Aluminum resistance in two cultivars of Zea mays L.: Root exudation of organic acids and influence of phosphorus nutrition

Root exudation of organic acids as Al-chelating compounds and P nutrition have been suggested to play a major role in Al-resistance in higher plants. Effects of Al exposure on maize plant growth, and organic acid root content and root exudation under various levels of P nutrition were examined. Sikuani, a Colombian maize cultivar tolerant to acid soils with high Al saturation, and Corso, a Swiss cultivar, were grown in sterile hydroponic conditions for 21 days. Al-caused inhibition of root growth was lower in Sikuani than in Corso. Al effect on plant growth was decreased with increasing P content in roots. Al content in roots increased with increasing P content and was higher in Sikuani than in Corso. When exposed to Al, the contents in root apices as well as the root exudation of citric and malic acids in Corso and citric, malic and succinic acids in Sikuani increased, and were higher in Sikuani than in Corso. Increased PEP carboxylase (PEPC) activity in root apices after Al exposure partially explained the variations of organic acid content in the roots. These Al-induced changes in PEPC activity, organic acid content and exudation were reduced in plants supplied with higher P concentrations during the 21 days prior to treatment. Increased secretion of organic acids after exposure to Al appeared to be specific to Al and was not totally explained by increased root content in organic acids.     

Environmental Control of Root Exudation of Low-Molecular Weight Organic Acids in Tropical Rainforests

The availability of phosphorus (P) can limit net primary production (NPP) in tropical rainforests growing on highly weathered soils. Although it is well known that plant roots release organic acids to acquire P from P-deficient soils, the importance of organic acid exudation in P-limited tropical rainforests has rarely been verified. Study sites were located in two tropical montane rainforests (a P-deficient older soil and a P-rich younger soil) and a tropical lowland rainforest on Mt. Kinabalu, Borneo to analyze environmental control of organic acid exudation with respect to soil P availability, tree genus, and NPP. We quantified root exudation of oxalic, citric, and malic acids using in situ methods in which live fine roots were placed in syringes containing nutrient solution. Exudation rates of organic acids were greatest in the P-deficient soil in the tropical montane rainforest. The carbon (C) fluxes of organic acid exudation in the P-deficient soil (0.7?mol?C?m^?2?month^?1) represented 16.6% of the aboveground NPP, which was greater than those in the P-rich soil (3.1%) and in the lowland rainforest (4.7%), which exhibited higher NPP. The exudation rates of organic acids increased with increasing root surface area and tip number. A shift in vegetation composition toward dominance by tree species exhibiting a larger root surface area might contribute to the higher organic acid exudation observed in P-deficient soil. Our results quantitatively showed that tree roots can release greater quantities of organic acids in response to P deficiency in tropical rainforests.     

Accumulation and transport of nickel in relation to organic acids in ryegrass and maize grown with different nickel levels

Difference in Ni tolerance/accumulation in plant genotypes might be used to identify or develop plants for remediation of high Ni soils. Ryegrass was shown to be more sensitive to Ni toxicity and accumulated much more Ni in shoots than maize. The objectives of this study were to examine the relationship of organic acids to Ni accumulation and xylem transport of Ni in ryegrass (Lolium perenne L.) and maize (Zea mays L.). The results showed that accumulation of Ni in shoots was 5 to 7 fold higher in ryegrass than in maize grown at 20 to 80 µM Ni, whereas Ni concentration in ryegrass roots was only 1 to 2 fold higher at 0.1 to 40 µM Ni and 1.5-fold lower at 80 µM Ni than that of maize roots. Xylem transport rates of Ni increased with increasing Ni supply for both species, and were about 2 to 7 times higher in ryegrass than in maize. Shoot concentrations of citric, malic, oxalic and cis-aconitic acids increased at Ni levels above 20 µM, and were about 2 to 6 times higher in ryegrass than in maize. Whereas, maize roots accumulated greater amount of malic, oxalic, and cis-aconitic acids than ryegrass roots, especially at Ni levels of 40-80 µM. The rate of Ni exudation by roots in the two species was significantly correlated with root Ni concentrations. It could be concluded that high Ni accumulation in shoots was closely related to high xylem transport rates of Ni and that the accumulation of organic acids, citric and malic acid in particular. A high root exudate rate of Ni and the enhanced accumulation of organic acids, malic acid in particular, in roots might be among the important factors which are associated with the tolerance of crops to toxic Ni levels.     

