Fe Resupply to Fe-deficient Sugar Beet Plants Leads to Rapid Changes in the Violaxanthin Cycle and other Photosynthetic Characteristics without Significant de novo Chlorophyll Synthesis

The effects of Fe resupply to Fe-deficient plants have been investigated in hydroponically-grown sugar beet. In the short-term (24 h) after Fe resupply, major changes were observed, although de novo chlorophyll (Chl) synthesis had not begun yet. Approximately 50% of the zeaxanthin was converted into violaxanthin, whereas the actual Photosystem II (PS II) efficiency increased by 69% and non-photochemical quenching (NPQ) and the amount of thermally dissipated energy decreased markedly (by 47% and 40%, respectively). At the same time, photosynthetic rate increased approximately by 50%. From one to two days after Fe resupply, there was a gradual increase in the leaf concentrations of Chl and other photosynthetic pigments, accompanied by a further conversion of zeaxanthin into violaxanthin, increases in actual PS II efficiency and photosynthetic rates and decreases in NPQ and the amount of thermally dissipated energy. At 72-96 h after Fe resupply, leaf pigment concentrations, photosynthetic rates and actual PS II efficiency had increased further, although both photosynthetic rate and leaf pigment concentrations were still lower than those found in Fe-sufficient leaves. Good correlations were observed between the amount of light thermally dissipated by the PS II antenna, NPQ and the antheraxanthin + zeaxanthin concentration after Fe resupply, confirming the photoprotective role of the xanthophyll cycle in Fe-deficient sugar beet leaves. Similar correlations were observed for lutein, suggesting a possible role of this pigment in photoprotection.     

Increases in phosphoenolpyruvate carboxylase activity in iron-deficient sugar beet roots: Analysis of spatial localization and post-translational modification

Root tips of Fe-deficient and Fe-sufficient sugar beet plants grown in hydroponics have been used to study the changes in the amount and activity of the cytosolic enzyme phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31). Phosphoenolpyruvate carboxylase activity in extracts of the yellow Fe-deficient root tips was, at pH 7.3, 30-fold higher (when expressed on a FW basis) and 7.1-fold higher (when expressed on a protein basis) than that found in the extracts of Fe-sufficient root tips. The amount of phosphoenolpyruvate carboxylase protein determined by immuno-blotting was, on a protein basis, 35-fold larger in the yellow zone of Fe-deficient root tips than in the Fe-sufficient root tips. The inhibition of the phosphoenolpyruvate carboxylase activity by 500 m malate was 41 and 58% in the extracts Fe-deficient and Fe-sufficient roots. The possibility that post-translational regulation of phosphoenolpyruvate carboxylase may occur mediated through phosphorylation, was studied by immunological detection of phosphoserine residues in root tip extracts.     

Using a dual-stable isotope tracer method to study the uptake, xylem transport and distribution of Fe and its chelating agent from stereoisomers of an Fe(III)-chelate used as fertilizer in Fe-deficient Strategy I plants

A dual-stable isotope tracer experiment was carried out with Fe-deficient sugar beet plants grown hydroponically and resupplied with differentially Fe labeled racemic and meso Fe(iii)-chelates of the ethylendiamine di(o-hydroxyphenylacetic) acid (o,oEDDHA). No short-term Fe isotope exchange reactions occurred in the nutrient solution and plants did not discriminate between (54)Fe and (57)Fe. After 3-6 h, stable Fe isotopes, chelating agents and chelates were analyzed in roots, xylem sap and leaves by ICP-MS and HPLC-ESI/TOFMS. Ferric chelate reductase rates, xylem transport and total uptake were 2-fold higher with the meso isomer than with the racemic one. Both chelating agent isomers were incorporated and distributed by plants at similar rates, in amounts one order of magnitude lower than those of Fe. After 6 h of Fe resupply, most of the Fe acquired was localized in roots, whereas most of the chelating agent was in leaves. In a separate experiment, Fe-deficient sugar beet and tomato plants were treated with different concentrations of Fe(iii)-o,oEDDHA (with a meso/racemic ratio of 1). The xylem sap Fe concentration at 24 h was unaffected by the chelate concentration, with xylem Fe(iii)-o,oEDDHA accounting for 1-18% of total Fe and xylem meso/racemic ratio close to 1. Although most of the Fe coming from Fe(iii)-o,oEDDHA was taken up through a reductive dissociative mechanism, a small part of the Fe may be taken up via non-dissociative mechanisms.     

