Immobilization of tissue iron on calcareous soil: differences between calcicole and calcifuge plants

Deficiency of P and sometimes of micronutrients, especially Fe, is of importance to the calcicole–calcifuge behaviour of plants. Calcifuge species are unable to solubilize these elements or keep them metabolically active in sufficient amounts on calcareous soils. To demonstrate if calcicole, calcifuge and ‘soil indifferent’ species differ in Fe nutrition dynamics, samples of such species were transplanted on a slightly acid silicate soil (pH BaCl₂ ca 4.0) and on a calcareous soil (pH BaCl₂ ca 7.2). Plants were grown in a computer‐controlled greenhouse at a soil moisture content of 50–60% water holding capacity and with additional light (ca 160 μE s^?1 m^?2, 12 h d^?1) if ambient light was <120 μE s^?1 m^?2.
The calcifuge species developed chlorosis when grown on the calcareous soil, whereas the other species did not. Calcareous‐soil grown plants had less 1,10‐phenanthroline extractable Fe in their leaf tissues than the silicate‐grown plants whereas total leaf Fe showed more species specific properties. The ratio of 1,10‐phenanthroline extractable to total Fe in the leaves was significantly lower in the calcifuges than in the calcicoles when grown on the calcareous soil. ‘Soil indifferent’ species did not differ much from the calcicoles. Root Fe, fractioned as DCB extractable ‘plaque’ on the root surface and Fe remaining in the root after DCB extraction, showed no distinct pattern of DCB‐Fe related to the different categories, but remaining root Fe tended to be lower in the calcifuges compared to the two other categories. Leaf colour estimated by a colour scale correlated well with chlorophyll a+b content measured in the leaves of two calcifuges. Leaf P concentrations did not differ between the different categories but were more species dependent.
We conclude that chlorosis in calcifuge species is related to an immobilization of Fe in physiologically less active forms in the tissue, if plants are forced to grow on a calcareous soil, whereas calcicole and ‘soil indifferent’ species are able to retain a much higher share of their leaf Fe in metabolically active form. This probably decreases the vitality and may exclude calcifuge plants from calcareous soil. We consider this property, previously almost unconsidered in an ecological context, as important to the calcifuge–calcicole behaviour of plants.     

Soluble inorganic tissue phosphorus and calcicole-calcifuge behaviour of plants

BACKGROUND AND AIMS: Natural and semi-natural, non-fertilized calcareous soils are consistently low in soluble and easily exchangeable phosphate. An over-utilization, or possibly an immobilization, of inorganic P in the tissues of calcifuge plants may take place, if such plants are forced to grow on a calcareous soil, though this has not been experimentally demonstrated. The objectives of this study are, therefore, to elucidate if calcifuge plants, when forced to develop on a calcareous soil, not only have lower total P (Ptot) concentrations in their leaves than calcicole plants grown on such soil, but also a lower proportion of Ptot as water-soluble, inorganic phosphate. Such differences may be of importance in understanding the calcicole-calcifuge behaviour of plants. MATERIALS AND METHODS: Plants of five calcicole and five calcifuge herbs and three calcicole and three calcifuge grasses were cultivated in a glasshouse on a moderately acid Cambisol and a calcareous Rendzic Leptosol using seeds of wild populations from southern Sweden. The calcifuges were: Corynephorus canescens, Deschampsia flexuosa, Holcus mollis, Digitalis purpurea, Lychnis viscaria, Rumex acetosella, Scleranthus annuus and Silene rupestris. The calcicoles were: Melica ciliata, Phleum phleoides, Sesleria caerulea, Arabis hirsuta, Sanguisorba minor, Scabiosa columbaria, Silene uniflora ssp. petraea and Veronica spicata. KEY RESULTS: At harvest, calcifuges had much lower leaf tissue concentrations of Ptot and Pi than calcicoles when grown on the calcareous soil, and biomass production of the calcifuges was poor on this soil. Moreover, the calcifuge herbs had, on average, a lower proportion of their Ptot as Pi than had the calcicole herbs. The calcifuge herbs were also unable to avoid excessive uptake of Ca from the calcareous soil. The calcifuge grasses maintained a similar proportion of Ptot as Pi as the calcicole grasses, but their growth was still poor on the calcareous soil. CONCLUSIONS: On calcareous soil, very little Pi in the tissues of calcifuge herbs is, at any time, available for use in various physiological functions. This is of importance to their photosynthesis, growth, competition and final survival on such soils.     

