Growth Medium and Phosphorus Supply Affect Cluster Root Formation and Citrate Exudation by Lupinus albus Grown in a Sand/Solution Split-Root System

We examined cluster root formation and root exudation by white lupin (Lupinus albus L. cv. Kiev Mutant) in response to growth medium and phosphorus supply in a sand/solution split-root system. The split-root system consisted of a nutrient solution compartment and a siliceous sand compartment. Phosphorus was applied at 1 (low-P plants) or 50 (high-P plants) μM as KH₂PO₄ to the solution compartment and at 10, 50 or 250 mg P kg⁻¹ as hydroxyapatite (Ca-P) to the sand compartment. In contrast to the high-P plants, P concentration and P uptake in the low-P plants increased with increasing P supply to the sand compartment. The NaHCO₃-extractable P was lower in the rhizosphere of the low-P plants than the high-P ones. The proton extrusion rate by the solution-grown roots of the low-P plants was higher than that of the high-P plants at the early growth stage. For the low-P plants, the proportion of dry root biomass allocated to cluster roots was higher in the solution compartment than that in the sand compartment. The citrate exudation increased in the sand compartment and decreased in the solution compartment with time, showing a lack of synchronization in citrate exudation by two root halves grown in different media. The cluster root proportion and citrate exudation in both compartments decreased with increasing shoot P concentration. An additional experiment with no P added to either root compartment showed that the proportion of cluster roots was about 9% lower in the sand than solution compartments. The results suggest that cluster root formation and citrate exudation can be significantly affected by the root growth medium in addition to being regulated by shoot P status. More P can be exploited from sparingly available Ca-P by the low-P plants than the high-P ones due to greater citrate exudation under P deficiency.     

Cluster-root formation and carboxylate release in three Lupinus species as dependent on phosphorus supply,internal phosphorus concentration and relative growth rate

Background and Aims

Some Lupinus species produce cluster roots in response to low plant phosphorus (P) status. The cause of variation in cluster-root formation among cluster-root-forming Lupinus species is unknown. The aim of this study was to investigate if cluster-root formation is, in part, dependent on different relative growth rates (RGRs) among Lupinus species when they show similar shoot P status.

Methods

Three cluster-root-forming Lupinus species, L. albus, L. pilosus and L. atlanticus, were grown in washed river sand at 0, 7·5, 15 or 40 mg P kg⁻¹ dry sand. Plants were harvested at 34, 42 or 62 d after sowing, and fresh and dry weight of leaves, stems, cluster roots and non-cluster roots of different ages were measured. The percentage of cluster roots, tissue P concentrations, root exudates and plant RGR were determined.

Key Results

Phosphorus treatments had major effects on cluster-root allocation, with a significant but incomplete suppression in L. albus and L. pilosus when P supply exceeded 15 mg P kg⁻¹ sand. Complete suppression was found in L. atlanticus at the highest P supply; this species never invested more than 20 % of its root weight in cluster roots. For L. pilosus and L. atlanticus, cluster-root formation was decreased at high internal P concentration, irrespective of RGR. For L. albus, there was a trend in the same direction, but this was not significant.

Conclusions

Cluster-root formation in all three Lupinus species was suppressed at high leaf P concentration, irrespective of RGR. Variation in cluster-root formation among the three species cannot be explained by species-specific variation in RGR or leaf P concentration.     