Effects of cadmium and lead on ferric chelate reductase activities in sugar beet roots

The effects of the heavy metals Cd and Pb on the activity of the enzyme ferric chelate reductase (FC-R, E.C. 1.6.99.13) have been studied in excised sugar beet root tips. The activity of this enzyme is markedly increased by iron deficiency. Metals were used as chloride salts or chelated with EDTA, and chemical speciation was carried out to predict the metal chemical species in equilibrium both in the ferric reductase assay and in the nutrient solutions. Three different heavy metal treatments were used. First, effects of Cd and Pb on the functioning of the FC-R were assessed in Fe-deficient plants, by including metals in the enzyme assay medium only. Results indicate that 50 μM CdCl₂ or Cd-EDTA did not affect FC-R activities even when assay time was as long as 2 h, whereas Pb slightly decreased enzyme activity only at concentrations of 2 mM. Second, short-time Cd and Pb pre-treatments (30–60 min) were imposed on intact Fe-deficient plants before carrying out the assay of FC-R activity. These short-term treatments induced significant decreases in the FC-R activities previously induced by Fe deficiency. With Cd, effects were more pronounced at higher concentrations, and they were stronger when Cd was in the free ion form than when present in the form of Cd-EDTA chelate. Third, prolonged Cd and Pb treatments were imposed on plants grown on 45 μM Fe-EDTA to assess the long-term effects of heavy metals on the induction of the FC-R enzyme. These long-term heavy metal treatments caused a significant increase in the root FC-R activities, indicating that Cd and Pb induce a deficiency in Fe in sugar beet that in turn elicits FC-R activity. The increases, however, are not as large as those found in total absence of Fe.     

Characterization of the Xanthophyll Cycle and Other Photosynthetic Pigment Changes Induced by Iron Deficiency in Sugar Beet (Beta vulgaris L.)

In this work we characterize the changes induced by iron deficiency in the pigment composition of sugar beet (Beta vulgaris L.) leaves. When sugar beet plants were grown hydroponically under limited iron supply, neoxanthin and β-carotene decreased concomitantly with chlorophyll a, whereas lutein and the carotenoids within the xanthophyll cycle were less affected. Iron deficiency caused major increases in the lutein/chlorophyll a and xanthophyll cycle pigments/chlorophyll a molar ratios. Xanthophyll cycle carotenoids in Fe-deficient plants underwent epoxidations and de-epoxidations in response to ambient light conditions. In dark adapted Fe-deficient plants most of the xanthophyll cycle pigment pool was in the epoxidated form violaxanthin. We show, both by HPLC and by in vivo 505 nanometers absorbance changes, that in Fe deficient plants and in response to light, the de-epoxidated forms antheraxanthin and zeaxanthin were rapidly formed at the expense of violaxanthin. Several hours after returning to dark, the xanthophyll cycle was shifted again toward violaxanthin. The ratio of variable to maximum chlorophyll fluorescence from intact leaves was decreased by iron deficiency. However, in iron deficient leaves this ratio was little affected by light conditions which displace the xanthophyll cycle toward epoxidation or de-epoxidation. This suggests that the functioning of the xanthophyll cycle is not necessarily linked to protection against excess light input.     

Metabolic responses in iron deficient tomato plants

The effects of Fe deficiency on different metabolic processes were characterized in roots, xylem sap and leaves of tomato. The total organic acid pool increased significantly with Fe deficiency in xylem sap and leaves of tomato plants, whereas it did not change in roots. However, the composition of the pool changed with Fe deficiency, with major increases in citrate concentrations in roots (20-fold), leaves (2-fold) and xylem sap (17-fold). The activity of phosphoenolpyruvate carboxylase, an enzyme leading to anaplerotic C fixation, increased 10-fold in root tip extracts with Fe deficiency, whereas no change was observed in leaf extracts. The activities of the organic acid synthesis-related enzymes malate dehydrogenase, citrate synthase, isocitrate dehydrogenase, fumarase and aconitase, as well as those of the enzymes lactate dehydrogenase and pyruvate carboxylase, increased with Fe deficiency in root extracts, whereas only citrate synthase increased significantly with Fe deficiency in leaf extracts. These results suggest that the enhanced C fixation capacity in Fe-deficient tomato roots may result in producing citrate that could be used for Fe xylem transport. Total pyridine nucleotide pools did not change significantly with Fe deficiency in roots or leaves, although NAD(P)H/NAD(P) ratios were lower in Fe-deficient roots than in controls. Rates of O(2) consumption were similar in Fe-deficient and Fe-sufficient roots, but the capacity of the alternative oxidase pathway was decreased by Fe deficiency. Also, increases in Fe reductase activity with Fe deficiency were only 2-fold higher when measured in tomato root tips. These values are significantly lower than those found in other plant species, where Fe deficiency leads to larger increases in organic acid synthesis-related enzyme activities and flavin accumulation. These data support the hypothesis that the extent of activation of different metabolic pathways, including carbon fixation via PEPC, organic acid synthesis-related enzymes and oxygen consumption is different among species, and this could modulate the different levels of efficiency in Strategy I plants.     

