essentiality 329 11. 2 Functions in Plants 330 11. 3 Forms and Sources of Iron in Soils 330 11. 4 Diagnosis of Iron Status in Plants 332 11. 4.1 Iron Deficiency 332 11. 4.2 Iron Toxicity 332 11. 5 Iron Concentration. expression of constitutive Fe III -chelate reductase isoforms in the root plas- malemma (58). Action of the Fe III -chelate reductase is the rate-limiting step of iron acquisition of Strategy 1 plants