Mechanisms of citrate transport and exchange in corn mitochondria

Previous work (Birnberg, Jayroe, Hanson 1982 Plant Physiol 70: 511-516) demonstrated that corn mitochondria (Zea mays L.) can accumulate citrate by a malate- and phosphate-independent proton symporter. This uptake and symport of other ions were investigated. Passive swelling experiments indicated that corn mitochondria can accumulate several other anions by proton symport, but only isocitrate is taken up nearly as effectively as citrate. At the optimal pH (4.5), active uptake of carrier-free [¹⁴C]citrate in 50 micromolar mersalyl is inhibited by fourteen anions, but only the I₅₀ (the concentration of inhibitor required to reduce uptake of carrier-free [¹⁴C]citrate by 50%) values of citrate (0.08 millimolar) and d-and l-isocitrate (0.5 millimolar) are less than 4 millimolar. Isocitrate is a competitive inhibitor of citrate uptake and [¹⁴C]isocitrate is accumulated with a K_m similar similar to its I₅₀. Valinomycin reduces net active citrate accumulation at pH 7.5, consistent with the relatively low V_max for citrate uptake. At pH 4.5, mersalyl reduces the rate of citrate uptake without changing the affinity of the carrier for citrate. Thus, the corn mitochondria have a high-affinity, mersalyl-insensitive carrier selective for citrate that also transports isocitrate.     

Carboxylate release of wheat, canola and 11 grain legume species as affected by phosphorus status

The capacity of plant roots to increase their carboxylate exudation at a low plant phosphorus (P) status is an adaptation to acquire sufficient P at low soil P availability. Our objective was to compare crop species in their adaptive response to a low-P availability, in order to gain knowledge to be used for improving crop P-acquisition efficiency from soils that are low in P or that have a high capacity to retain P. In the present screening study we compared 13 crop species, grown in sand at either 3 or 300 μM of P, and measured root mass ratio, cluster-root development, rhizosphere pH and carboxylate composition of root exudates. Root mass ratio decreased with increasing P supply for Triticum aestivum L., Brassica napus L., Cicer arietinum L. and Lens culinaris Medik., and increased only for Pisum sativum L., while the Lupinus species and Vicia faba L. were not responsive. Lupinus species that had the potential to produce root clusters either increased or decreased biomass allocation to clusters at 300 μM of P compared with allocation at 3 μM of P. All Lupinus species acidified their rhizosphere more than other species did, with average pH decreasing from 6.7 (control) to 4.3 for Lupinus pilosus L. and 5.9 for Lupinus atlanticus L.; B. napus maintained the most alkaline rhizosphere, averaging 7.4 at 300 μM of P. Rhizosphere carboxylate concentrations were lowest for T. aestivum, B. napus, V. faba, and L. culinaris than for the other species. Exuded carboxylates were mainly citrate and malate for all species, with the exception of L. culinaris and C. arietinum, which produced mainly citrate and malonate. Considerable variation in the concentration of exuded carboxylates and protons was found, even with a genus. Cluster-root forming species did not invariably have the highest concentrations of rhizosphere carboxylates. Lupinus species varied both in P-uptake and in the sensitivity of their cluster-root development to external P supply. Given the carbon cost of cluster roots, a greater plasticity in their formation and exudation (i.e. reduced investment in cluster roots and exudation at higher soil P, a negative feedback response) is a desirable trait for agricultural species that may have variable access to readily available P.     

Impact of phosphorus mineral source (Al–P or Fe–P) and pH on cluster-root formation and carboxylate exudation in Lupinus albus L.

Lupinus albus L. were grown in rhizoboxes containing a soil amended with sparingly available Fe–P or Al–P (100 μg P g⁻¹ soil/resin mixture). Root halves of individual plants were supplied with nutrient solution (minus P) buffered at either pH 5.5 or 7.5, to assess whether the source of mineral-bound P and/or pH influence cluster-root growth and carboxylate exudation. The P-amended soil was mixed 3:1 (w/w) with anion-exchange resins to allow rapid fixation of carboxylates. Treatments lasted 10 weeks. Forty percent and 30% of the root mass developed as cluster roots in plants grown on Fe–P and Al–P respectively, but cluster-root growth was the same on root-halves grown at pH 5.5 or 7.5. Mineral-bound P source (Al– or Fe–P) had no influence on the types of carboxylates measured in soil associated with cluster roots—citrate (and trace amounts of malate and fumarate) was the only major carboxylate detected. The [citrate] in the rhizosphere of cluster roots decreased with increased shoot P status (suggesting a systemic effect) and also, only for plants grown on Al–P, with decreased pH in the root environment (suggesting a local effect). In a separate experiment using anion exchange resins pre-loaded with malate or citrate, we measured malate (50%) and citrate (79%) recovery after 30 days in soil. We therefore, also conclude that measurements of [citrate] and [malate] at the root surface may be underestimated and would be greater than the 40- and 1.6-μmol g⁻¹ root DM, respectively estimated by us and others because of decomposition of carboxylates around roots prior to sampling.     