Spatial and temporal dynamics of the microbial community structure in the rhizosphere of cluster roots of white lupin (Lupinus albus L.)

White lupin was grown in a quartz sand–soil mix with poorly available Ca phosphate. The plants were harvested on days 21, 35 and 51 and DNA was extracted from the non-cluster roots, the young, mature and senescent cluster roots with adhering soil. Bacterial community structure was examined by PCR-DGGE of 16S rDNA, digitisation of the band patterns and multivariate analyses. In all root zones the bacterial community structure changed with plant age. The communities in the rhizosphere of the non-cluster roots were always different from those of the cluster roots. The bacterial communities of the cluster roots were cluster age and plant age dependent. The differences in bacterial community structure between the cluster root age classes were significant on days 35 and day 51 but not on d 21. A separate experiment, in which root exudates and samples for PCR-DGGE were collected simultaneously, showed that both bacterial and eukaryotic (18S rDNA) community structures change with organic acid exudation. While eukaryotic community structure of the cluster roots was correlated with citric acid exudation, bacterial community structure was correlated with cis-acconitic, citric and malic acid exudation.     

子叶磷在白羽扇豆缺磷适应性反应中的作用

实验用液体培养的方法,对比分析了在不同供磷条件下,白羽扇豆子叶中的磷对植物生长发育的影响,以及排根和根尖中有机酸积累和分泌的作用,结果表明,子叶中的磷能使白羽扇豆在完全缺磷２３d的环境中,不仅没有使干物质的积累减少,反而使干物质的积累略有增加,相反,如果没有子叶磷的供给,则使白羽扇豆在缺磷环境中产生强烈的抗胁迫反应,表现在干物质的积累明显下降,根系能产生大量的排根,排根能积累和分泌大量的柠檬酸,而根尖能积累和分泌萍果酸,在整个缺磷反应过程中,根尖中苹果酸的分泌要早于排根可柠檬酸的积累和分泌。     

Genotypic variability in phosphorus solubilizing activity of root exudates by pigeonpea grown in low-nutrient environments

A pot experiment confirmed that pigeonpea could efficiently utilize various sources of phosphorus (P) (aluminium phosphate, iron phosphate and apatite), irrespective of genotype. A qualitative assay method for iron (Fe)-P solubilizing activity showed that root exudates collected from P-deficient pigeonpea contained Fe-P solubilizing substances and that they were released mainly from root tips. Citric, malic, malonic, succinic and piscidic acids were identified in root exudates. Citric and piscidic acids release from roots was increased by low-P treatment in all the genotypes tested. The release rates of citric and piscidic acids were affected by the P concentration of shoots rather than that of roots. The pigeonpea roots released approximately 5–100 times more piscidic acid than citric acid depending on P stress status, plant age and genotype. When organic acids were added to Alfisols, citric acid was most capable of mobilizing P from the soil, followed by piscidic acid and malic acid. No correlation was found between genotypic variability in the release rates of citric and piscidic acids from the roots under low-P treatment at hydroponic culture and in the growth and P uptake of plants on Alfisols. Although citric and piscidic acids released from pigeonpea roots may play a partial role in solubilizing unavailable insoluble P in soils, the releases were thought to be an unsatisfactory strategy for explaining genotypic variation in low P availability of pigeonpea.     

Root carboxylate exudation capacity under phosphorus stress does not improve grain yield in green gram

Key message

Genetic variability in carboxylate exudation capacity along with improved root traits was a key mechanism for P-efficient green gram genotype to cope with P-stress but it did not increase grain yield.

