Factors related to iron uptake by dicotyledonous and monocotyledonous plants III. Competition between root and external factors for Fe

In comparison studies (11, 12), monocotyledonous corn (Zea mays L.) and oats (Avena byzantina C. Koch) did not respond to Fe stress as effectively nor to the same degree as the dicotyledonous soybeans (Glycine max (L.) Merr.) or tomatoes (Lycopersicon esculentum Mill.). Both the Fe‐inefficient and Fe‐efficient corn and oats developed Fe chlorosis; the Fe‐efficient dicotyledonous plants were green. In the present study, the method of inducing Fe stress was changed to make it less severe. Instead of using only NO₃‐N and no Fe to induce Fe stress (11, 12), both NH₄‐N and NO₃‐N were used along with varied concentrations of Fe. Iron stress was induced with BPDS (4,7‐diphenyl‐l, 10‐phenan‐throline disulfonic acid) and phosphate; both competed with the plant for Fe. Phosphate also inhibits reduction of Fe³⁺ to Fe²⁺ (12). This method of inducing Fe stress in the plants was less severe than using only NO₃‐N and no Fe in the nutrient solutions and we were able to measure a difference in Fe‐stress response for all four plant species (Fe‐inefficient and Fe‐efficient). At the lower Fe treatments, the roots of Fe‐efficient plants usually reduced more Fe³⁺ to Fe²⁺ than did the roots of Fe‐inefficient plants. The ‘inefficient’ ys₁ corn and TAM 0–312 oat roots did not compete with BPDS or phosphate for Fe as effectively as did the ‘efficient’ WF9 corn and Coker 227 oat roots. The same type mechanism for solubilization, absorption, and transport of Fe seems to function in both monocotyledenous and dicotyledenous plants but it is more effective and more readily detected in the dicot than in the monocot plants. The reactions involved in reduction of Fe³⁺ to Fe²⁺ seemed to be confined inside or at the root surface for the inefficient genotypes; the efficient genotypes alter the ambient medium to a greater degree.     

Phytosiderophore release as a criterion for genotypic evaluation of iron efficiency in oat

Iron (Fe) deficiency in small grains grown on calcareous soils results in reduced yields, is difficult and expensive to treat with fertilizer, and is complicated to overcome by genetic field screening due to heterogeneous soil and environmental conditions. Recently, phytosiderophore release has been linked to ability of species and genotypes to resist Fe‐deficiency chlorosis. We propose a laboratory technique to measure phytosiderophore release by Fe‐deficient oat (Avena sativa L.) genotypes as a selection method for Fe‐deficiency chlorosis resistance in oat. Plants were grown in Fe‐limiting nutrient solution and phytosiderophore release was measured on 11 days. Summations of daily phytosiderophore release by 17 oat genotypes correlate well with Fe‐deficiency chlorosis scores in the field (r = ‐0.70, p = 0.01). The proposed method consistently identified the genotypes most susceptible to Fe deficiency but did not clearly separate the moderately susceptible genotypes. In these latter genotypes, other factors such as active uptake sites, root growth rate, utilization of acquired Fe, or soil interactions may be modifying factors to phytosiderophore in Fe efficiency. Quantification of phytosiderophore provides a useful selection criterion for oat by eliminating the most inefficient types and with refinement, may become a powerful tool for identifying Fe efficiency in grass crops.     

Factors related to iron uptake by dicotyledonous and monocotyledonous plants. II. The reduction of Fe3+ as influenced by roots and inhibitors

In a companion paper (10), varieties of four plant species [two monocotyledons (oats and corn) and two dicotyledons (soybeans and tomato)] were shown to differ widely in their ability to respond to Fe‐stress. The ability of the more Fe‐efficient varieties was manifested by a lowering of the pH of the ambient medium of the root and/or by loss of reductants from the root. Both effects can enhance uptake of Fe by the roots, since Fe is taken up primarily, if not entirely, as Fe²⁺ ions. Thus, a given stressed plant has a means, under some degree of metabolic control, for modifying the root environment and, thereby, alleviating its chlorotic condition.