Some ecophysiological and historical approaches to species richness and calcicole/calcifuge behaviour — contribution to a debate

Species richness in vascular plants was related to the plants’ calcifuge or calcicole behaviour using documentation from forests and open-land vegetation at about one thousand sites in the southern parts of Sweden. It is concluded that vegetation of strongly acid soils (pH-KCl < 4.5) have fewer vascular plant species than comparable vegetation of other soils, whereas there are no consistent differences in species richness between slightly-moderately acid and calcareous sites. Low species richness is particularly related to high concentrations of Al³⁺ and H⁺ ions (either soil solution concentrations or concentrations of exchangeable ions), not to a lack of calcium carbonate. The majority of plant species are able to render the sparingly soluble phosphate, iron and manganese compounds of high-pH soils available, but they are unable to tolerate much Al³⁺ or H⁺. Acidicole (calcifuge) species have developed the power of tolerating Al³⁺ and H⁺, which may be considered a secondary property of plants, but they have lost the power of solubilizing critical mineral nutrients in high-pH soils. The reasons why these ecophysiological properties are inversely related in the current flora are obscure, difficult to account for experimentally and a main ecological problem. In areas with cool-temperate climates the flora was partly or mainly extinguished by the Pleistocene glaciations. Comparatively fewer calcifuge than calcicole species have, since then, had enough time to develop, and the number of calcifuges is lower, in spite of the fact that most natural and seminatural soils of these areas are nowadays acidic.     

Are Fe and P availabilities involved in determining the occurrence and distribution of Calluna vulgaris (L.) Hull in semi-arid grasslands on calcareous soils?

Calluna vulgaris (L.) Hull is primarily found on acid soils and is generally classified as a calcifuge species. Therefore, its occasional growth in semi-arid grassland on shallow calcareous soils gave rise to the question as to whether special soil conditions, deviating from the typical conditions in calcareous soils, enable this unusual occurrence. In an attempt to answer this question, we analysed selected soil factors, comparing plots where C. vulgaris was growing besides calcicole species (=CF plots) with neighbouring plots where only calcicole species were present (=CC plots). Main emphasis was placed on Fe and P availability because results from growth experiments indicate that the availability of either Fe or P to calcifuges causes calcifuge species to fail on calcareous soils. The results of our investigations do not support the hypothesis that the occurrence of C. vulgaris in semi-arid calcareous grasslands depends on higher Fe and/or P availability. Rather, its growth on carbonate-buffered soils shows that this species is not really calcifuge. Since the CF plots differ from the CC plots either by a lower inclination or by a more northerly exposition, we assume that the primary establishment and this species’ distribution pattern in the investigated semi-arid grassland are not dependent on soil chemical factors, but are governed by topography and its consequences for soil humidity and drought stress.     

Impact of nitrogen form on iron uptake and distribution in maize seedlings in solution culture

Comparative studies on the effect of nitrogen (N) form on iron (Fe) uptake and distribution in maize (Zea mays L. cv Yellow 417) were carried out through three related experiments with different pretreatments. Experiment 1: plants were precultured in nutrient solution with 1.0×10^–4 M FeEDTA for 6 d and then exposed to NO₃–N or NH₄–N solution with 1.0×10^–4 M FeEDTA or without for 7 d. Experiment 2: plants were precultured with ⁵⁹FeEDTA for 6 d and were then transferred to the solution with different N forms, and 0 and 1.0×10^–4 M FeEDTA for 8 d. Experiment 3: half of roots were supplied with ⁵⁹FeEDTA for 5 d and then cut off, with further culturing in treatment concentrations for 7 d. In comparison to the NH₄-fed plants, young leaves of the NO₃-fed plants showed severe chlorosis under Fe deficiency. Nitrate supply caused Fe accumulation in roots, while NH₄–N supply resulted in a higher Fe concentration in young leaves and a lower Fe concentration in roots. HCl-extractable (active) Fe was a good indicator reflecting Fe nutrition status in maize plants. Compared with NO₃-fed plants, a higher proportion of ⁵⁹Fe was observed in young leaves of the Fe-deficient plants fed with NH₄–N. Ammonium supply greatly improved ⁵⁹Fe retranslocation from primary leaves and stem to young leaves. Under Fe deficiency, about 25% of Fe in primary leaves of the NH₄-fed plants was mobilized and retranslocated to young leaves. Exogenous Fe supply decreased the efficiency of such ⁵⁹Fe retranslocation. The results suggest that Fe can be remobilized from old to young tissues in maize plants but the remobilization depends on the form of N supply as well as supply of exogenous Fe.     