Abstract

This study evaluates genotypic variability in green gram for total root carbon exudation under low phosphorus (P) using ¹⁴C and its relationship with root exuded carboxylates, growth and yield potential in contrasting genotypes. Forty-four genotypes grown hydroponically with low (2 μM) and sufficient (100 μM) P concentrations were exposed to ¹⁴CO₂ to screen for total root carbon exudation. Contrasting genotypes were employed to study carboxylate exudation and their performance in soil at two P levels. Based on relative ¹⁴C exudation and biomass, genotypes were categorized. Carboxylic acids were measured in exudates and root apices of contrasting genotypes belonging to efficient and inefficient categories. Oxalic and citric acids were released into the medium under low-P. PDM-139 (efficient) was highly efficient in carboxylate exudation as compared to ML-818 (inefficient). In low soil P, the reduction in biomass was higher in ML-818 as compared to PDM-139. Total leaf area and photosynthetic rate averaged for genotypes increased by 71 and 41 %, respectively, with P fertilization. Significantly, higher root surface area and volume were observed in PDM-139 under low soil P. Though the grain yield was higher in ML-818, the total plant biomass was significantly higher in PDM-139 indicating improved P uptake and its efficient translation into biomass. The higher carboxylate exudation capacity and improved root traits in the later genotype might be the possible adaptive mechanisms to cope with P-stress. However, it is not necessary that higher root exudation would result in higher grain yield.     

Shoot P status regulates cluster-root growth and citrate exudation in Lupinus albus grown with a divided root system

The present study was carried out to investigate whether the P concentration in the roots or the shoots controls the growth and citrate exudation of cluster roots in white lupin (Lupinus albus L). Foliar P application indicated that low P concentration in the shoots enhanced cluster‐root growth and citrate‐exudation rate more so than low P concentration in the roots. In the split‐root study, the P concentration in the shoots increased with increased P supply (1, 25 or 75 mmol m^?3 P), to the ‘privileged’ root halves. Roots ‘deprived’ of P invariably had the same low P concentrations, whereas those in the ‘privileged’ roots increased with increasing P supply (1, 25 or 75 mmol m^?3 P). Nevertheless, the proportion of the total root mass allocated to cluster roots, and the citrate‐exudation rates from the root halves were always similar on both root halves, irrespective of P supply, and decreased with increasing shoot P concentrations. Peak citrate exudation rates from developing cluster roots were significantly faster from cluster roots on the ‘deprived’ root halves when the ‘privileged’ half was exposed to 1 mmol m^?3 P as compared with 25 or 75 mmol m^?3 P. The possibility that changes in the concentrations of P fractions in the root halves influenced cluster‐root growth and citrate exudation was discounted, because there were no significant differences in insoluble organic P, ester‐P and inorganic P among all ‘deprived’ root halves. The results indicate that cluster‐root proportions and citrate exudation rates were regulated systemically by the P status of the shoot, and that P concentrations in the roots had little influence on growth and citrate exudation of cluster roots in L. albus.     

低磷胁迫下AM真菌对枳实生苗吸磷效应及根系分泌有机酸的影响

在温室沙培灭菌条件下,以Al-P为磷源、枳为试材、Glomus mosseae (G.m)和G.versiforme (G.v)为菌剂,研究低磷胁迫下AM真菌对枳实生苗干物重、吸磷效应及根系分泌有机酸的影响。结果表明,接种AM真菌显著增加枳地上部、地下部干物重,增幅16.79%~135.25%;同时显著增加其吸磷量,菌丝对植株的吸磷贡献率为17.04%~71.95%(G.m>G.v),施Al-P显著提高菌丝吸磷贡献率。接种AM真菌的根系分泌的有机酸种类与对照有所不同,未接种处理枳分泌的有机酸有草酸、苹果酸、乳酸、乙酸、顺丁烯二酸和柠檬酸等6种,而接种G.m的则检测到草酸、酒石酸、苹果酸、乳酸、乙酸、柠檬酸、丁二酸等7种,G.v处理的检测到酒石酸,接种处理均未检测到顺丁烯二酸;接种丛枝菌根真菌增加了枳根系分泌有机酸的量(比未接种处理增加19.80~56.87 mg/kg,且施用AlPO₄后有机酸含量显著增加(增加20.06~21.84 mg/kg);未接种植株根系仅分泌少量有机酸;接种植株根系分泌的有机酸以苹果酸(42.87%)、柠檬酸(39.22%)和草酸(12.06%)为主。     