The present investigation deals with environmental factors, particularly chemical inhibitors, modifying the effectiveness of the stress response. Without inhibitors, excised root samples of the four species exhibited a wide range of abilities to reduce Fe³⁺ to Fe²⁺. Roots of the dicotyledonous species reduced about twice as much Fe³⁺ as did equal weights of the monocotyledonous species. Iron‐efficient tomato, soybean, and oat roots reduced more Fe³⁺ than did roots of the Fe‐inefficient varieties. The two corn varieties were about equal in their effectiveness.

Comparable samples of roots were also exposed to chemicals that induce or aggravate Fe chlorosis. Those found to be very effective inhibitors of Fe³⁺ reduction by the roots included: hydroxide, orthophosphate, pyrophosphate, Cu²⁺ and Ni²⁺. Other ions (includ ing Mn²⁺, Zn²⁺ and molybdate) and ethyl ammonium phosphate also inhibited Fe³⁺ reduction but to a lesser degree. Citrate, however, enhanced Fe³⁺ reduction. The degree of inhibition or enhancement differed for each of the varieties. In general, the Fe‐efficient plants were best able to reduce Fe³⁺ in spite of the inhibitory influence of the imposed treatments. Thus, our findings indicated that inhibition of the Fe³⁺ ‐reduction process at, or near, the periphery of the root is an apparent cause of Fe chlorosis.     

Iron‐manganese interactions among clones of Nilegrass

Tetraploid clones of Nilegrass (Acroceras macrum, Stapf.) develop a chlorosis resembling iron (Fe) deficiency on acid (pH 5.0) soils in the Midlands of KwaZulu, Natal, South Africa. Hexaploid and pentaploid clones appear more resistant to the disorder. Iron deficiency would not be expected in such acid soils, but foliar sprays of Fe sulfate reduce the symptoms within 24 hours. Aluminum (Al) toxiciry has been ruled out as a cause of this chlorosis on the basis of soil tests. Manganese (Mn)‐induced Fe deficiency has been postulated. Six Nilegrass clones, differing in ploidy levels, were grown under low Fe or high Mn levels in nutrient solutions, in Mn‐toxic soil, in calcareous soil and in a standard potting soil at pH 7.0. Differential chlorosis symptoms, similar to those observed in the field, were reproduced in plants grown in low Fe or high Mn solutions, in neutral potting soil and in calcareous soil at pH 7.8. Based on plant symptoms and dry weights, the tetraploids were generally more sensitive to these conditions than hexaploid or pentaploid clones. However, in Mn‐toxic soil, plants had leaf tip necrosis rather than the chlorosis typical of Fe deficiency. When grown in a standard potting soil at pH 7.0, plants showing chlorosis accumulated higher concentrations of phosphorus (P), Al, copper (Cu), Mn, Fe, and zinc (Zn) than non‐chlorotic plants. Differential susceptibility to chlorosis is apparently associated with interference of such elements in Fe metabolism, and not with differential Fe concentrations in plant shoots. Additional studies are needed to determine the chemical states of Fe and Mn in root zones and within plant shoots of these clones. Resolution of the differential chlorosis phenomenon would contribute to fundamental knowledge in mineral nutrition and could be helpful in tailoring plant genotypes to fit problem soils.     

Iron deficiency stress response of various c‐3 and c‐4 grain crop genotypes: Strategy II mechanism evaluated

The relative amount of phytosiderophore produced by various Strategy II plants has been categorized as follows: barley (Hordeum vulgare L.) > wheat (Triticum aestivum L.) > oat (Avena byzantina C. Koch.) > rye (Secale cereale L.) >> corn (Zea mays L.) >> sorghum (Sorghum bicolor (L.) Moench) > rice (Oryza sativa L.). With the exception of rice, these plants developed under oxidized soil conditions, and the C‐3 species produce more phytosiderophore than C‐4 species under Fe‐deficiency stress. Iron‐efficient Coker 227 oat produced phytosiderophore in response to Fe‐deficiency stress, while Fe‐inefficient TAM 0–312 oat did not. Although Fe‐efficient WF9 corn and Fe‐inefficient ys₁ corn differed in their ability to obtain Fe, neither produced sufficient quantities of phytosiderophore to explain these differences. The objectives of this research were to determine: (a) if phytosiderophore production of Fe‐deficiency stressed C‐4 species millet (Panicum miliaceum L.) and corn is low or absent compared to identically stressed C‐3 species oat and barley, and (b) if native, inbred and hybrid corn cultivars differ in ability to produce and utilize phytosiderophores.