The effect of phosphorus on cluster-root formation and functioning of Embothrium coccineum (R. et J. Forst.)

Background and aims

Embothrium coccineum (R. et J. Forst.) is a Proteaceae species from the southern part of South America. South-central Chilean soils are younger and contain more phosphorus (P) than soils in Australia and South Africa, where Proteaceae are common. Phosphorus deficiency is the main factor promoting cluster-root formation in Proteaceae. It is not known, however, whether this also applies to E. coccineum, which grows on soils with higher P content.

Methods

Four-month-old seedlings were grown for 4 weeks in hydroponic cultures with 1 μM P or 50 μM P. The number of cluster roots, relative height increment, biomass distribution, cluster root/total root biomass ratio, foliar P concentration, root acid phosphatase activity and root carboxylate-exudation rates were determined.

Results

Seedlings growing at 50 μM P showed a 10?, 1.3? and 3.3-fold greater increase in relative height, total dry mass and foliar P concentration, respectively, compared with those grown at1 μM P. However, seedlings grown at 1 μM P showed a 5?, 16?, 1.7? and 1.3-fold greater number of cluster roots, cluster root/total root biomass ratio, phosphatase activity and total carboxylate exudation, respectively, as compared with those grown at 50 μM P.

Conclusions

A low P supply promotes the initiation, growth and metabolic activity of cluster roots which is in accordance with reports on Proteaceae species occurring in ancient and highly weathered soils.     

Effects of low pH and aluminum stresses on common beans (Phaseolus vulgaris) differing in low-phosphorus and photoperiod responses

Using common beans differing greatly in the response to photoperiod and low-phosphorus (P) stress, we investigated their responses to acidity and aluminum (Al) toxicity and the relationship between Al tolerance and organic acid exudation under Al or low P stress. A genotype Ginshi was found to be sensitive to low pH treatment. When exposed to pH 4.5, serious curvature in the root tips of cv. Ginshi was observed; however, it was completely corrected by the application of 5 or 10 μmol/L AlCl₃; increasing calcium (Ca) could ameliorate Al toxicity, but it could not correct root curvature at pH 4.5. Common beans showed significant differences in both root growth and Al tolerance, and the varieties from the Andes were more tolerant to Al toxicity than those from the Mesoamerican origin. In the presence of 50 μmol/L AlCl₃, all the common bean genotypes exuded citrate, and a significant difference in the amounts of citrate was observed among genotypes. The genotypes originated in the Mesoamerica tended to release more citrate than other origins in the presence of Al. The P-inefficient genotype DOR364 exuded more citrate than the P-efficient genotype G19833 in the presence of 50 μmol/L AlCl₃, whereas no organic acids were detected in root exudates under low-P stress. A reduction of citrate exudation in the DOR364, but a slight increase of citrate exudation in the G19833, was observed under Al stress after they were exposed to 6-d P starvation. These results suggest that different low-P or Al tolerance in common beans might not be associated with organic acid exudation.     

Does the combination of citrate and phytase exudation in <Emphasis Type="Italic">Nicotiana tabacum</Emphasis> promote the acquisition of endogenous soil organic phosphorus?

Background and Aims

Plant acquisition of endogenous forms of soil phosphorus (P) could reduce external P requirements in agricultural systems. This study investigated the interaction of citrate and phytase exudation in controlling the accumulation of P and depletion of soil organic P by transgenic Nicotiana tabacum plants.