Carboxylate metabolism in sugar beet plants grown with excess Zn

The effects of Zn excess on carboxylate metabolism were investigated in sugar beet (Beta vulgaris L.) plants grown hydroponically in a growth chamber. Root extracts of plants grown with 50 or 100 μM Zn in the nutrient solution showed increases in several enzymatic activities related to organic acid metabolism, including citrate synthase and phosphoenolpyruvate carboxylase, when compared to activities in control root extracts. Root citric and malic acid concentrations increased in plants grown with 100 μM Zn, but not in plants grown with 50 μM Zn. In the xylem sap, plants grown with 50 and 100 μM Zn showed increases in the concentrations of citrate and malate compared to the controls. Leaves of plants grown with 50 or 100 μM Zn showed increases in the concentrations of citric and malic acid and in the activities of citrate synthase and fumarase. Leaf isocitrate dehydrogenase increased only in plants grown with 50 μM Zn when compared to the controls. In plants grown with 300 μM Zn, the only enzyme showing activity increases in root extracts was citrate synthase, whereas the activities of other enzymes decreased compared to the controls, and root citrate concentrations increased. In the 300 μM Zn-grown plants, the xylem concentrations of citric and malic acids were higher than those of controls, whereas in leaf extracts the activity of fumarase increased markedly, and the leaf citric acid concentration was higher than in the controls. Based on our data, a metabolic model of the carboxylate metabolism in sugar beet plants grown under Zn excess is proposed.     

Rhizosphere carboxylate concentrations of chickpea are affected by genotype and soil type

We investigated whether concentrations of carboxylates in the rhizosphere of chickpea (Cicer arietinum L.) roots were related to soil phosphorus levels. In a field experiment, cultivar Sona was grown at two P levels on eight soil types at three locations. There were large differences in extractable (0.2 mM CaCl₂) rhizosphere carboxylate concentrations amongst the locations. The effect of P fertiliser was variable and carboxylate concentrations depended on soil type. To examine the effect of soil P in more detail, a glasshouse experiment was carried out, in which three cultivars (Heera, Sona and Tyson) were grown at four P levels on one soil type. The biomass of chickpea plants increased with increasing P level of the soil, and the root mass ratio decreased at the highest soil P level. However, rhizosphere concentrations of the carboxylates malonate, malate and citrate did not differ significantly between P treatments. This implied that there was no simple relation between available P and root exudation rates, in contrast to earlier results in studies using hydroponics. Cultivars differed in carboxylate concentration pattern: Sona and Tyson showed a tendency towards increased rhizosphere carboxylate concentrations at the second harvest, whereas the carboxylate concentration of Heera tended to decrease. It is hypothesised that chickpea roots always exude a basal level of carboxylates into the rhizosphere. They only increase carboxylate exudation considerably when the P availability is extremely low, which may occur in soils that strongly bind P.     

Differences between calcifuge and acidifuge plants in root exudation of low-molecular organic acids

The nature and quantity of low-molecular organic acids (LOAs) exuded by the roots of nine species of calcifuge and nine species of acidifuge wild plants from northern Europe were determined by ion chromatography. Particular attention was paid to differences between the calcifuge and the acidifuge species in the proportions of different LOAs in their root exudates. Great differences in mol% root exudation between the calcifuge and the acidifuge species were found in some acids. The calcifuge species exuded more acetic acid, the acidifuge species more oxalic acid and much more citric acid. In three calcifuge species, however, root exudation of oxalic acid was appreciable, whereas acetic acid exudation was low in these species. The phosphate- and Fe-solubilizing ability of eight LOAs in a rhizosphere limestone soil was also tested. Oxalic acid was the most efficient phosphate solubilizer and citric acid, by far, the most efficient Fe-solubilizer at the concentration (10 mM) tested. It might be hypothesized that acidifuge species use oxalate to solubilize phosphate and citrate to solubilize Fe, in limestone soil. The inability of calcifuge species to grow in limestone soil might, therefore, be due to low root exudation of these acids and, as a result, inability to solubilize phosphate and Fe in limestone soil.     

