Excess Cation Uptake Research Articles

White lupin (Lupinus albus L.) exposed to elevated atmospheric CO2 requires additional phosphorus for N2 fixation

PurposeThe steady rise of atmospheric CO2 concentrations enhances symbiotic N2 fixation and plant growth. However, it is largely unknown whether more P is required to enhance N2 fixation under elevated CO2. We examined if plants under elevated CO2 require more P to support enhanced N2 fixation.MethodsWhite lupin (Lupinus albus L. cv. Kiev) was grown in nutrient solution under ambient (380 ppm) and elevated (700 ppm) CO2 levels at 0, 2, 5, 12, 25 and 50 µM P. Nodulation, nodule function, N2 fixation and plant growth were assessed at 4 and 6 weeks post inoculation. Excess cation uptake and proton release were also analysed due to its importance in P acquisition.ResultsElevated CO2 increased the critical P concentration in nutrient solution for N2 fixation, indicating plants under elevated CO2 need to acquire additional P to support greater N2 fixation. Elevated CO2 improved N2 fixation in P-deficient plants by increasing specific nitrogenase activity. Elevated CO2 also enhanced excess cation uptake which increased proton release by roots.ConclusionsAs elevated CO2 increased the plant demand for P, it increased the critical P concentration in nutrient solution for N2 fixation, thereby plants require more P to mitigate the effect of P deficiency on N2 fixation. Despite this, elevated CO2 could enhance N2 fixation in P-limiting environments.

Aluminum-Enhanced Proton Release Associated with Plasma Membrane H+-Adenosine Triphosphatase Activity and Excess Cation Uptake in Tea (Camellia sinensis) Plant Roots

Cultivated tea (Camellia sinensis) plants acidify the rhizosphere, but the mechanisms responsible for rhizosphere acidification associated with aluminium (Al) have not been fully elucidated. The present study examined the effect of Al on root-induced rhizosphere acidification, plasma membrane H+-ATPase activity and the cation–anion balance of tea plants. The H+ exudation from tea plant roots with or without Al treatment was visualized using an agar sheet with bromocresol purple. Al strongly enhanced H+ exudation and the acidification of tea plant roots by Al was closely associated with plasma membrane H+-ATPase activity. Detection of H+-ATPase hydrolytic activity associated with plasma membranes of roots under different Al treatments using the two-phase partition system indicated that plasma membrane H+-ATPase activity increased with Al concentration. The Al content, amount of proton release and H+-ATPase activity in the roots of tea plants were quantified after 7 days with or without Al. The Al content, amount of proton release and H+-ATPase activity were significantly higher in roots with Al treatment than those without Al treatment. The value for the cation–anion balance showed that all treatments had an excess of cations relative anions, and the amount of excess cations uptake increased with the increasing Al concentration. The results suggested that Al-enhanced proton release was associated with plasma membrane H+-ATPase activity and excess cation uptake. Taken together, these findings provide insights into the contributing factors of soil acidification in tea plantation.

Soil Acidification as Affected by Phosphorus Sources and Interspecific Root Interactions between Wheat and Chickpea

A pot experiment was conducted to determine the effects of chickpea/wheat intercropping and two phosphorus (P) sources on soil acidification and to explore a new way of ameliorating soil acidification. Wheat and chickpea roots were grown in compartments separated either by a solid barrier to prevent any root interactions or by a nylon mesh (30 μm) to permit partial root interactions, or with no separation between the compartments. Two P sources were applied at 60 mg P kg−1 soil either as sodium phytate or ferrous phosphate (FePO4). The decline of soil pH after growing plants for 42 days was alleviated by supplying organic P or intercropping while receiving organic P. The ameliorating of soil acidification resulted mainly from a decrease in excess cations over anion uptake of both wheat and chickpea under phytate supply, compared to FePO4 supply. The excess cation uptake of chickpea was reduced by root interactions.