Methods

N. tabacum plant lines including wild-type, vector controls, transgenic plants with single-trait expression of a citrate transporter (A. thaliana frd3) or fungal phytases (phyA: A. niger, P. lycii) and crossed plant lines expressing both traits, were characterized for citrate efflux and phytase exudation. Monocultures and intercropped combinations of single-trait plants were grown in a low available P soil (12 weeks). Plant biomass, shoot P accumulation, rhizosphere soil pH and citrate-extractable-P fractions were determined. Land Equivalent Ratio and complementarity effect was determined in intercropped treatments and multiple-linear-regression was used to predict shoot P accumulation based on plant exudation and soil P depletion.

Results

Crossed plant lines with co-expression of citrate and phytase accumulated more shoot P than single-trait and intercropped plant treatments. Shoot P accumulation was predicted based on phytase-labile soil P, citrate efflux, and phytase activity (Rsq=0.58, P < .0001). Positive complementarity occurred between intercropped citrate- and phytase-exuding plants, with the greatest gains in shoot P occurring in plant treatments with A. niger phyA expression.

Conclusions

We show for the first time that trait synergism associated with the exudation of citrate and phytase by tobacco can be linked to the improved acquisition of P and the depletion of soil organic P.

     

Effects of phosphorus supply on growth, phosphate concentration and cluster-root formation in three Lupinus species

Background and Aims

In some lupin species, phosphate deficiency induces cluster-root formation, which enhances P uptake by increasing root surface area and, more importantly, the release of root exudates which enhances P availability.

Methods

Three species of Lupinus, L. albus, L. atlanticus and L. micranthus, with inherently different relative growth rates were cultivated under hydroponics in a greenhouse at four phosphate concentrations (1, 10, 50 and 150 µm) to compare the role of internal P in regulating cluster-root formation.

Key Results

The highest growth rate was observed in L. atlanticus, followed by L. albus and L. micranthus. At 1 µm P, cluster-root formation was markedly induced in all three species. The highest P uptake and accumulation was observed in L. micranthus, followed by L. atlanticus and then L. albus. Inhibition of cluster-root formation was severe at 10 µm P in L. atlanticus, but occurred stepwise with increasing P concentration in the root medium in L. albus.

Conclusions

In L. atlanticus and L. albus cluster-root formation was suppressed by P treatments above 10 µm, indicating a P-inducible regulating system for cluster-root formation, as expected. By contrast, production of cluster roots in L. micranthus, in spite of a high internal P concentration, indicated a lower sensitivity to P status, which allowed P-toxicity symptoms to develop.     

Developmental physiology of cluster-root carboxylate synthesis and exudation in harsh hakea. Expression of phosphoenolpyruvate carboxylase and the alternative oxidase

Harsh hakea (Hakea prostrata R.Br.) is a member of the Proteaceae family, which is highly represented on the extremely nutrient-impoverished soils in southwest Australia. When phosphorus is limiting, harsh hakea develops proteoid or cluster roots that release carboxylates that mobilize sparingly soluble phosphate in the rhizosphere. To investigate the physiology underlying the synthesis and exudation of carboxylates from cluster roots in Proteaceae, we measured O2 consumption, CO2 release, internal carboxylate concentrations and carboxylate exudation, and the abundance of the enzymes phosphoenolpyruvate carboxylase and alternative oxidase (AOX) over a 3-week time course of cluster-root development. Peak rates of citrate and malate exudation were observed from 12- to 13-d-old cluster roots, preceded by a reduction in cluster-root total protein levels and a reduced rate of O2 consumption. In harsh hakea, phosphoenolpyruvate carboxylase expression was relatively constant in cluster roots, regardless of developmental stage. During cluster-root maturation, however, the expression of AOX protein increased prior to the time when citrate and malate exudation peaked. This increase in AOX protein levels is presumably needed to allow a greater flow of electrons through the mitochondrial electron transport chain in the absence of rapid ATP turnover. Citrate and isocitrate synthesis and accumulation contributed in a major way to the subsequent burst of citrate and malate exudation. Phosphorus accumulated by harsh hakea cluster roots was remobilized during senescence as part of their efficient P cycling strategy for growth on nutrient impoverished soils.     