Iron availability in plant tissues-iron chlorosis on calcareous soils

The article describes factors and processes which lead to Fe chlorosis (lime chlorosis) in plants grown on calcareous soils. Such soils may contain high HCO₃ ^- concentrations in their soil solution, they are characterized by a high pH, and they rather tend to accumulate nitrate than ammonium because due to the high pH level ammonium nitrogen is rapidly nitrified and/or even may escape in form of volatile NH₃. Hence in these soils plant roots may be exposed to high nitrate and high bicarbonate concentrations. Both anion species are involved in the induction of Fe chlorosis.Physiological processes involved in Fe chlorosis occur in the roots and in the leaves. Even on calcareous soils and even in plants with chlorosis the Fe concentration in the roots is several times higher than the Fe concentration in the leaves. This shows that the Fe availability in the soil is not the critical process leading to chlorosis but rather the Fe uptake from the root apoplast into the cytosol of root cells. This situation applies to dicots as well as to monocots. Iron transport across the plasmamembrane is initiated by Fe^III reduction brought about by a plasmalemma located Fe^III reductase. Its activity is pH dependent and at alkaline pH supposed to be much depressed. Bicarbonate present in the root apoplast will neutralize the protons pumped out of the cytosol and together with nitrate which is taken up by a H⁺/nitrate cotransport high pH levels are provided which hamper or even block the Fe^III reduction.Frequently chlorotic leaves have higher Fe concentrations than green ones which phenomenon shows that chlorosis on calcareous soils is not only related to Fe uptake by roots and Fe translocation from the roots to the upper plant parts but also dependent on the efficiency of Fe in the leaves. It is hypothesized that also in the leaves Fe^III reduction and Fe uptake from the apoplast into the cytosol is affected by nitrate and bicarbonate in an analogous way as this is the case in the roots. This assumption was confirmed by the highly significant negative correlation between the leaf apoplast pH and the degree of iron chlorosis measured as leaf chlorophyll concentration. Depressing leaf apoplast pH by simply spraying chlorotic leaves with an acid led to a regreening of the leaves.     

CALCIUM AND PLANT GROWTH

Calcium as a plant nutrient is characterized by its relatively high content in the plant coupled with a requirement not much higher than that of a micro nutrient element and an exceedingly uneven occurrence in soils. The difficulties in defining its actions are accentuated by a weak biochemical activity. In ecological conditions the secondary consequences of variations in calcium content may be more striking than the direct ones. Electron-microscopical studies have revealed that calcium is required for formation and maintenance of lamellary systems in cell organellae, a fact which might suffice to explain its indispensability for meristematic growth. Calcium is required for cell elongation in both shoots and roots; the common experience that it inhibits shoot elongation is certainly due to calcium additions far above actual requirement. It must be assumed for a rational interpretation of cell elongation that the fundamental mechanism is the same in shoots and roots. The one action which can be ascribed with certainty to calcium is a stabilizing of the cell wall with an increase in rigidity, an effect which, with over-optimal supply, may lead to growth inhibitions. The function is, however, necessary for the normal organization of cell walls. Calcium has, on the contrary, no significant effect on the synthesis of cell wall compounds but appears to act on their proper incorporation into the cell wall. The growth-active calcium may be bound not only to pectins but also to proteins and nucleoproteids in or in close contact with the cell wall. The supposition that calcium interacts directly with auxin in the cell wall has not been verified and does not seem very probable. There are reasons to believe that the points of action of calcium and auxin in the cell wall differ, auxin inducing growth by wall loosening and calcium establishing new wall parts. For submerged organs it may be necessary to consider an indirect effect of calcium on growth by its regulation of cytoplasmic permeability and thus affecting the exudation of growth-active compounds. The ecological problem is to characterize calcifuges (acid soil plants) from calcicoles (base soil or calcareous soil plants). Growth inhibitions on acid soils depend upon poisoning by A1³⁺ and Mn²⁺. Opinions differ as to what extent this can be antagonized by calcium. Lime-induced chlorosis in calcifuges depends upon iron deficiency or iron inactivation in the plant. No acceptable explanation is given, but it might be related to an interaction of calcium carbonate, phosphorus, and iron. A hypothesis that it is linked to formation of organic acids is not tenable in the given form. Plants react to the calcium ions in the concentrations found in soils. Calcifuges have a low calcium-optimum for growth and show growth inhibition at high concentrations. Calcicoles have a high optimum for growth. Calcifuges are resistant to aluminium poisoning. Attempts made to explain the differences in calcium uptake and generally in salt uptake are tentative only, and relevant data are lacking.     