Although release of phytosiderophore for Fe‐stressed corn and millet was generally lower than oat, quantity of release was not always related to obtaining Fe and maintaining green plants. Barley maintained high leaf Fe and low chlorosis with a minor release of phytosiderophore. Oat with increased release acted similarly to barley, whereas a relatively high release of phytosiderophore from White maize did not effect Fe uptake or greening. Likewise, small amounts of phytosiderophore were produced by all corn types, but corn was generally unable to obtain adequate Fe from the growth medium. Two of the native corns, Coneso and Tepecintle, maintained relatively low chlorosis, but they differed in phytosiderophore release. Thus, it appears that the C‐4 plants studied herein generally release a lower amount of phytosiderophore than do C‐3 species, but overcoming Fe‐deficiency chlorosis is not guaranteed by such release. The Strategy II mechanism of mere release of phytosiderophore and consequential Fe acquisition appears simplistic. There is a need for understanding what other factors are involved.     

IRON EFFICIENCY IN KENTUCKY BLUEGRASS NOT RELATED TO PHYTOSIDEROPHORE RELEASE

Some Kentucky bluegrass (KBG; Poa pratensis L.) is susceptible to iron (Fe)-deficiency chlorosis. Under Fe-deficiency stress, phytosiderophore is produced and released by the roots of many grasses to solubilize soil Fe and enhance uptake. In other species, quantifying phytosiderophore screens for Fe-deficiency resistant cultivars. A hydroponic study was conducted at 1 and 10 μM solution Fe to variously stress ‘Baron’, ‘Award’, ‘Limousine’, and ‘Rugby II’ KBG cultivars. One μM Fe solution produced more Fe-deficiency stress in all cultivars compared to 10 μM, resulting in greater chlorosis and phytosiderophore release but reduced shoot and root Fe concentrations and shoot weight. Of the four cultivars, Baron was the most susceptible to Fe deficiency and exhibited severe Fe chlorosis and low shoot Fe but, surprisingly, produced the most phytosiderophore. These results imply that Fe-deficiency susceptibility in KBG may be less related to phytosiderophore release and more related to inefficient uptake or utilization mechanisms.     

Identification of dibutyl phthalate in the root exudate of Fe‐efficient (corchorus capsularis L.) subjected to Fe‐deficiency stress : Its action on Fe‐chlorosis recovery

The jute (Corchorus capsularis L.) cv. 3RC‐212 which is Fe‐efficient, was subjected to Fe‐deficiency stress, and the nutrient medium was examined for chemicals, when the plants became chlorotic and the pH was lowered to about 4. While phenolic acids could not be detected, DBP (dibutyl phthalate) was identified in the extract by means of TLC and HPLC. The effect of DBP and caffeic acid was studied in JRC‐212 and DBP was found to cause recovery of the plants from chlorosis in 5 days. The chemicals, PA (phthalic acid), a derivative of DBP (50 mg/1) were supplied to chlorotic plants of JRO‐632, an Fe‐inefficient jute cultivar, and both the chemicals were effective in chlorosis recovery. PA application caused more rapid greening than DBP.

Jute is the second crop species in which DBP is identified in the root exudate. The detection of DBP was first recorded in sorghum CSH‐7.     

Activities of Iron‐Containing Enzymes in Leaves of Two Tomato Genotypes Differing in Their Resistance to Fe Chlorosis