Physiological changes in white lupin associated with variation in root-zone CO2 concentration and cluster-root P mobilization

White lupin (Lupinus albus L.) mobilizes insoluble soil phosphorus through exudation of organic acids from ‘cluster’ roots. Organic acid synthesis requires anaplerotic carbon derived from dark CO₂ fixation involving PEP-carboxylase. We tested the hypothesis that variation in root-zone CO₂ concentration would influence organic acid synthesis and thus P mobilization. Root-zone CO₂ concentrations and soil FePO₄ concentrations supplied to sand-grown white lupin (cv. Kiev Mutant) were varied. More biomass accumulated in plants supplied with 360 µL L⁻¹ CO₂ to the root-zone, compared with those aerated with either 100 or 6000 µL L⁻¹ CO₂. Increased FePO₄ in the sand resulted in greater leaf P concentrations, but root-zone [CO₂] did not influence leaf P concentration. Suppression of cluster-root development in plants supplied with 100 µL L⁻¹ root-zone CO₂ was correlated with increased leaf [P]. However, at both 360 and 6000 µL L⁻¹ CO₂, cluster-root development was suppressed only at the highest leaf P concentration. Phloem sap [P] was significantly increased by greater [FePO₄] in the sand, but was reduced with increased root-zone [CO₂], and this may have triggered increased cluster-root initiation. Succinate was the major organic acid (carboxylate) in the phloem sap (minor components included malate, citrate, fumarate) and was increased at greater [FePO₄], suggesting that this shoot-derived carboxylate might provide an important source of organic acids for root metabolism. Since cluster root development was inhibited by increasing concentrations of FePO₄ in the sand, it is possible that succinate was utilized for the functioning of the root-nodules.     

Phosphorus acquisition characteristics of cotton (Gossypium hirsutum L.), wheat (Triticum aestivum L.) and white lupin (Lupinus albus L.) under P deficient conditions

A rhizobox experiment was conducted to examine the P acquisition characteristics of cotton (Gossypium hirsutum L.), wheat (Triticum aestivum L.) and white lupin (Lupinus albus L.) under P-deficient conditions. We aimed to identify whether cotton is physiologically efficient at acquiring P through release of protons, phosphatases or carboxylates. Plants were pre-grown in the upper compartment of rhizoboxes filled with a sand and soil mixture to create a dense root mat against a 53 μm polyester mesh. For each species, two P treatments (0 and 20 mg P kg^?1) were applied to the upper compartment in order to create P-deficient and P-sufficient plants. At harvest, the upper compartment with intact plants was used for collection of root exudates while the lower soil compartment was sliced into thin layers (1 mm) parallel to the rhizoplane. Noticeable carboxylates release was only detected for white lupin. All P-deficient plants showed a capacity to acidify their rhizosphere soil to a distance of 3 mm. The activity of acid phosphatase was significantly enhanced in the soil-root interfaces of P-stressed cotton and wheat. Under P-deficient conditions, the P depletion zone of cotton from the lower soil compartment was narrowest (<2 mm) among the species. Phosphorus fractionation of the rhizosphere soil showed that P utilized by cotton mainly come from NaHCO₃–P_i and NaOH–P_o pools while wheat and white lupin markedly depleted NaHCO₃–P_i and HCl–P pools, and the depletion zone extended to 3 mm. Wheat also depleted NaOH–P_o to a significant level irrespective of P supply. The study suggests that acquisition of soil P is enhanced through P mobilization by root exudates for white lupin, and possibly proton release and extensive roots for wheat under P deficiency. In contrast, the P acquisition of cotton was associated with increased activity of phosphatases in rhizosphere soil.     

Phosphorus deficiency does not enhance proton release by roots of soybean [Glycine max (L.) Murr.]