Effect of fertilization on exudation,dehydrogenase activity,iron-reducing populations and Fe++ formation in the rhizosphere of rice (Oryza sativa L.) in relation to iron toxicity

Summary To explain the mechanism of iron toxicity, greenhouse and growth chamber (¹⁴CO₂ atmosphere) experiments were carried out. In pot experiments (with a typical iron-toxic soil and a fertile clay) we studied the effect of N, P, K and Ca+Mg fertilization (alone or in combination) on dehydrogenase activity, Fe⁺⁺ formation, and the populations of iron-reducing bacteria in the rhizosphere of rice IR22 and IR42. Fe uptake by the plants was measured at regular intervals. Dehydrogenase activity, the number of N₂-fixing iron-reducing bacteria, and the formation and uptake of Fe⁺⁺ decreased with increased supply of K, Ca, and Mg. This effect was clearer with IR22 (susceptible to iron toxicity) than with IR42 (releatively tolerant). Increased exudation and Fe uptake by IR36 at low nutrient and high Fe supply were recorded in a growth chamber experiment. Nutritional conditions, exudation rate (a measure of metabolic root leakage), the iron-reducing activity of the rhizosphere, and Fe⁺⁺ uptake by wetland rice appear to be clearly related. Iron toxicity is considered a physiological disorder caused by multiple nutritional soil stress rather than by a low pH and high Fe supply per sé.     

Root traits and taxonomic affiliation of nine herbaceous species grown in glasshouse conditions

This study examines whether root traits differed between three major plant families (Asteraceae, Fabaceae and Poaceae) and whether they are related to root respiration and exudation. Nine traits related to biomass allocation, root topology, morphology, chemical composition and mycorrhizal colonisation were examined for nine C₃ herbaceous species grown in controlled conditions. Poaceae differed from Fabaceae for the whole set of root traits examined except mycorrhizal colonisation, while Asteraceae showed intermediate characteristics. As compared to Fabaceae, Poaceae allocated more biomass to roots; showed a more sparsely branched root system with a small average root diameter, a high root dry matter content and a low nitrogen concentration. Root respiration was weakly related to root mass ratio and root dry matter content; no significant relationship was found between root functions and root architecture or morphology. This study shows that plant classification based on taxonomic affiliation reflects differences in root system traits and functions. Whole root system traits do not allow strong predictions of root respiration and exudation, perhaps because these processes are more linked to fine root than to whole root system traits.     

Iron and phosphate uptake explains the calcifuge–calcicole behavior of the terricolous lichens Cladonia furcata subsp. furcata and C. rangiformis

Mechanisms causing the calcifuge–calcicole behavior of lichens are largely unexplored. Studying the case examples of two closely related terricolous lichens, the calcifuge Cladonia furcata subsp. furcata and the calcicole C. rangiformis, we found that preference for acidic or calcareous soils in these lichens is related to iron and phosphate uptake as in vascular plants. In laboratory studies, the calcicole species was more efficient in the intracellular uptake of Fe³⁺ and phosphate at pH 8 than the calcifuge species. At pH 3, intracellular uptake of Fe²⁺ in the calcicole species significantly exceeded that in the calcifuge species suggesting that calcicole lichens suffer from toxicity symptoms by excess Fe²⁺ at acidic sites. Though these observations parallel findings from calcifuge and calcicole vascular plants, mechanisms leading to the different iron and phosphate uptake characteristics in the studied calcifuge and calcicole lichens may differ from those in vascular plants and should be the topic of future research. A role of the depside atranorin in facilitating iron uptake by reducing Fe³⁺ in the apoplast is hypothesized.     