Citrate, Malate, and Succinate Concentration in Exudates from P-Sufficient and P-Stressed Medicago sativa L. Seedlings

Under certain stress conditions roots exude organic molecules, which may facilitate the uptake of nutrients. The objective of this research was to identify and measure the effect of low P upon the exudation of organic acids by roots of alfalfa (Medicago sativa L.) seedlings. Surface-sterilized alfalfa seeds were grown aseptically in sterile sand using an apparatus specially designed for the addition of ±P nutrient solutions and for the collection of root exudates. Citric, malic, and succinic acids were detected in the root exudates of 24-day-old alfalfa seedlings. Citrate exudation from the roots of P-stressed alfalfa was 182% that of plants receiving a complete nutrient solution. The increased release of citrate may provide a mechanism by which P-stressed plants enhance the availability of P in the rhizosphere.     

Roles of Organic Acids and Nitrate in the Long-Distance Transport of Cobalt in Xylem Saps of <Emphasis Type="Italic">Alyssum murale</Emphasis> and <Emphasis Type="Italic">Trifolium subterraneum</Emphasis>

Roles of organic acids and nitrate in the long-distance transport of cobalt (Co) in xylem saps of hyperaccumulator Alyssum murale and non-hyperaccumulator Trifolium subterraneum were studied under hydroponic conditions. Organic acids (oxalic, malic, malonic, citric, and fumaric) and nitrate in xylem sap samples were separated and determined simultaneously by reversed-phase high performance liquid chromatography after solid-phase extraction with nanosized hydroxyapatite. Results indicated that Co treatment significantly increased the concentrations of xylem oxalic and malic acids for the hyperaccumulator A. murale compared to the control but significantly decreased the concentrations of xylem nitrate and malonic acid; concentrations of citric acid in xylem sap samples did not show significant difference between the control and Co treatments. By analyzing the relationship between the concentrations of organic acids, nitrate, and concentrations of Co in xylem saps, it could be concluded that oxalic and malic acids in xylem saps seemed to participate in the long-distance Co translocation process, and citric acid did not relate to the xylem Co transport of A. murale and T. subterraneum. Our work might be very useful for understanding the mechanism of long-distance transport of heavy metals in hyperaccumulator.     

Physiological Aspects of Cluster Root Function and Development in Phosphorus-deficient White Lupin (Lupinus albus L.)

Cluster root formation in white lupin (Lupinus albus L.) isinduced mainly by phosphorus (P) starvation, and seems to beregulated by the endogenous P status of the plant. Increasedformation of cluster roots, when indole acetic acid is suppliedto the growth medium of P sufficient plants, and inhibitoryeffects of kinetin application suggest the involvement of endogenousphytohormones (auxins and cytokinins), which may act in an antagonisticmanner in the P-starvation response. Phosphorus deficiency-inducedadaptations of white lupin, involved in P acquisition and mobilizationof sparingly available P sources, are predominantly confinedto the cluster roots, and moreover to distinct stages duringtheir development. Increased accumulation and exudation of citrateand a concomitant release of protons were found to be mainlyrestricted to mature root clusters after prolonged culture (3–4weeks) under P-deficient conditions. Inhibition of citrate exudationby exogenous application of anion channel antagonists such asethacrynic- and anthracene-9-carboxylic acids may indicate involvementof an anion channel. Phosphorus deficiency-induced accumulationand subsequent exudation of citric acid seems to be a consequenceof both enhanced biosynthesis and reduced turnover of citricacid in the cluster root tissue, indicated by enhanced expressionof sucrose synthase, fructokinase, phosphoglucomutase, phosphoenol-pyruvatecarboxylase, but reduced activity of aconitase and slower rootrespiration. The release of acid phosphatase and of phenoliccompounds (isoflavonoids) as well as the induction of a putativehigh-affinity P uptake system was more highly expressed in juvenile,mature and even senescent cluster regions than in apical zonesof non-proteoid roots. An AFLP-cDNA library for cluster root-specificgene expression was constructed to assist in the identificationof further genes involved in cluster root development. Copyright2000 Annals of Botany Company Acid phosphatase, auxin, citric acid, cluster roots, cytokinin, Lupinus albus L., P acquisition, P uptake, root exudates     

