Excess cation uptake, and extrusion of protons and organic acid anions by Lupinus albus under phosphorus deficiency

In symbiotically-grown legumes, rhizosphere acidification may be caused by a high cation/anion uptake ratio and the excretion of organic acids, the relative importance of the two processes depending on the phosphorus nutritional status of the plants. The present study examined the effect of P deficiency on extrusions of H⁺ and organic acid anions (OA⁻) in relation to uptake of excess cations in N₂-fixing white lupin (cv. Kiev Mutant). Plants were grown for 49 days in nutrient solutions treated with 1, 5 or 25 mmol P m⁻³ Na₂HPO₄ in a phytotron room. The increased formation of cluster roots occurred prior to a decrease in plant growth in response to P deficiency. The number of cluster roots was negatively correlated with tissue P concentrations below 2.0 g kg⁻¹ in shoots and 3 g kg⁻¹ in roots. Cluster roots generally had higher concentrations of Mg, Ca, N, Cu, Fe, and Mn but lower concentrations of K than non-cluster roots. Extrusion of protons and OA⁻ (90% citrate and 10% malate) from roots was highly dependent on P supply. The amounts of H⁺ extruded per unit root biomass decreased with time during the experiment. On the equimolar basis, H⁺ extrusion by P-deficient plants (grown at 1 and 5 mmol P m⁻³) were, on average, 2–3-fold greater than OA⁻ exudation. The excess cation content in plants was generally the highest at 1 mmol P m⁻³ and decreased with increasing P supply. The ratio of H⁺ release to excess cation uptake increased with decreasing P supply. The results suggest that increased exudation of OA⁻ due to P deficiency is associated with H⁺ extrusion but contributes only a part of total acidification.     

Transfer cell formation in sugar beet roots induced by latent Fe deficiency

Leaves of Fe deficient sugar beets precultured in complete nutrient solution with Fe(III)EDTA remained green during the first 6 days of –Fe treatment when grown in a small nutrient solution volume (0.5 L/plant). After 3 days of –Fe treatment, roots placed in agar showed enhanced H⁺ release and ferric reduction at the tips of young laterals where short root hairs and transfer cells had developed. However, the H⁺ release was too weak to cause a pH decrease of the bulk nutrient solution. Nevertheless, the Fe stress response reactions probably lead to mobilization of Fe from the apoplasmic pool so that chlorosis development was prevented. Slight chlorosis symptoms appeared only after 4 more days of Fe deficiency and the pH of the bulk nutrient solution decreased to pH 4.5 simultaneously with renewed transfer cell formation and subsequent rapid regreening. In the 10 times higher volume of 5 L-Fe solution/plant, laterals with root hairs and transfer cells also showed localized acidification of the agar system. However, the protons released were so diluted that no pH decrease of the bulk solution was measurable. Instead, the leaves showed continuously increasing chlorosis with degenerated chloroplast ultrastructure. It is concluded that root hairs and transfer cells are not only formed under severe chlorosis but, instead, they seem to be an integral part of the adaptive response to latent Fe deficiency.     

稻、麦根系H~ 的分泌与介质磷水平的关系

水稻、小麦根系H~ 的分泌量随供磷水平的降低而增加,并存在明显的昼夜变化。在自然光照下H~ 分泌量随光强度增加而增多,同时强光比黑暗时H~ 分泌对磷供应水平更为敏感。磷供应不足还诱导水稻根系柠檬酸分泌量增加,而苹果酸则差异不明显。难溶性磷的溶解率与根系H~ 和柠檬酸分泌所导致的根际pH下降有密切联系。因此,在有效磷不足的条件下可明显提高稻、麦根际土壤中难溶性磷的利用率,其中丰产型小麦和粳稻品种对土壤中磷利用的根际效应更为显著。     