Responses of sugar beet roots to iron deficiency. Changes in carbon assimilation and oxygen use

Different root parts with or without increased iron-reducing activities have been studied in iron-deficient and iron-sufficient control sugar beet (Beta vulgaris L. Monohil hybrid). The distal root parts of iron-deficient plants, 0 to 5 mm from the root apex, were capable to reduce Fe(III)-chelates and contained concentrations of flavins near 700 microM, two characteristics absent in the 5 to 10 mm sections of iron-deficient plants and the whole root of iron-sufficient plants. Flavin-containing root tips had large pools of carboxylic acids and high activities of enzymes involved in organic acid metabolism. In iron-deficient yellow root tips there was a large increase in carbon fixation associated to an increase in phosphoenolpyruvate carboxylase activity. Part of this carbon was used, through an increase in mitochondrial activity, to increase the capacity to produce reducing power, whereas another part was exported via xylem. Root respiration was increased by iron deficiency. In sugar beet iron-deficient roots flavins would provide a suitable link between the increased capacity to produce reduced nucleotides and the plasma membrane associated ferric chelate reductase enzyme(s). Iron-deficient roots had a large oxygen consumption rate in the presence of cyanide and hydroxisalycilic acid, suggesting that the ferric chelate reductase enzyme is able to reduce oxygen in the absence of Fe(III)-chelates.     

Limiting Factors in Photosynthesis: VI. Regeneration of Ribulose 1,5-Bisphosphate Limits Photosynthesis at Low Photochemical Capacity

Earlier work (SE Taylor, N Terry [1984] Plant Physiol 75: 82-86) has shown that the rate of photosynthesis may be colimited by photosynthetic electron transport capacity, even at low intercellular CO₂ concentrations. Here we monitored leaf metabolites diurnally and the activities of key Calvin cycle enzymes in the leaves of three treatment groups of sugar beet (Beta vulgaris L.) plants representing three different in vivo photochemical capacities, i.e. Fe-sufficient (control) plants, moderately Fe-deficient, and severely Fe-deficient plants. The results show that the decrease in photosynthesis with Fe deficiency mediated reduction in photochemical capacity was through a reduction in ribulose 1,5-bisphosphate (RuBP) regeneration and not through a decrease in ribulose 1,5-bisphosphate carboxylase/oxygenase activity. Based on measurements of ATP and NADPH and triose phosphate/3-phosphoglycerate ratios in leaves, there was little evidence that photosynthesis and RuBP regeneration in Fe-deficient leaves were limited directly by the supply of ATP and NADPH. It appeared more likely that photochemical capacity influenced RuBP regeneration through modulation of enzymes in the photosynthetic carbon reduction cycle between fructose-6-phosphate and RuBP; in particular, the initial activity of ribulose-5-phosphate kinase was strongly diminished by Fe deficiency. Starch and sucrose levels changed independently of one another to some extent during the diurnal period (both increasing in the day and decreasing at night) but the average rates of starch or sucrose accumulation over the light period were each proportional to photochemical capacity and photosynthetic rate.     