Comparative Effects of Two Forage Species on Rhizosphere Acidification and Solubilization of Phosphate Rocks of Different Reactivity

ABSTRACT Dissolution of phosphate rocks (PR) in soils requires an adequate supply of acid (H+) and the removal of the dissolved products [calcium (Ca2 +) and dihydrogen phosphate (H2PO4 −)]. Plant roots may excrete H+ or OH− in quantities that are stoichiometrically equal to excess cation or anion uptake in order to maintain internal electroneutrality. Extrusion of H+ or OH− may affect rhizosphere pH and PR dissolution. Differences in rhizosphere acidity and solubilization of three PRs were compared with triple superphosphate between a grass (Brachiaria decumbens) and a legume (Stylosanthes guianensis) forage species at two pH levels (4.9 and 5.8) in a phosphorus (P)-deficient Ultisol with low Ca content. The experiment was performed in a growth chamber with pots designed to isolate rhizosphere and non-rhizosphere soil. Assessment of P solubility with chemical extractants led to ranking the PRs investigated as either low (Monte Fresco) or high solubility (Riecito and North Carolina). Solubilization of the PRs was influenced by both forage species and mineral composition of the PR. The low solubility PR had a higher content of calcite than the high solubility PRs, which led to increased soil pH values (> 7.0) and exchangeable Ca, and relatively little change in bicarbonate-extractable soil P. Rhizosphere soil pH decreased under Stylosanthes but increased under Brachiaria. The greater ability of Stylosanthes to acidify rhizosphere soil and solubilize PR relative to Brachiaria is attributed to differences between species in net ion uptake. Stylosanthes had an excess cation uptake, defined by a large Ca uptake and its dependence on N2 fixation, which induced a significant H+ extrusion from roots to maintain cell electroneutrality. Brachiaria had an excess of anion uptake, with nitrate (NO3 −) comprising 92% of total anion uptake. Nitrate and sulfate (SO4 2 −) reduction in Brachiaria root cells may have generated a significant amount of cytoplasmic hydroxide (OH−), which could have increased cytoplasmic pH and induced synthesis of organic acids and OH− extrusion from roots.

Root-induced acidification and excess cation uptake by N2-fixing Lupinus albus grown in phosphorus-deficient soil

White lupin plants (Lupinus albus L. cv. Kiev) were grown in soil columns under controlled conditions at 20/12 °C (12/12 h) for 76 d to investigate the effect of phosphorus (P) deficiency on root-induced acidification and excess cation uptake by N2-fixing plants. Phosphorus was added in each column as FePO4 at a level of 10 (limited P) or 200 μg P g−1 (adequate P). Supply of 10 μg P g−1 restricted plant growth from 58 d after sowing (DAS) and decreased P concentrations significantly in shoots from 49 DAS and in roots from 40 DAS compared with plants supplied with 200 μg P g−1. Phosphorus concentrations in shoots of plants receiving 10 μg P g−1 decreased steadily from 2.1 to 1.1 mg P g−1 dry weight from 40 to 76 DAS, but P concentrations in roots were constant with time. Total P uptake increased with time irrespective of P supply, and the P uptake by plants at 10 μg P g−1 was only 35–75% of that at 200 μg P g−1. Plants fed with 10 μg P g−1 had higher Ca and Mg concentrations but lower S concentration in shoots than the plants fed with 200 μg P g−1. The concentrations of excess cations in plants were higher at 10 μg P g−1 than 200 μg P g−1 after 49 DAS. Phosphorus deficiency decreased the pH of root exudate solution due to the enhanced release of protons (H+) from roots. The pH of root exudate solution decreased rapidly with time and dropped to the lowest (4.28) at 58 DAS in the 10 μg P g−1 treatment. The decreased pH of root exudate solution was correlated with the increased concentrations of excess cations in plants. The pH of root exudate solution showed a different pattern of change with time compared with citrate exudation, suggesting that exudation of citrate anions contributes only a part of total acidification, but excess cation uptake dominantly contributes net proton release from roots of plants grown in P-deficient soil. Plant tissue had a significant accumulation of citrate in the treatment of 10 μg P g−1 compared with 200 μg P g−1 after 67 DAS. The results suggest that P deficiency enhances the excess cation uptake and concomitant proton release, and non-synchronous processes are involved in tissue accumulation and root exudation of organic anions under P deficiency.