Rhizosphere acidification as a response to iron deficiency in bean plants

Iron deficiency in higher plants causes accumulation of salts of organic acids in the roots, the most characteristic being citrate. We show that citrate and malate accumulate in beans (Phaseolus vulgaris L. var Prélude), not because of a lack of the iron-containing enzyme aconitase (EC 4.2.1.3), but in close coupling to the extrusion of protons during rhizosphere acidification, one of the `Fe-efficiency' reactions of dicotyledonous plants. When proton excretion is induced in roots of control bean plants by addition of fusicoccin, only malate, not citrate, is accumulated. We propose that iron deficiency induces production of organic acids in the roots, which in beans leads to both proton excretion and an increased capacity to reduce ferric chelates via the induced electron transfer system in the root epidermis cells.     

Soil acidification and adaptations of plants and microorganisms in Bornean tropical forests

In tropical forest ecosystems, a paradoxical relationship is commonly observed between massive biomass production and low soil fertility (low pH). The loss and deficiency of soil phosphorus (P) and bases generally constrain biomass production; however, high productivity on nutrient-deficient soils of Bornean tropical forests is hypothesized to be maintained by plant and microorganism adaptation to an acidic soil environment. Proton budgets in the plant–soil system indicated that plants and microorganisms promote acidification to acquire bases, even in highly acidic tropical soils. The nitric and organic acids they produce contribute to the mobilization of basic cations and their uptake by plants. In response to soil P deficiency and the recalcitrance of lignin-rich organic matter, specific trees and fungi can release organic acids and enzymes for nutrient acquisition. Organic acids exuded by roots and rhizosphere microorganisms can promote the solubilization of P bonded to aluminum and iron oxides and its uptake by plants from P-poor soils. Lignin degradation, a rate-limiting step in organic matter decomposition, is specifically enhanced in acidic organic layers by lignin peroxidase, produced by white-rot fungi, which may solubilize recalcitrant lignin and release soluble aromatic substances into the soil solution. This dissolved organic matter functions in the transport of nitrogen, P, and basic cations in acidic soils without increasing leaching loss. In Bornean tropical forests, soil acidification is promoted by plants and microorganisms as a nutrient acquisition strategy, while plant roots and fungi can develop rhizosphere and enzymatic processes that promote tolerance of low pH.     

Differences in responses to iron deficiency among various cultivars of mungbean (Vigna radiata (L.) Wilczek)

Ten mungbean cultivars were evaluated for their resistance to iron deficiency in view of chlorosis symptoms, plant growth and seed yield under field conditions on a calcareous soil in Thailand. The KPS2 cultivar was highly susceptible; the KPS1, PSU1 and Pag-asa 1 cultivars were somewhat susceptible; the VC1163B cultivar was moderately tolerant; the CN36, CN60, UT1 and CNM-I cultivars were tolerant; and the CNM8509B cultivar was very tolerant to iron deficiency. Foliar application of a solution of 5 g L^-1 ferrous sulphate was effective in correcting chlorosis that was induced by iron deficiency, and it enhanced both the growth and the yield of susceptible cultivars. Compared with the susceptible cultivar KPS2, the tolerant cultivar UT1 had a greater ability to lower the pH of the nutrient solution in response to iron deficiency. The root-associated Fe³⁺-reduction activity of UT1 that had been grown in -Fe medium was similar to that of the plants grown in +Fe medium when the acidification of the medium occurred. Acidification of the medium in response to iron deficiency might contribute to the efficient solubilization of iron from calcareous soils, and it related more closely to the resistance to iron deficiency than Fe³⁺ reduction by roots in mungbean cultivars.     