Abstract

Two tomato (Lycopersicon esculentum Mill., cvs. Pakmor and Target) genotypes differing in resistance to iron (Fe) deficiency were grown in nutrient solution under controlled environmental conditions over 50 days to study the relationships between severity of leaf chlorosis, total concentration of Fe, and activities of Fe‐containing enzymes in leaves. The activities of Fe‐containing enzymes ascorbate peroxidase, catalase, and guaiacol peroxidase, and additionaly the activity of glutathione reductase, an enzyme that does not contain Fe, were measured. Plants were supplied with 2 × 10^?7 M (Fe deficient) and 10^?4 M (Fe sufficient) FeEDTA, respectively. Leaf chlorosis appeared more rapidly and severely in Target (Fe deficiency senstive genotype) than Pakmor (Fe deficiency resistant genotype). On day 50, Pakmor had 2‐fold more chlorophyll than Target under Fe deficiency, while at adequate supply of Fe the two genotypes were very similar in chlorophyll concentration. Despite distinct differences in development of leaf chlorosis and chlorophyll concentrations, Pakmor and Target were very similar in concentrations of total Fe under Fe deficiency. In contrast to Fe concentration, activities of Fe‐containing enzymes were closely related to the severity of leaf chlorosis. The Fe‐containing enzymes studied, especially catalase, showed a close relationship with the concentration of chlorophyll and thus differential sensitivity of tomato genotypes to Fe deficiency. Glutathione reductase did not show relationship between Fe deficiency chlorosis and enzyme activity. The results confirm that measurement of Fe‐containing enzymes in leaves is more reliable than the total concentration of Fe for characterization of Fe nutritional status of plants and for assessing genotypical differences in resistance to Fe deficiency. It appears that Fe deficiency‐resistant genotype contains more physiologically available Fe in tissues than the genotype with high sensitivity to Fe deficiency.     

Influences of ultra‐violet (UV)‐blue light radiation on the growth of cotton. I. Effect on iron nutrition and iron stress response

Cool white fluorescent (CWF) light reduces Fe³⁺ to Fe²⁺ while low pressure sodium (LPS) light does not. Cotton plants grown under CWF light are green, while those yrown under LPS light develop a chlorosis very similar to the chlorosis that develops when the plants are deficient in iron (Fe). It could be that CWF light (which has ultra violet) makes iron more available for plant use by maintaining more Fe²⁺ in the plant. Two of the factors commonly induced by Fe‐stress in dicotyledonous plants‐‐hydroyen ions and reductants released by the roots‐‐were measured as indicators of the Fe‐deficiency stress response mechanism in M8 cotton.

The plants were grown under LPS and CWF light in nutrient solutions containing either NO₃‐N or NH₄‐N as the source of nitrogen, and also in a fertilized alkaline soil. Leaf chlorophyll concentration varied significantly in plants grown under the two light sources as follows: CWF+Fe > LPS+Fe > CWF‐Fe ≥ LPS‐Fe. The leaf nitrate and root Fe concentrations were significantly greater and leaf Fe was generally lower in plants grown under LPS than CWF light. Hydrogen ions were extruded by Fe‐deficiency stressed roots grown under either LPS or CWF light, but “reductants”; were extruded only by the plants grown under CWF light. In tests demonstrating the ability of light to reduce Fe³⁺ to Fe²⁺ in solutions, enough ultra violet penetrated the chlorotic leaf of LPS yrown plants to reduce some Fe³⁺ in a beaker below, but no reduction was evident through a yreen CWF grown leaf.

The chlorosis that developed in these cotton plants appeared to be induced by a response to the source of liyht and not by the fertilizer added. It seems possible that ultra violet liyht could affect the reduction of Fe³⁺ to Fe²⁺ in leaves and thus control the availability of this iron to biological systems requiring iron in the plant.     

Behavior of iron‐inefficient plants when grown in combinations of calcareous and noncalcareous soils

Abstract

When Fe‐inefficient plants were grown in mixtures of calcareous Hacienda loam soil and noncalcareous Yolo loam soil compared with plants grown in unmixed soils, characteristics and composition of the plants including Fe deficiency were generally intermediate to those with either soil alone. Noncalcareous soil adjacent to calcareous soil allowed PI 54619–5–1 soybeans (Glycine max L.) to obtain sufficient Fe.     

An iron chelating compound released by barley roots in response to Fe‐deficiency stress

The excretion of phytosiderophores by barley (Hordeum vulgare L.) has recently been documented and a major difference in the Fe‐stress response of gramineous species and dicotyledonous species proposed. However, currently used methods of quantifying and measuring phytosiderophore are tedious or require specialized equipment and a cultivar easily accessible to U.S. scientists is needed. The objectives of this study were (a) to determine if “Steptoe”; and “Europa”; (used as a control cultivar) barleys would release Fe³⁺ solubilizing compounds in response to Fe‐deficiency stress and (b) to develop a technique to determine the efficiency of solubilization of Fe(OH)₃ by the released chelating substances. Two cultivars of barley were place under Fe‐stressed (‐Fe) and nonstressed (+Fe) conditions in modified Hoagland solutions (14 L). The solutions were periodically monitored for H⁺ and reductant release from the roots and plants were rated daily for chlorosis development. Periodic (6 or 7 harvests) evaluation of the release of Fe³⁺ solubilizing substances was performed as herein described. Neither H⁺ nor reductant extrusion occurred with either cultivar during Fe stress. However, Fe³⁺ solubilizing substances were released by both cultivars at relatively high levels under Fe‐stress conditions compared to the nonstressed plants. A convenient technique was developed to measure the release of Fe solubilizing substances released by barley roots.     