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 N 2-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 Na 2HPO 4 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.

Mineral ion, nitrogen and organic solute pattern in sedges(Carex Spp.) – a contribution to the physiotype concept. I. Field samples

Summary Fifty species of Carex from different Austrian habitats have been investigated to elucidate their physiological plasticity with respect to different ecological conditions. The plants were collected in their natural habitats and divided into five ecological groups according to the respective soils (silicate soil, limestone, serpentine, flysch-sandstone, saline soil). Inorganic ions, organic acids, low molecular weight carbohydrates (sugars and sugar alcohols), amino acids, betaines and total N were measured quantitatively together with selected inorganic soil characteristics. No substantial differences could be found in the solute patterns between saline, dry and wet conditions. In particular, proline, betaines and sugar alcohols – known to act as “compatible solutes” or “cytoplasmic osmotica” in several salt- or water-stressed plants – were not accumulated in Carex species growing under conditions of low soil water potential. In addition, physiological data were subjected to cluster analysis. With reference to the chosen parameters, the investigated species could not be assigned to the five ecological groups and showed no considerable physiological plasticity. Thus, the genus represents a homogeneous “physiotype” showing high K/Ca ratios as well as high contents of mono- and disaccharides and γ-aminobutyric acid. K and sugars are the main osmotic solutes in Carex species. Concerning organic acids, mainly citrate, malate and malonate are involved in balancing excess cation uptake. It is concluded that the ecological success of the genus Carex may be largely attributable to an excellent regulation of the ion uptake processes as well as ion balance within cells.

H+/OH- Excretion and Nutrient Uptake in Upper and Lower Parts of Lupin (Lupinus angustifolius L.) Root Systems

The cultivation of narrow-leafed lupins (Lupinus angustifolius L.) increase rates of subsoil acidification, and this is thought to be partly related to their pattern of nutrient uptake and H+/OH- excretion. The main hypothesis of this study was that H+ and OH- excretion is not distributed evenly over the entire length of the root system but is limited to zones where excess cation or anion uptake occur. Seedlings of nodulated lupins were grown in solution culture using vertically split pots that allowed the upper and lower zones of the root system to be supplied with varying concentrations of K+ and NO-3. Net H+/OH- excretion was equated to the addition of NaOH/HCl required to maintain a constant pH in the nutrient solution during a 4-d treatment period and nutrient uptake was measured by depletion from solution in each zone of the split pots.The excess of cation over anion uptake was positively correlated with H+ excretion in each rooting zone. In zones where K+ was supplied at 1200 μM, cation uptake was dominated by K+ and up to twice as much H+ was excreted than in zones where K+ was absent. In zones where NO-3 was supplied at 750 μM, the anion/cation uptake was balanced, however H+ excretion continued to occur in the zone. When NO-3 was supplied at 5000 μM, anion uptake exceeded cation uptake but there was no OH- excretion. Organic acid anions may be excreted by lupins to maintain their internal electroneutrality when anion uptake exceeds cation uptake. Rhizosphere pH would not increase unless the pKa of the excreted organic anions was greater than the external pH.