Ethylene Production by Fe-deficient Roots and its Involvement in the Regulation of Fe-deficiency Stress Responses by Strategy I Plants

Species that showed marked morphological and physiological responsesby their roots to Fe-deficiency (Strategy I plants) were comparedwith others that do not exhibit these responses (Strategy IIplants). Roots from Fe-deficient cucumber (Cucumis sativusL.‘Ashley’), tomato (Lycopersicon esculentumMill.T3238FER) and pea (Pisum sativumL. ‘Sparkle’) plantsproduced more ethylene than those of Fe-sufficient plants. Thehigher production of ethylene in Fe-deficient cucumber and peaplants occurred before Fe-deficient plants showed chlorosissymptoms and was parallel to the occurrence of Fe-deficiencystress responses. The addition of either the ethylene precursorACC, 1-aminocyclopropane-1-carboxylic acid, or the ethylenereleasing substance, Ethephon, to several Fe-sufficient StrategyI plants [cucumber, tomato, pea, sugar beet (Beta vulgarisL.),Arabidopsis(Arabidopsis thaliana(L.) Heynh ‘Columbia’), plantago(Plantago lanceolataL.)] promoted some of their Fe-deficiencystress responses: enhanced root ferric-reducing capacity andswollen root tips. By contrast, Fe-deficient roots from severalStrategy II plants [maize (Zea maysL. ‘Funo’), wheat(Triticum aestivumL. ‘Yécora’), barley (HordeumvulgareL. ‘Barbarrosa’)] did not produce more ethylenethan the Fe-sufficient ones. Furthermore, ACC had no effecton the reducing capacity of these Strategy II plants and, exceptin barley, did not promote swelling of root tips. In conclusion,results suggest that ethylene is involved in the regulationof Fe-deficiency stress responses by Strategy I plants.Copyright1999 Annals of Botany Company. Arabidopsis (Arabidopsis thaliana(L.) Heynch), barley (Hordeum vulgareL.), cucumber (Cucumis sativusL.), ethylene, iron deficiency, maize (Zea maysL.), pea (Pisum sativumL.), plantago (Plantago lanceolataL.), ferric-reducing capacity, sugar beet (Beta vulgarisL.), tomato (Lycopersicon esculentumMill.), wheat (Triticum aestivumL.).     

Nitrogen availability affects hydraulic conductivity of rice roots,possibly through changes in aquaporin gene expression

Background and aims

Nitrogen (N) availability affects water uptake from the roots, which decreases upon N deprivation and increases upon resupply. The aim of this study was to reveal possible mechanisms of regulation of water transport in roots through physiological and morphological adaptations to N availability.

Methods

The effects of continuous N deprivation and following resupply on root morphology, osmotic hydraulic conductivity, and expression of genes for aquaporins (water channels) were examined in rice (Oryza sativa L.) plants. The effect of local N availability was examined by using a split-root system.

Results

N deprivation decreased the expression of root-specific aquaporin genes, whereas N resupply increased their expression. Changes in aquaporin gene expression were correlated with changes in hydraulic conductivity. N deprivation increased dry matter allocation to the roots. In a split-root experiment, the expression of root-specific aquaporin genes was down-regulated in the N-deprived half, whereas it was up-regulated in the N-supplied half.

Conclusion

Our results suggest that expression of genes for root-specific aquaporins underlies the changes in conductivity during continuous N deprivation and resupply. Rice plants seem to adapt to N availability through coordinated adjustment of root proliferation and abundance of aquaporins.     

Leaf responses to iron nutrition and low cadmium in peanut: anatomical properties in relation to gas exchange

Aims

This study evaluated how iron nutrition affect leaf anatomical and photosynthetic responses to low cadmium and its accumulation in peanut plants.

Methods

Seedlings were treated with Cd (0 and 0.2 μM CdCl₂) and Fe (0, 10, 25, 50 or 100 μM EDTA-Na₂Fe) in hydroponic culture.

Results

Cadmium accumulation is highest in Fe-deficient plants, and dramatically decreased with increasing Fe supply. The biomass, gas exchange, and reflectance indices were highest at 25 μM Fe²⁺ treatments, indicating the concentration is favorable for the growth of peanut plants. Both Fe deficiency and Cd exposure impair photosynthesis and reduce reflectance indices. However, they show different effects on leaf anatomical traits. Fe deficiency induces more and smaller stomata in the leaf surface, but does not affect the inner structure. Low Cd results in a thicker lamina with smaller stomata, thicker palisade and spongy tissues, and lower palisade to spongy thickness ratio. The stomatal length and length/width ratio in the upper epidermis, spongy tissue thickness, and palisade to spongy thickness ratio were closely correlated with net photosynthetic rate, stomatal conductance, and transpiration rate.

Conclusions

Cd accumulation rather than Fe deficiency alters leaf anatomy that may increase water use efficiency but inhibit photosynthesis.     