The role of nodules in the tolerance of common bean to iron deficiency

Iron is vital for the establishment and function of symbiotic root nodules of legumes. Although abundant in the environment, Fe is often a limiting nutrient for plant growth due to its low solubility and availability in some soils. We have studied the mechanism of iron uptake in the root nodules of common bean to evaluate the role of nodules in physiological responses to iron deficiency. Based on experiments using full or partial submergence of nodulated roots in the nutrient solution, our results show that the nodules were affected only slightly under iron deficiency, especially when the nodules were submerged in nutrient solution in the tolerant cultivar. In addition, fully submerged root nodules showed enhanced acidification of the nutrient solution and showed higher ferric chelate reductase activity than that of partially submerged roots in plants cultivated under Fe deficiency. The main results obtained in this work suggest that in addition to preferential Fe allocation from the root system to the nodules, this symbiotic organ probably develops some mechanisms to respond to iron deficiency. These mechanisms were implied especially in nodule Fe absorption efficiency and in the ability of this organ to take up Fe directly from the medium.     

Localization and capacity of proton pumps in roots of intact sunflower plants

Proton extrusion by roots of intact sunflower plants (Helianthus annuus L.) was studied in nutrient solutions or in agar media with a pH indicator. Proton extrusion was enhanced by either iron deficiency, addition of fusicoccin, or single salt solutions of ammonium or potassium salts. The three types of proton extrusion differ in both localization along the roots and capacity. From their sensitivity to ATPase inhibitors it seems justified to characterize them as proton pumps driven by plasma membrane APTases.

Enhanced proton extrusion induced by preferential cation uptake from (NH₄)₂SO₄ or K₂SO₄ was uniformly distributed over the whole root system. In contrast, the enhancement effect of fusicoccin was confined to the basal root zones and that of iron deficiency to the apical root zones. Also the rates of proton extrusion per unit of root fresh weight differed remarkably and increased in the order: Fusicoccin K₂SO₄ < (NH₄)₂SO₄ < iron deficiency.

Under iron deficiency the average values of proton extrusion for the whole root system are 5.6 micromoles H⁺ per gram fresh weight per hour; however, for the apical root zones values of about 28 micromoles H⁺ can be calculated. This high capacity is most probably related to the iron deficiency-induced formation of rhizodermal transfer cells in the apical root zones. It can be assumed that the various types of root-induced acidification of the rhizosphere are of considerable ecological importance for the plant-soil relationships in general and for mobilization of mineral nutrients from sparingly soluble sources in particular.

     

Root excretion of carboxylic acids and protons in phosphorus-deficient plants

Phosphorus deficiency-induced metabolic changes related to exudation of carboxylic acids and protons were compared in roots of wheat (Triticum aestivum L. cv Haro), tomato (Lycopersicon esculentum L., cv. Moneymaker), chickpea (Cicer arietinum) and white lupin (Lupinus albus L. cv. Amiga), grown in a hydroponic culture system. P deficiency strongly increased the net release of protons from roots of tomato, chickpea and white lupin, but only small effects were observed in wheat. Release of protons coincided with increased exudation of carboxylic acids in roots of chickpea and white lupin, but not in those of tomato and wheat. P deficiency-induced exudation of carboxylic acids in chickpea and white lupin was associated with a larger increase of carboxylic acid concentrations in the roots and lower accumulation of carboxylates in the shoot tissue compared to that in wheat and tomato. - Citric acid was one of the major organic acids accumulated in the roots of all investigated species in response to P deficiency, and this was associated with increased activity and enzyme protein levels of PEP carboxylase, which is required for biosynthesis of citrate. Accumulation of citric acid was most pronounced in the roots of P-deficient white lupin, chickpea and tomato. Increased PEP carboxylase activity in the roots of these plants coincided with decreased activity of aconitase, which is involved in the breakdown of citric acid in the TCA cycle. In the roots of P-deficient wheat plants, however, the activities of both PEP carboxylase and aconitase were enhanced, which was associated with little accumulation of citric acid. The results suggest that P deficiency-induced exudation of carboxylic acids depends on the ability to accumulate carboxylic acids in the root tissue, which in turn is determined by biosynthesis, degradation and partitioning of carboxylic acids or related precursors between roots and shoot. In some plant species such as white lupin, there are indications for a specific transport mechanism (anion channel), involved in root exudation of extraordinary high amounts of citric acid. This revised version was published online in June 2006 with corrections to the Cover Date.     