Cultivar differences for tolerance to Fe and Zn deficiency: A comparison of two maize hybrids and their parents

The Fe and Zn deficiency tolerances for two high yielding maize (Zea mays L.) hybrids (G‐2 and G‐5) and their parent cultivars were examined by growing them in nutrient solutions. The results indicated the occurrence of heterosis for Zn deficiency tolerance in G‐5, and to a lesser extent in G‐2. Each cultivar was susceptible to Fe deficiency and did not show signs of recovery from chlorosis. The symptoms of Fe deficiency were distinct from those for Zn deficiency. Plant growth was affected more by Fe deficiency than by Zn deficiency. The roots of cultivars were reduced in growth under Fe deficiency conditions.     

Excretion of dibutyl phthalate by sorghum roots under Fe‐stress: Evidence of its action on chlorosis recovery

Sorghum (Sorghum bicolor L. Moench) cv. CSH‐7, an Fe‐efficient hybrid was grown and subjected to Fe‐deficiency stress. The nutrient medium was extracted for isolation of reductant chemicals. By means of thin layer chromatography, I.R. spectrum and HPLC analysis, dibutyl phthalate was identified as the principal component. This chemical was not found in the nutrient medium extracted before the onset of chlorosis or in that after the plants recovered from chlorosis. Furthermore, synthetic dibutyl phthalate and that obtained from the exudate when supplied to the nutrient medium caused greening of chlorotic sorghum in 24 hours. Evidence that the root medium of the Fe‐efficient sorghum can induce recovery of an Fe‐inefficient sorghum grown together, has also been obtained. It is concluded that dibutyl phthalate released by the Fe‐efficient sorghum subjected to stress, is responsible for making Fe available for utilisation. The mechanism of action of dibutyl phthalate on chlorosis recovery is still an open question.     

Root and Leaf Ferric Chelate Reductase Activity in Pond Apple and Soursop

Abstract

Root and leaf ferric chelate reductase (FCR) activity in Annona glabra L. (pond apple), native to subtropical wetland habitats and Annona muricata L. (soursop), native to nonwetland tropical habitats, was determined under iron (Fe)-sufficient and Fe-deficient conditions. One-year-old seedlings of each species were grown with 2, 22.5, or 45 µM Fe in a nutrient solution. The degree of tolerance of Fe deficiency was evaluated by determining root and leaf FCR activity, leaf chlorophyll index, Fe concentration in recently mature leaves, and plant growth. Root FCR activity was generally lower in soursop than in pond apple. Eighty days after plants were put in nutrient solutions, leaf FCR activity of each species was lower in plants grown with low Fe concentrations (2 µM) than in plants grown with high (22.5 or 45 µM) Fe concentrations in the nutrient solution. Leaves of pond apple grown without Fe became chlorotic within 6 weeks. The Fe level in the nutrient solution had no effect on fresh and dry weights of soursop. Lack of Fe decreased the leaf chlorophyll index and Fe concentration in recently matured leaves less in soursop than in pond apple. The rapid development of leaf chlorosis in low Fe conditions and low root and leaf FCR activities of pond apple are probably related to its native origin in wetland areas, where there is sufficient soluble Fe for adequate plant growth and development. The higher leaf FCR activity and slower growth rate of soursop compared to pond apple may explain why soursop did not exhibit leaf chlorosis even under low Fe conditions.     

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.     