Iron Deficiency Stress Response of Epipremnum aureum and Philodendron scandens Subspecies Oxycardium

Summary Philodendron [ Philodendron scandens Koch, C. and H. Sello subspecies oxycardium (Schott) Bunt.] grown in Hoagland Solution Number 1, with all N supplied as nitrate, lowered solution pH from 5.1 to below 3 when the solution contained no Fe, 0.1 mM FeEDTA, or 6.3 mM Fe 2 O 3 . Philodendron with no Fe became severely chlorotic, while plants with Fe 2 O 3 or FeEDTA remained green. Reductant levels in the philodendron solutions did not differ regardless of Fe treatment. Pothos [ Epipremnum aureum (Linden and Andre) Bunt.] caused the solution pH to rise above 7 with FeEDTA, but the pH dropped with no Fe and at least initially with Fe 2 O 3 . With no Fe, pothos became severely chlorotic. With Fe 2 O 3 , pothos became slightly chlorotic but regreened as the pH dropped. After regreening of plants, the solution pH increased to above 7. Solutions of chlorotic pothos plants had reductant concentrations 10 to 25 times as high as those of Fe-sufficient plants. A macronutrient charge balance of leaf tissue indicated excess anion uptake only for the pothos receiving FeEDTA, which was consistent with the rise in nutrient solution pH. All other pothos and all philodendron treatments had excess cation uptake, which was consistent with the drop in nutrient solution pH in these treatments. Pothos had a strong Strategy I Fe-deficiency stress response, i.e. pH decrease and production of Fe reductant. Philodendron had some aspects of a Strategy I Fe-deficiency stress response but was unique because it lowered the pH of an all nitrate-N solution, regardless of Fe availability or plant Fe status.

Relative ability of a range of crop species to use phosphate rock and monocalcium phosphate as P sources when grown in soil

AbstractGlasshouse experiments were conducted to compare the ability of a range of crop species to use the medium reactive Jordan phosphate rock (JPR), the highly reactive North Carolina phosphate rock (NCPR) and the highly soluble monocalcium phosphate (MCP) as P sources in a P‐deficient silt loam when supplied with nitrate‐N. Growth of wheat and barley resulted in excess anion uptake, a consequent rise in rhizosphere pH and yield increased in the following order: control < JPR < NCPR < MCP. Growth of buckwheat resulted in excess cation uptake, and as a result it used NCPR as effectively as MCP; the yield order was as follows: control < JPR < NCPR = MCP. Lupin, when supplied with nitrate, raised its rhizosphere pH and yield followed the same order as that for wheat and barley: control < JPR < NCPR < MCP. In contrast, when not supplied with nitrate and relying substantially on biological N2 fixation, lupin absorbed a cation excess, lowered rhizosphere pH and the yield order was control < JPR = NCPR = MCP. Although rape and kale absorbed an anion excess and raised rhizosphere pH they were surprisingly effective at using phosphate rock as a P source; the yield order was control < JPR = MCP < NCPR. This was attributed to the known ability of rape to acidify its rhizosphere in a localised area just behind the root tip in response to P deficiency. The opportunity appears to exist for the strategic use of phosphate rock at times within arable rotations when species are present that can effectively use such P sources.

Active ion uptake and maintenance of cation-anion balance: A critical examination of their role in regulating rhizosphere pH

The processes responsible for maintenance of cation-anion balance in plants and their relation to active ion accumulation and changes in rhizosphere pH are outlined and discussed. The major processes involved are: (1) accumulation and degradation of organic acids which occur in the plant mainly as organic acid anions (and their transfer within the plant) and (2) extrusion of H+ or OH− into the rhizosphere. The relative importance of the two processes is determined by the size of the excess anion or cation uptake. Indeed, plants typically absorb unequal quantities of nutritive cations (NH4++Ca2++ Mg2++K++Na+) and anions (NO3−+Cl−+SO42−+H2PO4−) and charge balance is maintained by excretion of an amount of H+ or OH− which is stoichiometrically equal to the respective excess cation or anion uptake. The mechanisms and processes by which H+ and in particular OH− ions are excreted in response to unequal cation-anion uptake are, however, poorly understood.

