Relative Contribution of Apices and Mature Tissues to ABA Synthesis in Droughted Maize Root Systems

We investigated whether different parts of maize root systemscould contribute to ABA synthesis, and whether a previous cycleof soil dehydration-rehydration would modify the ability ofroots to synthesize ABA. Maize (Zea mays L.) root tissues, i.e.mature primary root sections, young and unbranched primary rootsections, secondary roots and primary root tips, from both wellwatered plants and previously drought-rewatered plants, weresubjected to different degrees of dehydration and their ABAconcentration changes were assayed. All categories of rootsfrom always well watered plants, including mature tissues containingno apex, could synthesize ABA when dehydrated. Mature primaryroot sections and their previously associated secondary rootsaccumulated less ABA in response to dehydration than the youngprimary roots and primary root tips did, and their ABA accumulationwas not substantial until dehydration was below 65% of relativewater content (RWC). Previous soil dehydration-rehydration cyclessubstantially reduced ABA accumulation in these roots in responseto dehydration again. Young primary root sections and primaryroot tips accumulated ABA much more sensitively in responseto dehydration than mature root sections, although considerablevariations existed among different batches of young primaryroot sections. Results are discussed in the context of the relativecontribution of different categories of roots to ABA synthesiswhen the root system is in drying soil. We concluded that primaryroot apices should not contribute by more than 2% to the totalABA synthesis by the root system. (Received December 15, 1995; Accepted April 19, 1996)     

Effect of indoleacetic acid,abscisic acid,root tips and coleoptile tips on growth and curvature of maize roots

The effect of externally applied indoleacetic acid (IAA) and abscisic acid (ABA) on the growth of roots of Zea mays L. was measured. Donor blocks of agar with IAA or ABA were placed laterally on the roots and root curvature was measured. When IAA was applied to vertical roots, a curvature directed toward the donor block was observed. This curvature corresponded to a growth inhibition at the side of the root where the donor was applied. When IAA was applied to horizontal roots from the upper side, normal geotropic downward bending was delayed or totally inhibited. The extent of retardation and the inhibition of curvature were found to depend on the concentration of IAA in the donor block. ABA neither induced curvature in vertical roots nor inhibited geotropic curvature in horizontal roots; thus the growth of roots was not inhibited by ABA. However, when, instead of donor blocks, root tips or coleoptile tips were placed onto vertical roots, a curvature of the roots was observed.Abbreviations ABA abscisic acid - IAA 3-indoleacetic acid     

Increased Synthesis of ABA in Partially Dehydrated Root Tips and ABA Transport from Roots to Leaves

Zhang, J. and Davies, W. J. 1987. Increased synthesis of ABAin partially dehydrated root tips and ABA transport from rootsto leaves.—J. exp. Bot. 38: 2015–2023. Isolated root tips of pea (Pisum sativum L. cv. Feltham First)and Commelina communis L. were air-dried until they lost between10% and 40% of their fresh weight, followed by a period of incubationat these reduced water contents. These treatments resulted inincreased ABA production, suggesting that root tips of bothspecies have the capacity to synthesize ABA in increased amountswhen water deficits develop in the root. The ABA concentrationin pea roots increased linearly as turgors fell below about0·15 M Pa and relative water contents (R WC) fell below90%. Commelina roots produced more ABA when RWC fell below asimilar value but the threshold turgor for increased ABA productionin Commelina roots was around 0·30 MPa. Roots of intact plants loaded with ABA as a result of incubationin solutions of varying concentrations provided ABA to leaveswhich resulted in increased ABA concentrations in the leaveswhen these were assayed several hours later. This occurred whenthese roots were not contributing substantially to transpirationalflux. Leaves on shoots that were enclosed and darkened and thereforenot transpiring, did not accumulate ABA from ‘loaded’roots. A role for root-sourced ABA in root-to-shoot communication ofthe effects of soil drying is discussed. Key words: ABA, roots, water relations     

Abscisic Acid Response of Corn (<Emphasis Type="Italic">Zea mays</Emphasis> L.) Roots and Protoplasts to Lanthanum

Lanthanum ions antagonize calcium and are used as a Ca²⁺ channel blocker but their direct effects are unknown. We investigated lanthanum effects on endogenous abscisic acid (ABA) levels in protoplasts and intact primary roots of Zea mays L. Application of 1 mM La³⁺ reduced primary root elongation, caused swelling of root tips, and essentially doubled the ABA content in intact roots but decreased ABA in root protoplasts in a concentration-dependent manner. Osmotic stress increased ABA level in protoplasts more than in intact roots. Temporal ABA changes in response to La³⁺ treatment indicate that La³⁺ affects root growth at least partially via ABA pathway.     