Response of lupins (Lupinus angustifolius L.) and peas (Pisum sativum L.) to Fe deficiency induced by low concentrations of Fe in solution or by addition of HCO3 -

Lupins appear to be more sensitive than peas to Fe deficiency. However, when grown in nutrient solutions between pH 5–6, little difference existed between them in their ability to acidify the solution or to release Fe^III reducing compounds. This experiment was aimed at determining whether differences between species which occurred when Fe deficiency was induced by withholding Fe from an acid solution, are maintained when Fe deficiency is induced by addition of HCO₃ ^-. Lupins and peas were grown in nutrient solutions at 0, 2 and 6 μM of Fe^III EDDHA and either with or without HCO₃ ^- (6 mM). Bicarbonate induced symptoms of Fe deficiency (chlorosis) in both lupins and peas, and markedly decreased the growth of shoots. Symptoms appeared sooner and were more severe in lupins than in peas. Growing plants without HCO₃ ^-, but at the lowest Fe level, decreased the growth and Fe concentration of shoots of lupins but did not induce chlorosis. Growing peas in this treatment, decreased Fe concentrations, but to a lesser extent than in lupins, and did not decrease growth. H⁺-ion extrusion and release of Fe^III reducing compounds was greater in lupins than in peas. Bicarbonate also decreased the growth of roots of lupins but increased the growth of roots of peas. Results indicate that when Fe deficiency is induced by HCO₃ ^-, then the response of lupins and peas are similar to their response in acid solution culture. Differences between species therefore could not be explained by their relative abilities to acidify or release Fe^III reducing compounds. Greater control of the distribution of Fe within the shoots, the presence of a pool of Fe within the roots, a lower threshold for Fe uptake, or a higher content of seed-Fe, may therefore be the reason for the lower sensitivity of peas than lupins to Fe deficiency.     

Cluster-root formation, carboxylate exudation and proton release of Lupinus pilosus Murr. as affected by medium pH and P deficiency

The study examined the interactive effect of pH and P supply on cluster-root formation, carboxylate exudation and proton release by an alkaline-tolerant lupin species (Lupinus pilosus Murr.) in nutrient solution. The plants were exposed to 1 (P₁, deficient) and 50 μM P (P₅₀, adequate) for 34 days in nutrient solution at either pH 5.6 or 7.8. Plant biomass was not influenced by pH at P₁, but at P₅₀ shoot and root dry weights were 23 and 18% higher, respectively, at pH 7.8 than at pH 5.6. There was no significant difference in plant biomass between two P treatments regardless of medium pH. Phosphorus deficiency increased significantly the number of the second-order lateral roots compared with the P₅₀ treatment. Both total root length and specific root length of plants grown at pH 5.6 were higher than those at pH 7.8 regardless of P supply. Cluster roots were formed at P₁, but cluster-root number was 2-fold higher at pH 7.8 than pH 5.6. Roots released 16 and 31% more protons at pH 5.6 and 7.8, respectively, in P₁ than in P₅₀ treatments, and the rate of proton release followed the similar pattern. At pH 5.6, citrate exudation rate was 0.39 μmol g⁻¹ root DW h⁻¹ at P₁, but was under the detection limit at P₅₀; at pH 7.8, it was 2.4-fold higher in P₁ than in P₅₀ plants. High pH significantly increased citrate exudation rate in comparison to pH 5.6. The uptake of anions P and S was inhibited at P₁ and high pH increased cations Na, Mg and Ca uptake. The results suggested that enhanced cluster-root formation, proton release and citrate exudation may account for the mechanism of efficient P acquisition by alkaline-tolerant L. pilosus well adapted to calcareous soils. Cluster-root formation and citrate exudation in L. pilosus can be altered by medium pH and P deficiency. Phosphorus deficiency-induced proton release may be associated with the reduced anion uptake, but high pH-induced proton release may be partly attributed to increased cation uptake.     

Differing organic Acid exudation pattern explains calcifuge and acidifuge behaviour of plants

Many vascular plant species are unable to colonize calcareous sites. Thus, the floristic composition of adjacent limestone and acid silicate soils differs greatly. The inability of calcifuge plants to establish in limestone sites seems related to a low capacity of such plants to solubilize and absorb Fe or phosphate from these soils. Until now, mechanisms regulating this differing ability of plants to colonize limestone sites have not been elucidated. We propose that contrasting exudation of low-molecular organic acids is a major mechanism involved and show that germinating seeds and young seedlings of limestone plants exude considerably more di- and tricarboxylic acids than calcifuges, which mainly exude monocarboxylic acids. The tricarboxylic citric acid is a powerful extractor of Fe, and the dicarboxylic oxalic acid a very effective extractor of phosphate from limestone soils. Monocarboxylic acids are very weak in these respects. The study is based on ten species from limestone soils and ten species from acid silicate soils.     