Important factors in nutrient availability: Root morphology and physiology

AbstractNutrient availability in the soil is not only determined by chemical, physical, and micro‐biological factors but can be actively influenced by the plant root system. Various components of root activity have been studied in our laboratory during the last 10 years. The major findings are summarized and discussed with special emphasis on root morphology, cation‐anion imbalance and proton release, the mechanism of proton release and anion secretion. Large species differences in root morphology were found between red clover and rye‐grass. Root morphology was also found to be controlled by environmental and physiological factors. Experimental results obtained with different plant species suggest that proton release by plant roots is not primarily a consequence of excess cation uptake to achieve charge balance in the plant, but rather that active proton extrusion by ATPase activity creates a charge imbalance and a pH gradient which drive nutrient uptake. The pH gradient across the plasmalemma of root cells was also found to be essential for the retention of organic anions by plant roots.

Short‐term effects of Al on nutrient uptake, H+ efflux, root respiration and nitrate reductase activity of two sorghum genotypes differing in al‐susceptibility

Abstract To investigate whether effects of Al in the sorghum genotypes SCO283 (Al‐tolerant) and NB9040 (Al‐sensitive) are primary effects of Al toxicity, a short‐term experiment was carried out. After 21 days of pregrowth in an Al‐free nutrient solution, plants were exposed to nutrient solutions without or with 10 mg 1‐1 Al for 96 h. Nitrogen was supplied as NH4NO3. Twenty four h after Al supply, the relative uptake of all nutrient cations and anions, except NH4 +, declined to values varying between 0.8 (K+) and 0.2 (Ca2+, Mg2+). In contrast, the uptake of NH4 + was stimulated by Al in both sorghum lines. Total cation uptake and uptake of total nitrogen were balanced by the stimulation of NH4 + uptake. As a consequence, excess cation uptake and proton efflux were increased by Al, especially in the Al‐sensitive line, NB9040. Time relationships of nutrient uptake (except for NH4 +) and root respiration showed close resemblance, i.e. a decline within 24 h followed by a stable phase. This could indicate that ...

Chemical Properties of the Rhizosphere in an Acid Subsoil

AbstractA method of corn seedling culture was developed which allowed growth of the root systems in samples of an acid subsoil (pH 4.7‐5.0) for at least 2 weeks after seedling emergence, without adding additional water to the subsoil. This method was used to culture corn inbred and corn hybrid seedlings in samples of acid subsoil in two separate greenhouse studies. Bulk (non‐rhizosphere) soil and rhizosphere soil samples obtained in these studies were assayed for pH, KCl‐extractable Ca, KCl‐extractable Al, and water‐extractable Ca. The pH of rhizosphere samples was less than that of the bulk samples for all inbreds and hybrids. This pH decrease was 0.16 in one experiment with inbreds, and 0.11 in another experiment with hybrids. In comparison to the bulk soil, the rhizosphere soil contained 0.16 cmolc kg−1 more KCl‐extractable Al, 0.13 cmolc kg−1 less KCl‐extractable Ca, 0.023 cmolc kg−1 more water‐extractable Ca, and 0.10 cmolc kg−1 less total Ca. The difference in the sum of KCl‐extractable Al plus Ca between bulk and rhizosphere soils was not statistically significant. The results of both studies are consistent with the hypothesis that Ca uptake by the root depleted the Ca in the rhizosphere, that H ions were released by the root in response to excess cation (vs. anion) uptake and that this excess H+ dissolved basic Al compounds, which increased the KCl‐extractable Al. The lack of statistically significant differences between rhizosphere and bulk soil parameters among hybrids/inbreds indicate that differential pH changes in the rhizosphere are not the chief mechanism of any differential Al tolerance among these corn inbreds and hybrids.

Cation‐anion uptake balance and proton efflux in plant efficiency for iron

Abstract Iron efficiency in plants appears to be related to several phenomena which differentially function in different species and cultivars of plants although 2 or more of the mechanisms may be coupled and present in the same individual. Proton expulsion from roots is important in iron efficiency in dicots and it may or may not be associated with excess cation uptake where a cation is exchanged for H+. When protons are expelled, the residual OH‐ in roots must be oxidized but this oxidation releases electrons for redox reactions which may.be coupled with enzymatic Fe+++ reduction. If this set of reactions is related to catylase or peroxidase, the presence of these enzymes in plants would be more rational. Some plants, particularly monocots, do not expel protons unless much of the nitrogen is in the NH4+ form. Such plants then cannot acidify or reduce the rhizosphere. There is evidence that these monocot plants produce phytosiderophores which function in iron mobilization and reduction. Monocots take up ...