Spatial and temporal variation in organic acid anion exudation and nutrient anion uptake in the rhizosphere of Lupinus albus L.

We investigated in situ the temporal patterns and spatial extent of organic acid anion exudation into the rhizosphere solution of Lupinus albus, and its relation with the nutrient anions phosphate, nitrate and sulfate by means of a rhizobox micro suction cup method under P sufficient conditions. We compared the soil solution in the rhizosphere of cluster roots with that in the vicinity of normal roots, nodules and bulk soil. Compared to the other rhizosphere and soil compartments, concentrations of organic acid anions were higher in the vicinity of cluster roots during the exudative burst (citrate, oxalate) and nodules (acetate, malate), while concentrations of inorganic nutrient anions were highest in the bulk soil. Both active cluster roots and nodules were most efficient in taking up nitrate and phosphate. The intensity of citrate exudation by cluster roots was highly variable. The overall temporal patterns during the lifetime of cluster roots were overlaid by a diurnal pattern, i.e. in most cases, the exudation burst consisted of one or more peaks occurring in the afternoon. Multiple exudation peaks occurred daily or were separated by 1 or 2 days. Although citrate concentrations decreased with distance from the cluster root apex, they were still significantly higher at a distance of 6 to 10 mm than in the bulk soil. Phosphate concentrations were extremely variable in the proximity of cluster roots. While our results indicate that under P sufficient conditions cluster roots take up phosphate during their entire life time, the influence of citrate exudation on phosphate mobilization from soil could not be assessed conclusively because of the complex interactions between P uptake, organic acid anion exudation and P mobilization. However, we observed indications of P mobilization concurrent with the highest measured citrate concentrations. In conclusion, this study provides semiquantitative in situ data on the reactivity of different root segments of L. albus L. in terms of root exudation and nutrient uptake under nutrient sufficient conditions, in particular on the temporal variability during the lifetime of cluster roots.     

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.     

Effect of Pseudomonas fluorescens on the root exudates of two tomato mutants differently sensitive to Fe chlorosis

Plants with different Fe-mobilization properties are known to differ in the amount and kind of Fe-reducing and Fe-chelating compounds exuded by their roots. Although rhizosphere bacteria are known to affect the exudation of organic compounds by the plant roots, their effect on the root exudates of plants differing in Fe-mobilization properties is not known. We studied the effect of Pseudomonas fluorescens, on the exudation of sugars and organic and amino acids by roots of an iron chlorosis-resistant (T3238FER) and a chlorosis-susceptible (T3238fer) tomato mutant. Under sterile conditions two tomato mutants grew equally well and did not differ in the total amount of sugars and organic acid exuded by their roots. More amino acids, however, were exuded by the roots of T3238FER than T323fer. Mutants differed in the amount of oxalic acid and the amino acids Ala, Asp, Gaba, Gln, Gly, His, Hyl, Ile, Leu, Lys, Phe, Pro, and Val exuded by their roots into sterile rooting media. Addition of P. fluorescens to the rooting medium did not affect the growth of T3238FER but stimulated the root growth of chlorosis-susceptible T3238fer, reduced the amounts of glucose, arabinose and fructose but increased the amount of sucrose, reduced the amounts of fumaric, malic and oxalic acid but increased the amounts of citric and succinic acid in the rooting media of both mutants. P. fluorescens resulted in the following changes in the amino acids in the rooting media: reduced the amounts of Gly, Leu, and Lys in T3238FER, and of Asp, Gln, Hyp, and Ile in T3238fer, and increased the amounts of Cys, Glu, His, Hyp, Ile, Phe and Tyr in T3238FER and of Ala, Glu, His, Phe, and Ser in T323fer—in cases more than 40-fold. These differential effects of P. fluorescens in altering the pattern of organic and amino acids compounds with some Fe-chelating properties detected in the rooting medium of these two mutants may indicate that the differences in Fe-chlorosis susceptibility of these tomato mutants may be the result of, or modified by, the interactions between plant roots and rhizosphere microorganisms. We postulate that the Fe-chlorosis susceptibility in plants may be the product of the interactions between soil microorganisms and plant roots, and may not be solely related to the plant per se.     