Little work has been done on root exudation in soybean under P deficiency. This study examined the effect of P supply on release of protons and carboxylates by roots of soybean (Glycine max Heinong 35), and to correlate the release with excess uptake of cations over anions. Plants were either reliant on N₂ fixation or supplied with nitrate and were grown in nutrient solution with 1–50 μM P for 7 weeks. Release of protons and carboxylates from roots, and concentrations of Ca, Mg, K, Na, P, S, Cl and N in plants were measured weekly from week 4. Unlike in many other species, P deficiency decreased proton release per unit root biomass in N₂-fixing plants and increased release of hydroxyl ions in nitrate-fed soybean. While P deficiency generally decreased uptake of K, Ca, Mg, S, Cl and P, it increased nitrate uptake per unit root biomass. Irrespective of P supply, amounts of protons released correlated well with excess uptake of cations over anions by the roots. Phosphorus deficiency increased release of carboxylates but the amounts released were small. The results suggest that soybean displays strategies of P acquisition through decreasing proton release which favors P mobilization in acid soils, and increasing root-to-shoot ratio and specific root length.     

Investigations into the exploitation of heterogeneous soils by Lupinus albus L. and L. pilosus Murr. and the effect upon plant growth

In calcareous soils, genotypes of Lupinus albus L. generally grow poorly, resulting in stunted plants that often develop lime-induced chlorosis. In contrast, some genotypes of L. pilosus Murr. occur naturally in calcareous soils without developing any visible symptoms of stress. Some genotypic variation for tolerance to calcareous soil does exist in L. albus and the tolerance mechanisms need to be determined. The adaptation through root system morphological plasticity of L. albus and L. pilosus, to heterogeneous limed soil profiles (pH 7.8) containing either patches of acid (non-limed) soil, or vertically split between acid and limed soil, was investigated. When grown in the presence of patches of acid soil, L. albus had a 52% greater shoot dry weight and visibly greener leaves compared with plants grown in the homogeneous limed soil. Total root dry matter in the acid-soil patches was greater than in the control limed-soil patches. This was due to a four-fold increase in the cluster root mass, accounting for 95% of the root dry matter in the acid-soil patch. Although these cluster roots secreted no more citric acid per unit mass than those in the limed soil did, their greater mass resulted in a higher citrate concentration in the surrounding soil. L. pilosus responded to the patches of acid soil in a manner comparable with L. albus. When grown in the homogeneous limed soil, L. pilosus had a greater maximum net CO₂ assimilation rate (P_max) than L. albus, however, the P_max of both species increased after they had accessed a patch of acid soil. Differences were apparent between the L. albus genotypes grown in soil profiles split vertically into limed and acid soil. A genotype by soil interaction occurred in the partitioning between soils of the cluster roots. The genotype La 674 was comparable with L. pilosus and produced over 11% of its cluster roots in the limed soil, whereas the other genotypes produced only 1–3% of their cluster roots in the limed soil. These results indicate L. pilosus is better adapted to the limed soil than L. albus, but that both species respond to a heterogeneous soil by producing mainly cluster roots in an acid-soil patch. This revised version was published online in June 2006 with corrections to the Cover Date.     

Organic acids differ in enhancing phosphorus uptake by Triticum aestivum L.—effects of rhizosphere concentration and counterion

Organic acids such as citrate and oxalate have been implicated in enhancing many rhizosphere processes including nutrient acquisition. This study was conducted to determine the importance of organic acid type and concentration on rhizosphere P mobilization and subsequent uptake by wheat (Triticum aestivum L.) roots and its translocation to shoots. A single wheat plant was grown in soil-filled rhizosphere microcosms and allowed to pass through a KH₂PO₄ ³³P-isotopically labeled patch of calcareous soil. Two days after ³³P-injection, citrate and oxalate at concentrations of 1 mM and 10 mM were injected into the microcosms at the same patch every day over a period of 4 days. Oxalate resulted in a several-fold enhancement in plant ³³P accumulation, while citrate had no such effect. In comparison with oxalate, high rates of citrate mineralization were observed suggesting that this reduced its potential to enhance plant ³³P acquisition. This study concludes that organic acids cause an increase in P mobilization and P uptake by wheat but that this response is highly organic acid specific.     