Response of root respiration and root exudation to alterations in root C supply and demand in wheat

Rising atmospheric CO₂ concentrations have highlighted the importance of being able to understand and predict C fluxes in plant-soil systems. We investigated the responses of the two fluxes contributing to below-ground efflux of plant root-dependent CO₂, root respiration and rhizomicrobial respiration of root exudates. Wheat (Triticum aestivum L., var. Consort) plants were grown in hydroponics at 20°C, pulse-labelled with ¹⁴CO₂ and subjected to two regimes of temperature and light (12 h photoperiod or darkness at either 15°C or 25°C), to alter plant C supply and demand. Root respiration was increased by temperature with a Q ₁₀ of 1.6. Root exudation was, in itself, unaltered by temperature, however, it was reduced when C supply to the roots was reduced and demand for C for respiration was increased by elevated temperature. The rate of exudation responded much more rapidly to the restriction of C input than did respiration and was approximately four times more sensitive to the decline in C supply than respiration. Although temporal responses of exudation and respiration were treatment dependent, at the end of the experimental period (2 days) the relative proportion of C lost by the two processes was conserved despite differences in the magnitude of total root C loss. Approximately 77% of total C and 67% of ¹⁴C lost from roots was accounted for by root respiration. The ratio of exudate specific activity to CO₂ specific activity converged to a common value for all treatments of 2, suggesting that exudates and respired CO₂were not composed of C of the same age. The results suggest that the contributions of root and rhizomicrobial respiration to root-dependent below-ground respiration are conserved and highlight the dangers in estimating short-term respiration and exudation only from measurements of labelled C. The differences in responses over time and in the age of C lost may ultimately prove useful in improving estimates of root and rhizomicrobial respiration.     

Effect of primary leaves on 59Fe uptake by roots and 59Fe distribution in the shoot of iron sufficient and iron deficient bean (Phaseolus vulgaris L.) plants

A study has been made on the effect of primary leaves on iron (Fe) distribution in the shoot. Bean (Phaseolus vulgaris L.) seedlings were precultured in nutrient solution with 8×10^-5 M FeEDTA for 4 days, and then grown further with either 8×10^-5 M FeEDTA (+Fe) or without Fe supply (-Fe) for another 5 days. Thereafter, both +Fe and -Fe plants were treated in three different ways: undisturbed; one primary leaf removed; or one primary leaf shaded, starting two hours before supply ⁵⁹FeEDTA to the roots. The +Fe plants were supplied with 8×10^-5 M ⁵⁹FeEDTA, and the -Fe plants with only 1×10^-6 M ⁵⁹FeEDTA. After 1 to 8 hour uptake periods, plants were harvested and ⁵⁹Fe in different organs was determined. Removal or shading of one primary leaf did not affect ⁵⁹Fe uptake by roots and ⁵⁹Fe translocation to the shoot in +Fe plants. In the -Fe plants, however, removal of one primary leaf decreased ⁵⁹Fe uptake by roots, whereas shading of one primary leaf had no effect on ⁵⁹Fe uptake but slightly enhanced ⁵⁹Fe translocation from roots to the shoot. The quantity of ⁵⁹Fe in primary leaves was positively correlated with quantity of ⁵⁹Fe in the stem in the -Fepplants, but not in the +Fe plants. In both, the +Fe and -Fe plants, the quantity of ⁵⁹Fe in the shoot apex was positively correlated with ⁵⁹Fe in primary leaves. The results suggest that irrespective of the Fe nutritional status of plants, the source of Fe for the shoot apex is Fe retranslocated from primary leaves.     