Genotypical differences in responses to iron deficiency between sensitive and resistant cultivars of chickpea (Cicer arietinum)

Differences in responses to iron deficiency between two chickpea cultivars, NP-62 and K-850, were examined. The apical leaves of NP-62 quickly showed symptoms of iron-deficiency chlorosis when grown on an iron-free medium. By contrast, K-850 showed no visible symptoms on the same medium. Iron contents of the apical leaves of these two cultivars were similar during the first 7 days after they were transferred to the iron-free medium in spite of a marked difference in root-associated Fe³⁺-reduction activity. The susceptibility to iron-deficiency chlorosis observed in NP-62 was not attributable to the poor Fe³⁺-reduction activity of roots but to the inefficient utilization of iron within leaves under conditions when the supply of iron was limited.     

Iron deficiency induces cluster (proteoid) root formation in Casuarina glauca

The effect of iron deficiency, phosphorus, NaHCO₃, chelator supply and nitrogen source on the formation of cluster (proteoid) roots was investigated in Casuarina glauca growing in water culture. The addition of iron-binding chelators (e.g. EDDHA, DTPA, EDTA) or increase in nutrient solution pH with NaHCO₃ resulted in the formation of cluster roots when plants were grown in solution lacking iron. Phosphorus supply even at a concentration of 500 µM did not inhibit cluster root formation if EDDHA was added to the iron-deficient medium. Cluster root formation was influenced significantly by nitrogen source and occurred only in nitrate-fed plants.C. glauca seemed to be very sensitive to iron deficiency as shown by plant chlorosis when grown on alkaline soil. The symptoms of chlorosis decreased as the chlorophyll content in shoots and the number of cluster roots increased, suggesting that the alleviation of iron deficiency in plant tissues was correlated with cluster root formation. It appears that iron deficiency is more important than phosphorus deficiency in inducing the formation of cluster roots in C. glauca.     

MECHANISMS OF ALUMINUM TOLERANCE IN TRITICUM AESTIVUM L. (WHEAT). I. DIFFERENTIAL PH INDUCED BY WINTER CULTIVARS IN NUTRIENT SOLUTIONS

Twenty winter cultivars of Triticum aestivum L. (wheat) were grown in solution culture with and without aluminum (Al) (74 μM, 2.0 mg L^-1) for 14 days. Exposure to Al increased root growth of the most tolerant cultivar, while both root and shoot growth were depressed in all other cultivars. On the basis of a root tolerance index (RTI = weight of roots grown with Al/weight of roots grown without Al), cultivar tolerance to Al ranged 9-fold, from 0.13 ± 0.01 to 1.16 ± 0.10. Symptoms of Al toxicity were most evident on roots. Aluminum-affected roots were relatively short and thick and had numerous undeveloped laterals. Leaves of some cultivars showed chlorosis resembling iron deficiency, and others showed purple stems typical of phosphate deficiency. Plants of all cultivars grown with and without Al depressed the pH of nutrient solutions, presumably until NH₄⁺ was depleted, at which point the pH increased. Cultivar tolerance, expressed both as the root tolerance index and a shoot tolerance index, was negatively correlated with the negative log of the mean hydrogen ion (H⁺) concentration, the minimum pH, and the slope of the pH decline, each calculated from pH data collected during the first 9 days of the experimental period before any sharp rises in pH occurred. These results are consistent with the hypothesis that the Al tolerance of a given cultivar is a function of its ability to resist acidification of the nutrient solution and hence to limit the solubility and toxicity of Al.     

Effect of salt on physiological responses of barley to iron deficiency.