Iron and copper nutrition-dependent changes in protein expression in a tomato wild type and the nicotianamine-free mutant chloronerva.

The nicotianamine-deficient mutant chloronerva resembles phenotypically an Fe-deficient plant despite the high accumulation of Fe in the leaves, whereas if suffers from Cu deficiency in the shoot. Two-dimensional electrophoretic separation of proteins from root tips and leaves of wild-type Lycopersicon esculentum Mill. cv Bonner Beste and the mutant grown with and without Fe showed a number of consistent differences. In root tips of the Fe-deficient wild type and the Fe-sufficient as well as the Fe-deficient mutant, the expression of glyceraldehyde-3-phosphate dehydrogenase, formate dehydrogenase, and ascorbate peroxidase was increased. In leaves of the Fe-sufficient and -deficient mutant, Cu-containing chloroplastic and cytosolic superoxide dismutase (Cu-Zn) and plastocyanin (Cu) were nearly absent. This low plastocyanin content could be restored by supplying Cu via the xylem, but the superoxide dismutase levels could not be increased by this treatment. The differences in the protein patterns between wild type and mutant indicate that the apparent Fe deficiency of mutant plants led to an increase in enzymes involved in anaerobic metabolism as well as enzymes involved in stress defense. The biosynthesis of plastocyanin was diminished in mutant leaves, but it was differentially induced by increased Cu content.     

Iron deficiency decreases the Fe(III)-chelate reducing activity of leaf protoplasts

The ferric-chelate reductase (FC-R) activity of mesophyll protoplasts isolated from Fe-sufficient (control) and Fe-deficient sugar beet (Beta vulgaris L.) leaves has been characterized. Measurements were made in an ionic environment similar to that in the apoplastic space of the sugar beet mesophyll cells. The FC-R activity of Fe-sufficient and Fe-deficient protoplasts was dependent on light. Fe deficiency decreased markedly the FC-R activity per protoplast surface unit. The optimal pH for the activity of the FC-R in mesophyll protoplasts was in the range 5.5 to 6.0, typical of the apoplastic space. Beyond pH 6.0, the activity of the FC-R in mesophyll protoplasts decreased markedly in both Fe-sufficient and Fe-deficient protoplasts. These data suggest that both the intrinsic decrease in FC-R activity per protoplast surface and a possible shift in the pH of the apoplastic space could lead to the accumulation of physiologically inactive Fe pools in chlorotic leaves.     

Effect of iron plaque outside roots on nutrient uptake by rice (Oryza sativa L.). Zinc uptake by Fe-deficient rice

This solution culture study examined the effect of the deposition of iron plaque on zinc uptake by Fe-deficient rice plants. Different amounts of iron plaque were induced by adding Fe(OH)₃ at 0, 10, 20, 30, and 50 mg Fe/L in the nutrient solution. After 24 h of growth, the amount of iron plaque was correlated positively with the Fe(OH)₃ addition to the nutrient solution. Increasing iron plaque up to 12.1 g/kg root dry weight increased zinc concentration in shoots by 42% compared to that at 0.16 g/kg root dry weight. Increasing the amount of iron plaque further decreased zinc concentration. When the amounts of iron plaque reached 24.9 g/kg root dry weight, zinc concentration in shoots was lower than that in shoots without iron plaque, implying that the plaque became a barrier for zinc uptake. While rice plants were pre-cultured in –Fe and +Fe nutrient solution in order to produce the Fe-deficient and Fe-sufficient plants and then Fe(OH)₃ was added at 20, 30, and 50 mg Fe/L in nutrient solution, zinc concentrations in shoots of Fe-deficient plants were 54, 48, and 43 mg/kg, respectively, in contrast to 32, 35, and 40 mg/kg zinc in shoots of Fe-sufficient rice plants. Furthermore, Fe(OH)₃ addition at 20 mg Fe/L and increasing zinc concentration from 0.065 to 0.65 mg Zn/L in nutrient solution increased zinc uptake more in Fe-deficient plants than in Fe-sufficient plant. The results suggested that root exudates of Fe-deficient plants, especially phytosiderophores, could enhance zinc uptake by rice plants with iron plaque up to a particular amount of Fe.     