Phosphorus efficiency of wheat and sugar beet seedlings grown in soils with mainly calcium,or iron and aluminium phosphate

Phosphorus is often limiting crop growth in soils low in P supplying capacity. The objective of this study was to investigate whether there are differences in P efficiency between sugar beet and wheat and to search for the plant properties responsible for different P efficiencies encountered and furthermore to see whether the kind of P binding in soil affects the P efficiency of crops. For this a pot experiment with an Oxisol with P mainly bound to Fe and Al (Fe/Al-P) and a Luvisol with P mainly bound to Ca (Ca-P) was run with increasing P fertilizer levels from 0 to 400 mg kg^–1 in a climate chamber. Shoot dry weights of wheat and sugar beet increased strongly with P application in both soils. Both crops, despite their large differences in plant properties, had the same P efficiency in both soils. Therefore none of the species was especially able to use either Fe/Al-P or Ca-P. Wheat relied on a somewhat lower internal requirement, a large root system (high root/shoot ratio) and a low shoot growth rate with a low influx while sugar beet with a small root system and a large shoot growth rate relied on a 5 to 10 times higher influx. A mechanistic mathematical model for calculation of uptake and transport of nutrients in the rhizosphere was used to assess the influence of morphological and physiological root properties on P influx. A comparison of calculated and measured P influx showed that prediction by the model is reasonably accurate for Luvisol. For Oxisol, the predicted P influx was much less than the observed one, even when P influx by root hairs was considered. A sensitivity analysis showed that physiological uptake parameters like I _max, K _m, and C_{L min} had no major influence on predicted influx. The greatest influence on influx had the P soil solution concentration C _{L i}. It is assumed that both species had used mechanisms to increase P availability in the rhizosphere similar to an increase of C _{L i}. Such mechanisms could be the exudation of organic acids, which are known as a sorption competitor to phosphate bound to Fe/Al-oxides or humic-Fe-(Al) complexes or to build soluble complexes with Fe and P. The close agreement between calculated and measured P influx in the Luvisol even at P deficiency indicates that root exudates were not able to mobilize Ca-bound P, whereas Fe/Al-P could be mobilized easily.     

Isoelectric Focusing of Plant Plasma Membrane Proteins : Further Evidence that a 55 Kilodalton Polypeptide Is Associated with beta-1,3-Glucan Synthase Activity from Pea

The role of the root apoplasm for iron acquisition was studied in wheat (Triticum aestivum L. cv Ares) grown in nutrient solution under controlled environmental conditions. To obtain different levels of Fe in the root apoplasm, plants were supplied in the dark for 5 hours (preloading period) with various ⁵⁹Fe-labeled Fe compounds [Fe(III) hydroxide; microbial siderophores: Fe rhodotorulic acid (FeRDA) and ferrioxamin (FeDesferal³), and synthetic Fe chelate (FeEDDHA)], each at a concentration of 5 micromolar. Large pools of apoplasmic Fe were formed after supplying Fe(III) hydroxide or FeRDA, but no such pools were observed after supplying FeDesferal or FeEDDHA. Depending on plant Fe nutritional status (preculture ± 0.1 millimolar FeEDTA), apoplasmic Fe was used to different extent for translocation to the shoot. Under Fe deficiency, a much greater fraction of the apoplasmic Fe was utilized than in Fe-sufficient plants, as a result of the different rates of phytosiderophore release. Because of the diurnal rhythm in release of phytosiderophores in Fe-deficient plants, the utilization of the apoplasmic Fe for translocation into the shoot started 2 hours after onset of the light period and was dependent on the concentration of Fe in the apoplasm, which followed the order: Fe(III) hydroxide FeRDA FeDesferal = FeEDDHA. From these results, it can be concluded that in soil-grown plants the apoplasmic Fe pool loaded by various indigenous Fe compounds such as siderophores in the soil solution can be an important Fe source in graminaceous species, particularly during periods of limited Fe supply from the soil.     

Absorption and transfer of fe and mn in germinating sorghum

The absorption of Fe from FeSO₄, FeEDTA, and FeEDDHA (ferric ethylenediaminedi (o-hydroxyphenylacetate)), and Mn from MnSO₄, MnEDTA, and MnEDDHA, by germinating sorghum (Sorghum vulgarie Pers. var. M 35-1) was studied. The seeds were found to absorb Fe and Mn from all the sources, and these ions moved to the scutellum, shoot, and root. EDDHA facilitated greater translocation of Fe and Mn from the seed to the shoot and root. The translocation of Fe was more towards the root than to the shoot, whereas it was the reverse in the case of Mn.     

Effects of pH and bicarbonate on the nutrient status and growth of three Lupinus species

Aims

High pH, and high bicarbonate (HCO₃⁻) and calcium (Ca) availability characterise calcareous soils. High [Ca] only partially explains why some Lupinus species are calcifuge, so we explored high [HCO₃⁻] and high pH.