Iron chlorosis is very common on alkaline soils such as calcareous ones, since iron availability is limited by high pH. Under these conditions of iron deficiency, graminaceous plant species induce special mechanisms for iron acquisition, involving enhanced release of iron chelators called phytosiderophores. On the other hand, it is known that most of salt soils have alkaline pH. So, plants growing on this kind of soils are often subjected simultaneously to salinity and iron deficiency. This work aimed at (i) studying the physiological responses of barley (Hordeum vulgare L.) to iron deficiency, and (ii) evaluating the effect of salt on the iron nutrition and the phytosiderophore release. For this purpose, seedlings of Hordeum vulgare L. were cultivated under controlled conditions, either in a complete nutrient solution with or without NaCl, or in an iron free nutrient solution containing or not NaCl. The plant morphological aspect, chlorophyll content of young leaves, iron status, biomass production, and phytosiderophore release by roots were assessed. Plants subjected to Fe deficiency exhibited a severe chlorosis, accompanied by a significant biomass reduction. These plants developed more lateral roots than the control with a highly stimulated phytosiderophore release. However, the latter was greatly diminished when iron deficiency was associated to salinity. A depressive effect of salt on iron acquisition in plants subjected only to salt stress which was also observed and further confirmed by the important decrease of efficiency in iron acquisition. These results suggest that salinity may reduce capacity of plants to acquire iron from alkaline soils by inhibiting phytosiderophore release.     

Evaluation of strategy I mechanisms in iron efficient and inefficient maize cultivars

The iron (Fe) efficient maize cultivar WF9 and the Fe inefficient maize mutant ys1 were grown in nutrient solutions with varied Fe supply. Changes in pH, reducing capacity of the roots and the release of Fe(III) reducing compounds were monitored over a period of 11 days. In both cultivars under Fe deficiency, there was no increased release of protons or reducing compounds and no increased reducing capacity of the roots. Indeed, the Fe(III) reduction of the roots of both cultivars tended to be higher in Fe sufficient plants. In contrast to some recent reports, these results demonstrate that the Fe efficient maize cultivar WF9 does not respond to Fe deficiency by strategy I mechanisms. This suggests that differences in Fe efficiency between the two cultivars are probably due to use of strategy II mechanisms for Fe acquisition.     

Can plants exposed to SO2 excrete sulfuric acid through the roots?

Hydroponically grown pea plants (Pisum sativum L., cv. Kleine Rheinländerin) and barley seedlings (Hordeum vulgare L., cv. Gerbel) were fumigated for several days with 1 or 2 μl l^?1 SO₂. Both species accumulated sulfate during fumigation, although the nutrient medium lacked sulfate. In pea, SO₂-dependent sulfate accumulation in different plant parts accounted for 60 percent of the SO₂ sulfur which, as calculated from a determination of boundary and stomatal flux resistances had entered the leaves. Up to 55% of the air-borne sulfate was translocated from pea leaves to roots during the period of fumigation, but no or only little sulfate was excreted into the nutrient solution. In contrast, barley retained sulfate in the leaves, and sulfate translocation from shoot to the root system could not be observed. In both species, protons were excreted by the roots. In fumigated plants, proton loss was higher than in untreated controls in pea, but not in barley. In pea, SO₂-dependent proton loss into the medium accounted for up to 50% of the sulfuric acid formed from SO₂. Proton excretion was strongly dependent on potassium availability in the nutrient medium. Cation uptake by the plants during fumigation was sufficient to compensate for proton loss, suggesting proton/cation exchange at the interface between root and medium. We conclude that by oxidation to sulfuric acid, plants are capable of detoxifying SO₂ taken up by the leaves. Depending on plant species, either both protons and sulfate anions can be exported from the leaves, or the proton load on leaf cells can be relieved by proton/cation exchange at the plasmalemma. Finally, the problem of airborne plant acidification may be solved by proton/cation exchange at the level of roots. The burden of acidification is then shifted from the plant to the nutrient medium. Appreciable amounts of sulfate can be excreted neither by pea nor by barley plants.     