Helix–coil transition of poly(α,L‐glutamic acid) at an interface: Correlation with static and dynamic membrane properties

AbstractWhen adsorbed from an aqueous dilute solution at high pH into the pores of an inert cellulose acetate filter, poly(α,L‐glutamic acid) remains strongly anchored to the pore walls. The existence of the helix–coil transition for the adsorbed polypeptide in a certain pH range is evidenced by static and dynamic membrane properties displayed by the “activated” filter, such as excess cation uptake, membrane potential, and hyraulic permeability. In particular, the variations of the hydrodynamic thickeness present a sigmoidal shape characteristic of the helix–coil transition at the interface, a transition apparently less sharp than in solution.

Ion Balance, Uptake, and Transport Processes in N2-Fixing and Nitrate- and Urea-Dependent Soybean Plants

The objective of this study was to examine the influence of N(2) fixation and NO(3) (-)-N and urea-N assimilation on ion balance, uptake, and transport processes in soybean (Glycine max L. Merr.).Inoculated plants were grown in Perlite supplied daily with nutrient solutions which contained zero-N, 10 and 20 millimolar NO(3) (-)-N, and 10 and 20 millimolar urea-N, and they were sampled 41, 76, and 151 days after transplanting. Total uptake of inorganic cations and anions was determined by analysis of tissue for K(+), Ca(2+), Mg(2+), Na(+), total N from NO(3) (-), total S, H(2)PO(4) (-), and Cl(-). Differences in total inorganic cations (C) and inorganic anions (A) in plant tissue were used to estimate total carboxylate content.Internal OH(-) generation resulting from excess cation uptake (net H(+) excretion) by the roots accounted for more than 89% of the carboxylate accumulation in N(2)- and urea-fed plants, while OH(-) generation resulting from SO(4) (2-) reduction accounted for less than 11%. Shoots contained over 89% of the total plant carboxylate content. Malate balanced about 75% of the excess inorganic cationic charge of the xylem sap; allantoate and aspartate balanced most of the remaining charge. These results indicate that carboxylates (primarily malate) are synthesized in roots of N(2)- and urea-fed plants and transported to the shoots in the xylem to maintain charge balance. The high malate concentration resulted in the C/N weight ratio of xylem sap from N(2)-fed plants being >2.0, even though 83% of the N was transported as allantoin and allantoic acid which have a C/N ratio of 1.0. The data emphasize that C and N content of N compounds should not be the sole basis for calculating the C/N weight ratio of xylem sap.The C-to-A uptake ratio for plants supplied 10 millimolar NO(3) (-) ranged from 1.24 to 1.57 during development, indicating that internal OH(-) was generated both by excess cation uptake and by NO(3) (-) and SO(4) (2-) reduction. The C-to-A uptake ratio for 20 millimolar NO(3) (-) -fed plants ranged from 0.86 to 0.96 during development, indicating a small net OH(-) efflux from the roots for support of excess anion uptake. On a seasonal basis, only 15% of the OH(-) generated during NO(3) (-) and SO(4) (2-) reduction was associated with OH(-) efflux (excess anion uptake), while 85% was associated with carboxylate accumulation. The malate concentration in xylem sap from plants supplied 20 millimolar NO(3) (-) was only one-third that of N(2)- and urea-fed plants; however, it did balance 75% of the excess inorganic cationic charge. Potassium, recycling to accommodate excess anion uptake by 20 millimolar NO(3)-fed plants, was calculated to involve at most 17% of the total K(+) absorbed during the 41- to 76-day growth interval.