Methods

We grew six Lupinus genotypes in hydroponics with pH 5, 6.5 and 8^a (adjusted by KOH), and 8^b (adjusted by KHCO₃). Leaf symptoms and areas, root appearance and biomass were recorded; whole leaf and root nutrient concentrations, and leaf cellular phosphorus (P), Ca and potassium (K) concentrations were determined using elemental X-ray microanalysis.

Results

Chlorosis was observed in young leaves at high pH for L. angustifolius and L. cosentinii, and P deficiency at high pH for all genotypes. High pH decreased iron (Fe) and zinc (Zn) uptake in all genotypes. It also decreased lateral root growth, the uptake of P, K, Ca, and manganese (Mn) by all sensitive species; and translocation of P, Fe, Zn, Mn, and Ca to leaves in most sensitive species. However, leaf [Ca], leaf [K], [K] within each measured cell type, and translocation of K and Ca to leaves of L. pilosus and L. cosentinii at pH 8 were greater than at pH 5 and 6.5. Compared with pH 8^a, all L. angustifolius genotypes translocated more P, Fe, Zn, Mn and K from roots to leaves at pH 8^b. High pH did not affect the leaf cell types that accumulated P and Ca, but decreased the leaf cellular [P].

Conclusions

Lupinus angustifolius and L. cosentinii were sensitive to high [HCO₃⁻] and/or high pH; L. pilosus was relatively tolerant. High pH decreased lateral root growth and nutrient uptake, inhibiting growth of sensitive species. High [HCO₃⁻] diminished the negative effect of pH 8 on nutrient translocation to leaves in most L. angustifolius genotypes. This knowledge provides critical insights into the habits of Lupinus species to guide breeding of calcicole plants.

     

Solubility and dissolution of iron oxides

In most soils, FeIII oxides (group name) are the common source of Fe for plant nutrition. Since this Fe has to be supplied via solution, the solubility and the dissolution rate of the Fe oxides are essential for the Fe supply. Hydrolysis constants and solubility products (K_sp) describing the effect of pH on FeIII ion concentration in solution are available for the well-known Fe oxides occurring in soils such as goethite, hematite, ferrihydrite. K_sp values are usually extremely low ((Fe³⁺)·(OH)³=10^–37–10^–44). However, for each mineral type, K_sp may increase by several orders of magnitude with decreasing crystal size and it decreases with increasing Al substitution assuming ideal solid solution between the pure end-members. Based on such calculations a poorly crystalline goethite with a crystal size of 5 nm may well reach the solubility of ferrihydrite. The variations in K_sp are of relevance for soils because crystal size and Al substitution of soil Fe oxides vary considerably and can now be determined relatively easily.The concentration of Fe²⁺ in soil solutions is often much higher than that of Fe(III) ions. Therefore, redox potential strongly influences the activity of FeII. At a given pH and E_h, the activity of FeII is higher the higher K_sp of the FeIII oxide and thus also varies with the type of Fe oxide present.Besides the solubility, it is the dissolution rate which governs the supply of soluble Fe to the plant roots. Dissolution of Fe oxides takes place either by protonation, complexation or, most important, by reduction. Numerous dissolution rate studies with various FeIII oxides were conducted in strong mineral acids (protonation) and they have shown that besides the Fe oxide species, crystal size and/or crystal order and substitution are important determinative factors. For example, in soils, small amounts of a more highly soluble meta- or instable Fe oxide such as ferrihydrite with a large specific surface (several hundred m²g^–1) may be essential for the Fe supply to the plant root. Its higher dissolution rate can also be used to quantify its amount in soils. Ferrihydrite can be an important component in soils with high amounts of organic matter and/or active redox dynamics, whereas highly aerated and strongly weathered soils are usually very low in ferrihydrite. On the other hand, dissolution rates of goethites decrease as their Al substitution increases.Much less information exists on the rate of reductive and chelative dissolution of Fe oxides which generally simulate soil conditions better than dissolution by protonation. Here again, type of oxide, crystal size and substitution are important factors. Organic anions such as oxalate, which are adsorbed at the surface, may weaken the Fe³⁺-O bonds and thereby increase reductive dissolution. As often observed in weathering, the dissolution features of the crystals appear to follow zones of weakness in the crystal.