Characteristics of Fe‐deficiency‐induced acidification in subterranean clover

Iron-deficiency-induced acidification is one of the important reactions of plant Fe-deficiency-stress response, but the overall understanding of this reaction is limited. The characteristics of Fe-deficiency-induced acidification of subterranean clover (subclover) (Trifolium brachycalycinum Katzn. and Morley cv. Koala) were studied in this paper. Plants were grown hydroponically under -Fe conditions, and Fe-deficiency-induced acidification was determined using pH-stat, back-titration and chemical equilibrium procedures. Fe-deficiency-induced acidification was undetectable during the first day after Fe-deficiency stress initiation, but the maximum acidification rate was attained by the second day, when plants exhibited visual chlorosis symptoms. The acidification rate was relatively constant with increasing Fe-deficiency chlorosis, suggesting that a critical level of Fe deficiency was needed to trigger acidification, but that once the acidification process was initiated, the intensity of acidification was independent of severity of Fe deficiency. Net H⁺-release (PR) rate determined using a chemical equilibrium method and net acidity release (AR) rate determined using a back-titration method were practically identical, indicating that Fe-deficiency-induced acidification involved almost entirely the release of free H⁺, not organic acid. In the assay temperature range of 5 to 35°C, PR rate was highest at about 20°C. Net acidity release rate was almost totally inhibited at pH values ≤4.5 and increased with increasing assay pH up to pH 9. The pH effect occurred within 30 min of incubation initiation, implying that the effect of pH is probably on the activity of H⁺ transport through the plasma membrane, not on the quantity of responsible protein(s). Cations were required in the incubation solution for Fe-deficiency-induced acidification. Divalent cations in the assay solution resulted in a higher AR rate than monovalent cations, and essential cations resulted in a higher AR rate than non-essential cations, indicating that the relative effectiveness of cations is related to the efficiency of their absorption by plant roots. These results are discussed in relation to their practical significance and the mechanisms of Fe-deficiency-induced acidification.     

Shovel roots: a unique stress-avoiding developmental strategy of the legume plant Hedysarum coronarium L.

Hedysarum coronarium (sulla) is a legume native to the Mediterranean basin, known for its broad tolerance to various environmental stresses, and its ability to thrive without signs of chlorosis when growing in arid and alkaline soils up to pH 9.6. A unique but poorly known morphological feature of its root system is the production of “shovels”, modified lateral roots that acquire a curved and flattened shape. A combined structural and functional analysis was undertaken to define the nature and role of the shovel roots using various microscopy techniques, histochemical stains, STEM - energy dispersive X-ray microanalysis, infrared spectroscopy, and plant cultivation in different conditions. We found that sulla displays remarkable unique rhizosphere-buffering properties at both ends of the pH scale, and that shovels act as efficient calcium-absorbing organs that accumulate this cation intracellularly as insoluble crystalline salts. Such bioaccumulation results in a localized depletion of CaCO₃ from the soil. As a consequence of this removal of the pivotal carbonate buffering system, the iron-solubilizing acidification activities of the roots can become effective. Further tests revealed that the factor triggering shovel development is exposure of roots to iron oxide. This signal, reporting at once both iron presence and alkalinity, assures the availability of iron nutrient reserves upon acidification of the local microenvironment surrounding the roots. These findings, besides casting light on a novel and unique botanical phenomenon, offer the potential to exploit sulla’s model and genes for the improvement of other crops to sustain productivity in a scenario of climate warming and increasing desertification.     