Induction of the Root Cell Plasma Membrane Ferric Reductase (An Exclusive Role for Fe and Cu)

Induction of ferric reductase activity in dicots and nongrass monocots is a well-recognized response to Fe deficiency. Recent evidence has shown that Cu deficiency also induces plasma membrane Fe reduction. In this study we investigated whether other nutrient deficiencies could also induce ferric reductase activity in roots of pea (Pisum sativum L. cv Sparkle) seedlings. Of the nutrient deficiencies tested (K, Mg, Ca, Mn, Zn, Fe, and Cu), only Cu and Fe deficiencies elicited a response. Cu deficiency induced an activity intermediate between Fe-deficient and control plant activities. To ascertain whether the same reductase is induced by Fe and Cu deficiency, concentration- and pH-dependent kinetics of root ferric reduction were compared in plants grown under control, -Fe, and -Cu conditions. Additionally, rhizosphere acidification, another process induced by Fe deficiency, was quantified in pea seedlings grown under the three regimes. Control, Fe-deficient, and Cu-deficient plants exhibited no major differences in pH optima or Km for the kinetics of ferric reduction. However, the Vmax for ferric reduction was dramatically influenced by plant nutrient status, increasing 16- to 38-fold under Fe deficiency and 1.5- to 4-fold under Cu deficiency, compared with that of control plants. These results are consistent with a model in which varying amounts of the same enzyme are deployed on the plasma membrane in response to plant Fe or Cu status. Rhizosphere acidification rates in the Cu-deficient plants were similarly intermediate between those of the control and Fe-deficient plants. These results suggest that Cu deficiency induces the same responses induced by Fe deficiency in peas.     

Ethylene is differentially regulated during sugar beet germination and affects early root growth in a dose-dependent manner

Main conclusion

By integrating molecular, biochemical, and physiological data, ethylene biosynthesis in sugar beet was shown to be differentially regulated, affecting root elongation in a concentration-dependent manner. There is a close relation between ethylene production and seedling growth of sugar beet (Beta vulgaris L.), yet the exact function of ethylene during this early developmental stage is still unclear. While ethylene is mostly considered to be a root growth inhibitor, we found that external 1-aminocyclopropane-1-carboxylic acid (ACC) regulates root growth in sugar beet in a concentration-dependent manner: low concentrations stimulate root growth while high concentrations inhibit root growth. These results reveal that ethylene action during root elongation is strongly concentration dependent. Furthermore our detailed study of ethylene biosynthesis kinetics revealed a very strict gene regulation pattern of ACC synthase (ACS) and ACC oxidase (ACO), in which ACS is the rate liming step during sugar beet seedling development.     

Response of barley plants to Fe deficiency and Cd contamination as affected by S starvation

Both Fe deficiency and Cd exposure induce rapid changes in the S nutritional requirement of plants. The aim of this work was to characterize the strategies adopted by plants to cope with both Fe deficiency (release of phytosiderophores) and Cd contamination [production of glutathione (GSH) and phytochelatins] when grown under conditions of limited S supply. Experiments were performed in hydroponics, using barley plants grown under S sufficiency (1.2 mM sulphate) and S deficiency (0 mM sulphate), with or without Fe(III)-EDTA at 0.08 mM for 11 d and subsequently exposed to 0.05 mM Cd for 24 h or 72 h. In S-sufficient plants, Fe deficiency enhanced both root and shoot Cd concentrations and increased GSH and phytochelatin levels. In S-deficient plants, Fe starvation caused a slight increase in Cd concentration, but this change was accompanied neither by an increase in GSH nor by an accumulation of phytochelatins. Release of phytosiderophores, only detectable in Fe-deficient plants, was strongly decreased by S deficiency and further reduced after Cd treatment. In roots Cd exposure increased the expression of the high affinity sulphate transporter gene (HvST1) regardless of the S supply, and the expression of the Fe deficiency-responsive genes, HvYS1 and HvIDS2, irrespective of Fe supply. In conclusion, adequate S availability is necessary to cope with Fe deficiency and Cd toxicity in barley plants. Moreover, it appears that in Fe-deficient plants grown in the presence of Cd with limited S supply, sulphur may be preferentially employed in the pathway for biosynthesis of phytosiderophores, rather than for phytochelatin production.     