Growth and Energy Generation by Lactococcus lactis subsp. lactis biovar diacetylactis during Citrate Metabolism

Growth of Lactococcus lactis subsp. lactis biovar diacetylactis was observed on media with citrate as the only energy source. At pH 5.6, steady state was achieved in a chemostat on a citrate-containing medium in the absence of a carbohydrate. Under these conditions, pyruvate, acetate, and some acetoin and butanediol were the main fermentation products. This indicated that energy was conserved in L. lactis subsp. lactis biovar diacetylactis during citrate metabolism and presumably during the conversion of citrate into pyruvate. The presumed energy-conserving step, decarboxylation of oxaloacetate, was studied in detail. Oxaloacetate decarboxylase was purified to homogeneity and characterized. The enzyme has a native molecular mass of approximately 300 kDa and consists of three subunits of 52, 34, and 12 kDa. The enzyme is apparently not sodium dependent and does not contain a biotin moiety, and it seems to be different from the energy-generating oxaloacetate decarboxylase from Klebsiella pneumoniae. Energy-depleted L. lactis subsp. lactis biovar diacetylactis cells generated a membrane potential and a pH gradient immediately upon addition of citrate, whereas ATP formation was slow and limited. In contrast, lactose energization resulted in rapid ATP formation and gradual generation of a proton motive force. These data were confirmed during studies on amino acid uptake. α-Aminoisobutyrate uptake was rapid but glutamate uptake was slow in citrate-energized cells, whereas lactose-energized cells showed the reverse tendency. These data suggest that, in L. lactis subsp. lactis bv. diacetylactis, a proton motive force could be generated during citrate metabolism as a result of electrogenic citrate uptake or citrate/product exchange together with proton consumption by the intracellular oxaloacetate decarboxylase.     

pH affects ammonium,nitrate and proton fluxes in the apical region of conifer and soybean roots

The effect of pH on nitrate and ammonium uptake in the high‐affinity transport system and low‐affinity transport system ranges was compared in two conifers and one crop species. Many conifers grow on acidic soils, thus their preference for ammonium vs nitrate uptake can differ from that of crop plants, and the effect of pH on nitrogen (N) uptake may differ. Proton, ammonium and nitrate net fluxes were measured at seedling root tips and 5, 10, 20 and 30 mm from the tips using a non‐invasive microelectrode ion flux measurement system in solutions of 50 or 1500 µM NH₄NO₃ at pH 4 and 7. In Glycine max and Pinus contorta, efflux of protons was observed at pH 7 while pH 4 resulted in net proton uptake in some root regions. Pseudotsuga menziesii roots consistently showed proton efflux behind the root tip, and thus appear better adapted to maintain proton efflux in acid soils. P. menziesii's ability to maintain ammonium uptake at low pH may relate to its ability to maintain proton efflux. In all three species, net nitrate uptake was greatest at neutral pH. Net ammonium uptake in G. max and net nitrate uptake in P. menziesii were greatly reduced at pH 4, particularly at high N concentration, thus N concentration should be considered when determining optimum pH for N uptake. In P. menziesii and G. max, net N uptake was greater in 1500 than 50 µM NH₄NO₃ solution, but flux profiles of all ions varied among species.     

Effects of reduced seawater pH on fertilisation, embryogenesis and larval development in the Antarctic seastar Odontaster validus