Organic Acids and Ionic Balance in Xylem Exudate of Wheat during Nitrate or Sulfate Absorption

Experiments were designed to study the importance of organic acids as counterions for K(+) translocation in the xylem during excess cation uptake. A comparison was made of xylem exudate from wheat seedlings treated 72 hours with either 1.0 millimolar KNO(3) or 0.5 millimolar K(2)SO(4), both in the presence of 0.2 millimolar CaSO(4). Exudation from KNO(3) plants had twice the volume and twice the K(+) and Ca(2+) fluxes or rate of delivery to shoots, as K(2)SO(4) plants. Malate flux was 25% higher in K(2)SO(4) than in KNO(3) exudate. Malate was the principal anion accompanying K(+) or Ca(2+) in K(2)SO(4) treatment, while in the KNO(3) treatment, NO(3) (-) was the principal anion. The contribution of SO(4) (2-) was negligible in both treatments. In a second experiment, exudate was collected every 4 hours during the daytime throughout a 72-hour treatment with KNO(3). Malate was the only anion present in exudate at first, just after the CaSO(4) pretreatment had ended. Malate concentration decreased and NO(3) (-) concentration increased with time and these concentrations were negatively correlated. By 62 hours, NO(3) (-) represented 80% of exudate anions. K(+) and NO(3) (-) concentrations in exudate were strongly correlated with K(+) and NO(3) (-) uptake, respectively. The first 36 hours of absorption from KNO(3) solution resembled the continuous absorption of K(2)SO(4), in that malate was the principal counterion for translocation of K(+).

Malate synthesis and metabolism during photosynthesis in Egeria densa planch

During pulse-chase labelling experiments at air levels of CO2 (pH 4.5; 13 μM CO2 (aq.)) Egeria densa Planch. fixed 20% of its photosynthetic carbon by β-carbboxylation into the C-4 of malate. Further label, which entered the malate pool via the three carbon precursors, was derived from secondary photosynthetic products (sugar monophosphates or glycollate pathway intermediates) not directly from 3-phosphoglyceric acid.Tissue concentrations of malate increased by 80–200 μmol (g dry wt.)−1 during 8 h photosynthesis and although tracer data indicated that some turnover of malate occurred in the light, net decline of the malate pool only took place in the dark. Photosynthetically fixed 14C was transferred from malate to citrate plus isocitrate in the dark and eventually lost from the plant, suggesting that respiration is responsible for the breakdown of malate at night.It is proposed that malate synthesis in Egeria is not related to the synthetic or energy fixing roles of photosynthesis but may be required to balance excess cation uptake in the light.

Effect of Macronutrient Ions on the Availability of Iron from a Chelated Source

AbstractThe uptake of Fe from FeEDDHA [ferric chelate of ethylenediamine di (o‐hydroxyphenylacetic acid)] during periods of 1 to 2 days was studied with 10‐ to 11‐day‐old corn plants (Zea mays L.) grown in nutrient solutions. The Fe in roots and tops was markedly affected by the composition of the nutrient solution. Cations stimulated Fe in plants in the following order: K> Na > Mg > Ca, and anions SO4 > Cl > NO3. The stimulation was the greater the more the pH declined during the absorption experiment; hence it was concluded that the effect of nutrient solution composition on Fe nutrition must be attributed to the imbalance of cation and anion uptake rather than to a specific nutrient. The pH decline and, in general, the associated stimulation of Fe uptake were enhanced by increasing concentrations of the interfering ions and by time. However, when the pH fell toward a value of 4, injury to roots occurred and Fe uptake was reduced in spite of a higher supply. The composition of the nutrient solution was also effective in long‐term experiments in regulating Fe uptake. A decrease in pH resulting from excess cation uptake favors the formation of less dissociated forms of the free ligand, namely of (ligand‐H)3‐, (ligand‐H2)2‐, etc., instead of the reactive ligand, (ligand)4‐ and equilibrium can be maintained only if more FeEDDHA dissociates, liberating Fe3+ for uptake.