Physiological adaptations to phosphorus deficiency during proteoid root development in white lupin

Release of large amounts of citric acid from specialized root clusters (proteoid roots) of phosphorus (P)-deficient white lupin (Lupinus albus L.) is an efficient strategy for chemical mobilization of sparingly available P sources in the rhizosphere. The present study demonstrates that increased accumulation and exudation of citric acid and a concomitant release of protons were predominantly restricted to mature root clusters in the later stages of P deficiency. Inhibition of citrate exudation by exogenous application of anion-channel blockers such as ethacrynic- and anthracene-9-carboxylic acids may indicate involvement of an anion channel. Phosphorus-deficiency-induced accumulation and subsequent exudation of citric acid seem to be a consequence of both increased biosynthesis and reduced metabolization of citric acid in the proteoid root tissue, indicated by increased in-vitro activity and enzyme protein levels of phosphoenolpyruvate carboxylase (EC 4.1.1.31), and reduced activity of aconitase (EC 4.2.1.3) and root respiration. Similar to citric acid, acid phosphatase, which is secreted by roots and involved in the mobilization of the organic soil P fraction, was released predominantly from proteoid roots of P-deficient plants. Also ³³Pi uptake per unit root fresh-weight was increased by approximately 50% in juvenile and mature proteoid root clusters compared to apical segments of non-proteoid roots. Kinetic studies revealed a K _m of 30.7 μM for Pi uptake of non-proteoid root apices in P-sufficient plants, versus K _m values of 8.5–8.6 μM for non-proteoid and juvenile proteoid roots under P-deficient conditions, suggesting the induction of a high-affinity Pi-uptake system. Obviously, P-deficiency-induced adaptations of white lupin, involved in P acquisition and mobilization of sparingly available P sources, are predominantly confined to proteoid roots, and moreover to distinct stages during proteoid root development. Received: 10 September 1998 / Accepted: 22 December 1998     

Influence of arbuscular mycorrhiza on organic solutes in maize leaves under salt stress

A pot experiment was conducted to examine the effect of the arbuscular mycorrhizal (AM) fungus, Glomus mosseae, on plant biomass and organic solute accumulation in maize leaves. Maize plants were grown in sand and soil mixture with three NaCl levels (0, 0.5, and 1.0 g kg⁻¹ dry substrate) for 55 days, after 15 days of establishment under non-saline conditions. At all salinity levels, mycorrhizal plants had higher biomass and higher accumulation of organic solutes in leaves, which were dominated by soluble sugars, reducing sugars, soluble protein, and organic acids in both mycorrhizal and non-mycorrhizal plants. The relative abundance of free amino acids and proline in total organic solutes was lower in mycorrhizal than in non-mycorrhizal plants, while that of reducing sugars was higher. In addition, the AM symbiosis raised the concentrations of soluble sugars, reducing sugars, soluble protein, total organic acids, oxalic acid, fumaric acid, acetic acid, malic acid, and citric acid and decreased the concentrations of total free amino acids, proline, formic acid, and succinic acid in maize leaves. In mycorrhizal plants, the dominant organic acid was oxalic acid, while in non-mycorrhizal plants, the dominant organic acid was succinic acid. All the results presented here indicate that the accumulation of organic solutes in leaves is a specific physiological response of maize plants to the AM symbiosis, which could mitigate the negative impact of soil salinity on plant productivity.