The effects of ocean acidification will be pronounced in high-latitude marine communities, although little is known on how reproduction in free-spawning polar invertebrates will respond. Using the circum-Antarctic sea star Odontaster validus, we examined fertilisation, larval survival and development under a controlled seawater treatment (temperature = ?0.5 °C, pH 8.1, pCO₂(aq) = 326.6 μatm, TA = 2,274.2 μmol kg soln^?1), two near-future pH treatments (pH 7.8 and 7.6) and an extreme treatment (pH 7.0). At a sperm concentration of 3.5 × 10⁵ sperm ml^?1, percentage of fertilisation was 98–90 % across a pH 8.1–7.0 range. At near-future pH ranges (pH 7.8 and 7.6), fertilisation was not significantly lower than in the control pH 8.1 except at the lowest sperm concentration (2.2 × 10³ sperm ml^?1) where fertilisation was reduced to 60 and 61 % in pH 7.6 and 7.8, respectively. Larval survival was not significantly affected by a decrease in pH of 0.3 units, but at pH 7.6 survival was significantly reduced. This difference was apparent at 9 days, and at the end of the experiment at 58 days, survival was 55 % compared with 85 % in the ambient treatment. Near-future changes to pH yielded smaller larvae, a result of both subtle differences in their morphology and slowed development rates, while larvae at pH 7.0 showed evidence of abnormal development. O. validus fertilisation and larval success declines in seawater pH conditions expected in coastal Antarctica over the coming decades, although the responses observed are within the range observed in warmer-water echinoderms.     

Cation binding to beta-casein. A comparison of electrostatic models

The binding of cations of β-casein at pH 6.6 was considered previously. Available for three sodium concentiations, I = 0.04, 0.08, or 0.16 M are: [1] proton releases between I and [2] for each I, as calcium activity is increased, correlated sequences of monomer net charge, proton release, site bound calcium and protein Solvation- Models for ion binding are examined. Critical considerations are the intrinsic binding constants between hydrogen[H], calcium[Ca] and sodium[Na] ions and phosphate[P] and caiboxyIate[C] sites, and the effects of electrostatic interaction between sites as influenced by spatial fixed charge distribution, ionic strength and dielectric constant [D]. Anticipated intrinsic binding constants are k_H,P^o = 3 × 10⁶, k_Ca,P^o = 120, k_Na,P^o = 1, k_H,C^o = 7 × 10⁴ and k_Ca,C^o = 5.6Distributed charge models, either surface or volume, are inadequate since any reasonable monomer size yields fixed charge densities requiring k_H,P^o and k_Ca,C^o which are too low when the maximum in D is 75. Also, with increasing calcium binding, calculated proton release is only 0.4 to 0.5 of that observed.Discrete charge models accept anticipated k^o and yield calculated sequences of calcium binding and proton release which are in good agreement with those observed provided that: (1) using the known amino acid sequence of the phosphate-containing acidic peptide portion of the molecule, pep tide fixed charge is distributed at the lowest I so as to minimize electrostatic free energy; (2) in the region of fixed charge, D is approximately 5; (3) the distances between peptide fixed charges decrease with increasing ionic strength or calcium binding and (4) while protein is in solution, the acidic peptide and the remainder of the molecule are essentially electrostatically independent.     

Effect of Drug Loading Method on Drug Content and Drug Release from Calcium Pectinate Gel Beads

Drug-loaded calcium pectinate gel (CaPG) beads were prepared by either mixing, absorption, or swelling method. The effects of drug loading method as well as the drug loading factors (i.e., drug concentration, soaking time in drug solution, type of solvent) on drug content and drug release were investigated. The amount of drug uptake (i.e., drug content) into CaPG beads increased as the initial drug concentration increased and varied depending on the loading method. The in vitro release studies in 0.1 N hydrochloric acid (HCl) and pH 6.8 buffer indicated that the drug loading method affected drug release and release parameter, time for 50% of drug release (T ₅₀). The mixing method provided a faster drug release and lower T ₅₀ than the absorption method and swelling method, respectively. This is probably due to higher drug content in CaPG beads. The increased concentration of drug in soaking solution and soaking time resulted in higher drug content and thus faster drug release (lower in T ₅₀ values). When using 0.1 N HCl as solvent for soaking instead of water, the drug release was slower owing to the increase in molecular tortuosity of CaPG beads. The drug release was also affected by pH of the release medium in which drug release in 0.1 N HCl was faster than in pH 6.8 buffer.