The Substrate Utilization and Concentration of 14C Photosynthates in Citronella under Fe Deficiency

Changes in the utilization pattern of primary substrate, viz. [U-14C] acetate, 14CO2 and [U-14C] saccharose, and the contents of 14C fixation products in photosynthetic metabolites (sugars, amino acids, and organic acids) were determined in Fe-deficient citronella in relation to the essential oil accumulation. There was an overall decrease in photosynthetic efficiency of the Fe-deficient plants as evidenced by lower levels of incorporation into the sugar fraction and essential oil after 14CO2 had been supplied. When acetate and saccharose were fed to the Fe-deficient plants, despite a higher incorporation of label into sugars, amino acids, and organic acids, there was a lower incorporation of these metabolites into essential oils than in control plants. Thus, the availability of precursors and the translocation to a site of synthesis/accumulation, severely affected by Fe deficiency, is equally important for the essential oil biosynthesis in citronella.     

Iron deficiency enhances the levels of ascorbate,glutathione, and related enzymes in sugar beet roots

Summary.  The effects of Fe deficiency stress on the levels of ascorbate and glutathione, and on the activities of the enzymes ferric chelate reductase, glutathione reductase (EC 1.6.4.2), ascorbate free-radical reductase (EC 1.6.5.4) and ascorbate peroxidase (EC 1.11.1.11), have been investigated in sugar beet (Beta vulgaris L.) roots. Plasma membrane vesicles and cytosolic fractions were isolated from the roots of the plants grown in nutrient solutions in the absence or presence of Fe for two weeks. Plants responded to Fe deficiency not only with a 20-fold increase in root ferric chelate reductase activity, but also with moderately increased levels of the general reductants ascorbate (2-fold) and glutathione (1.6-fold). The enzymes of the ascorbate-glutathione cycle in roots were also affected by Fe deficiency. Glutathione reductase activity was enhanced 1.4-fold with Fe deficiency, associated to an increased ratio of reduced to oxidized glutathione, from 3.1 to 5.2. The plasma membrane fraction from iron-deficient roots showed 1.7-fold higher ascorbate free-radical reductase activity, whereas in the cytosolic fraction the enzyme activity was not affected by Fe deficiency. The activity of the cytosolic hemoprotein ascorbate peroxidase decreased approximately by 50% with Fe deprivation. These results show that sugar beet responds to Fe deficiency with metabolic changes affecting components of the ascorbate-glutathione cycle in root cells. This suggests that the ascorbate-glutathione cycle would play certain roles in the general Fe deficiency stress responses in strategy I plants. Received November 19, 2001; accepted September 30, 2002; published online April 2, 2003 RID="*" ID="*" Correspondence and reprints: Departamento de Nutrición Vegetal, Estación Experimental de Aula Dei, CSIC, Apartado 202, 50080 Zaragoza, Spain.     

Carboxylate metabolism changes induced by Fe deficiency in barley, a Strategy II plant species

The effects of iron (Fe) deficiency on carboxylate metabolism were investigated in barley (Hordeum vulgare L.) using two cultivars, Steptoe and Morex, which differ in their Fe efficiency response. In both cultivars, root extracts of plants grown in Fe-deficient conditions showed higher activities of enzymes related to organic acid metabolism, including citrate synthase, malate dehydrogenase and phosphoenolpyruvate carboxylase, compared to activities measured in root extracts of Fe-sufficient plants. Accordingly, the concentration of total carboxylates was higher in Fe-deficient roots of both cultivars, with citrate concentration showing the greatest increase. In xylem sap, the concentration of total carboxylates was also higher with Fe deficiency in both cultivars, with citrate and malate being the major organic acids. Leaf extracts of Fe-deficient plants also showed increases in citric acid concentration and in the activities of glucose-6-phosphate dehydrogenase and fumarase activities, and decreases in aconitase activity. Our results indicate that changes in root carboxylate metabolism previously reported in Strategy I species also occur in barley, a Strategy II plant species, supporting the existence of anaplerotic carbon fixation via increases in the root activities of these enzymes, with citrate playing a major role. However, these changes occur less intensively than in Strategy I plants. Activities of the anaerobic metabolism enzymes pyruvate decarboxylase and lactate dehydrogenase did not change in barley roots with Fe deficiency, in contrast to what occurs in Strategy I plants, suggesting that these changes may be Strategy I-specific. No significant differences were observed in overall carboxylate metabolism between cultivars, for plants challenged with high or low Fe treatments, suggesting that carboxylate metabolism changes are not behind the Fe-efficiency differences between these cultivars. Citrate synthase was the only measured enzyme with constitutively higher activity in Steptoe relative to Morex leaf extracts.