Modified procedures for commercial adaptation of root iron‐reducing capacity as a screening technique

Abstract

Genotypic evaluation is critical to development of soybean [Glycine max (L.) Merr.] cultivars with genetic resistance to Fe‐deficiency chlorosis. Root Fe³⁺ reducing activity is correlated with genotypic resistance to Fe chlorosis measured in field nurseries, and thus may be a reliable method for identifying chlorosis‐resistat genotypes. However, to develop methods useful for large‐scale screening, several modifications of the previously published procedure for measuring root Fe³⁺ reducing activity were investigated. Several hydroponic experiments were conducted to test proposed modifications. It was determined that: (a) different genotypes may be grown together in the same nutrient solution without affecting Fe³⁺ reduction, (b) genotype separation is maximized by growth in CaCO₃ buffered solution (37.5 mg L^?1), (c) a labor‐intensive elongation step can be eliminated, and (d) denotype evaluation can be accomplished without introducing Fe into the hydroponic solutions. These refinements to the procedure should allow its adaptation and use in soybean breeding programs.     

A greenhouse technique for screening Trifolium seedlings for iron‐deficiency chlorosis 1

Susceptible Trifolium plants often exhibit symptoms of iron (Fe)‐deficiency chlorosis when grown on high pH, calcareous soils. A greenhouse method was developed to screen seedlings for Fe‐deficiency chlorosis. ‘Yuchi’ arrowleaf (T. vesiculosum Savi.) and ‘Dixie’ crimson (I. incarnatum L.) clover seedlings were grown in “Super Cell”; Cone‐tainers in six calcareous Texas soils differing in Fe and selected other chemical characteristics. At the fourth trifoliolate leaf stage, chlorosis was induced by saturating the soil for a minimum of 2 weeks. The soils differed in their capacity to induce chlorosis in both clovers. Yuchi was more susceptible than Dixie, showing a higher percentage of chlorosis in five of the six soils. The results indicate that this screening method would be a useful tool for studying Fe‐deficiency chlorosis in Trifolium spp.     

Iron inefficient and efficient oat cultivars. II. Characterization of phytosiderophore released in response to Fe deficiency stress

Abstract

Iron‐inefficient TAM 0–312 and Fe‐efficient Coker 227 oats (Strategy II plants) differ in their release of phytosiderophore in response to iron‐deficiency stress—the Fe‐efficient Coker 227 releases a phytosiderophore whereas the Fe‐inefficient TAM 0–312 does not. The phytosiderophore released by Coker 227 oats in response to Fe‐deficiency stress does not appear to transport Fe into the plant as Fe phytosiderophore. When the Fe‐inefficient TAM 0–312 and Fe‐efficient Coker 227 oats were subjected to Fe supplied as Fe²⁺(BPDS)₃, Fe³⁺HEDTA, as Fe³⁺EDDHA, Coker 227 utilized the Fe more efficiently than TAM 0–312 in every case. Both cultivars reduced Fe³⁺ as FeCl₃ to form Fe²⁺(BPOS)₃ and responded better to this form of Fe than Fe supplied as the ferric chelate. Reduction of Fe³⁺ at the root appears to be a factor that facilitates iron uptake by Coker 227 oats and the release of a phytosiderophore appears to make more Fe available at the root that can be reduced and transported to plant tops.     

Differential expression of iron deficiency‐induced genes in barley genotypes with differing manganese efficiency

The expression of two barley genes, Ids1 and Ids2, that were induced specifically by iron (Fe) deficiency stress in solution culture, was examined in two barley genotypes differing in manganese (Mn) efficiency. Plants were grown in a calcareous soil supplied with two levels of Mn (15 and 100 mg/kg soil). Ids1 was expressed at equal levels in the roots of both genotypes, and this expression was not affected by Mn supply. These results suggest that the expression of Ids1 probably does not contribute to Mn efficiency. A contrasting result was obtained for Ids2, which was expressed at a higher level in the roots of the Mn‐inefficient genotype than in the Mn‐efficient genotype. However, the expression levels also were not affected by Mn supply. The differential expression of Ids2 may indicate that this gene plays a role both in the Fe deficiency response and in the Mn efficiency mechanism. An interesting observation made on the time course of expression of the two genes. Initially, both genes had low expression in two week old plants and then much higher expression in three week old plants. The timing of this increase probably relates to the exhaustion of the seed Fe reserves. Therefore, our results indicate a need to consider the effect of seed nutrient content in research on the molecular basis for micronutrient acquisition.     

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.