Water-extractable humic substances enhance iron deficiency responses by Fe-deficient cucumber plants

The ability of Fe-deficient cucumber plants to use iron complexed to a water-extractable humic substances fraction (WEHS), was investigated. Seven-day-old Fe-deficient plants were transferred to a nutrient solution supplemented daily for 5 days with 0.2 μM Fe as Fe-WEHS (5 μg org. C mL^-1), Fe-EDTA, Fe-citrate or FeCl₃. These treatments all allowed re-greening of the leaf tissue, and partial recovery of dry matter accumulation, chlorophyll and iron contents. However, the recovery was faster in plants supplied with Fe-WEHS and was already evident 48 h after Fe supply. The addition of 0.2 μM Fe to the nutrient solution caused also a partial recovery of the dry matter and iron accumulation in roots of Fe-deficient cucumber plants, particularly in those supplied with Fe-WEHS. The addition of WEHS alone (5 μg org. C mL^-1, 0.04 μM Fe) to the nutrient solution slightly but significantly increased iron and chlorophyll contents in leaves of Fe-deficient plants; in these plants, dry matter accumulation in leaves and roots was comparable or even higher than that measured in plants treated with Fe-citrate or FeCl₃. After addition of the different iron sources for 5 days to Fe-deficient roots, morphological modifications (proliferation of lateral roots, increase in the diameter of the sub-apical zones and amplified root-hair formation) and physiological responses (enhanced Fe(III)-chelate reductase and acidification of the nutrient solution) induced by Fe deficiency, were still evident, particularly in plants treated with the humic molecules. The presence of WEHS caused also a further acidification of the nutrient medium by Fe-deficient plants. The Fe-WEHS complex (1 μM Fe) could be reduced by intact cucumber roots, at rates of reduction higher than those measured for Fe-EDTA at equimolar iron concentration. Plasma membrane vesicles, purified by two-phase partition from root microsomes of Fe-deficient plants, were also able to reduce Fe-WEHS. Results show that Fe-deficient cucumber plants can use iron complexed to water soluble humic substances, at least in part via reduction of complexed Fe(III) by the plasma membrane Fe(III)-chelate reductase of root cells. In addition, the stimulating effect of humic substances on H⁺ release might be of relevance for the overall response of the plants to iron shortage. This revised version was published online in June 2006 with corrections to the Cover Date.     

Contribution of organic acids to the acidification of the rhizosphere of maize seedlings

The participation of organic acids in the process of soil acidification was related to other H⁺ pumping processes. The ratio between efflux of organic acids and proton secretion of maize roots was determined with the use of a pH-stat combined with a collecting system for organic acids. Changes in the composition of carboxylic acids influenced by nitrogen supply were monitored by HPLC and via enzymatic conversion. The following substances were found to be secreted by maize roots: glycolate, glyoxylate, fumarate, 2-oxoglutarate and oxalate. Malate, however, could not be detected. There is no organic acid dominantly secreted by the roots, but changes are observed during aging which might result from deficiencies of nutrients e.g. P. Fertilization of N-deficient plants with urea leads to a significant change in the composition of acids secreted. In this case, oxalate was additionally detected with a concomitant increase in glyoxylate, indicating important changes in metabolism. Acidification of the rhizosphere is predominantly maintained by secretion of protons, not by efflux of organic acids, which contributed 0.2 to 0.3% to this process only. The role of organic acids in nutrient uptake is discussed.     

Iron-nutritional aspects of the ionic balance of plants

Summary The effect of iron on the ionic balance of several plant species and cultivars was studied. Those plants which normally excrete relatively low amounts of hydroxyl ions respond to iron stress by lowering the pH of the nutrient medium and decreasing anion uptake. These plants may be considered as being Fe efficient. Plants which normally excrete relatively high amounts of hydroxyl ions and which continue to increase the pH of a nutrient medium when under iron stress may be considered as Fe inefficient.Iron deficiency tends to increase carboxylate accumulation and to decrease anion uptake. When cation uptake is depressed by iron deficiency this is mainly a non specific depression of potassium uptake, on the other hand when iron stress stimulates cation uptake this is mainly due to a specific stimulation of the divalent cations Ca and Mg.Additional key words: Iron, efficiency of uptake, ionic balance, hydroxyl and hydrogen ion excretion.