ABA and proline accumulation in leaves and roots of wild (<Emphasis Type="Italic">Hordeum spontaneum</Emphasis>) and cultivated (<Emphasis Type="Italic">Hordeum vulgare</Emphasis> ‘Maresi’) barley genotypes under water deficit conditions

The purpose of the study was to examine water stress-induced changes in the ABA and proline contents in roots and leaves of a potentially more resistant wild accession of Hordeum spontaneum and the modern cultivar Maresi (Hordeum vulgare). Leaves of H. spontaneum had higher contents of constitutive ABA and proline in comparison to those of ‘Maresi’. A moderate water deficit resulted only in root dehydration, which was higher in ‘Maresi’. Increases of water deficit in roots coincided with an increase of ABA content in roots, followed by that in leaves. The level of proline increased only in leaves and only in the case of H. spontaneum. Under conditions of severe water stress, the root dehydration levels were similar in the both genotypes, whereas leaf dehydration was higher in ‘Maresi’. H. spontaneum, as compared to ‘Maresi’ showed an earlier increase of ABA content in the roots and accumulated more ABA in the leaves. Free proline levels in the roots increased in both genotypes but H. spontaneum exhibited a 2-fold higher proline accumulation than ‘Maresi’. In H. spontaneum the accumulation of proline in the leaves occurred noticeably earlier and to a higher extent than in ‘Maresi’. A possible connection of these modifications with water stress resistance of the investigated genotypes is discussed in this paper.     

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.     

Accumulation rate of ABA in detached maize roots correlates with root water potential regardless of age and branching order

How much ABA can be supplied by the roots is a key issue for modelling the ABA-mediated influence of drought on shoot physiology. We quantified accumulation rates of ABA ( S _ABA) in maize roots that were detached from well-watered plants and dehydrated to various extents by air-drying. S _ABA was estimated from changes in ABA content in root segments incubated at constant relative water content (RWC). Categories of root segments, differing in age and branching order, were compared (root branches, and nodal roots subdivided into root tips, subapical unbranched sections, and mature sections). All categories of roots accumulated ABA, including turgid and mature tissues containing no apex. S _ABA measured in turgid roots changed with root age and among root categories. This variability was largely accounted for by differences in water content among different categories of turgid roots. The response of S _ABA to changes in root water potential ( Ψ _root) induced by dehydration was common to root tips, nodal roots and branches of several ages, while this was not the case if root dehydration was expressed in terms of RWC. Differences among root categories in the response of S _ABA to RWC were due to different RWC values among categories at a given Ψ _root, and not to differences in the response of S _ABA to Ψ _root.     

Growth,graviresponsiveness and abscisic-acid content of Zea mays seedlings treated with Fluridone

Ten-d-old seedlings of Zea mays L. cv. Tx 5855 treated with 1-methyl-3-phenyl-5-(3-[trifluoromethyl]phenyl)-4-(1H)-pyridinone (Fluridone) were analyzed for abscisic acid (ABA) content using high-performance liquid chromatography with an analysis sensitivity of 2.5 ng ABA g^-1 fresh weight (FW). Seedlings were divided into three portions: leaves, detipped roots, and root tips (terminal 1.5 mm). Control plants (water treatment only; no Fluridone) were characterized by the following amounts of ABA: leaves, 0.114±0.024 (standard deviation) g ABA g^-1 FW; detipped roots, 0.260±0.039±g ABA g^-1 FW; root tips, no ABA detected. We did not detect any ABA in tissues of Fluridone-treated plants. Primary roots of treated and untreated seedlings were strongly graviresponsive, with no significant differences between the curvatures or the growth rates of primary roots of Fluridone-treated and control seedlings. These results indicate that 1) Fluridone completely inhibits ABA synthesis, and 2) ABA is not necessary for positive gravitropism by primary roots of Zea mays.Abbreviations ABA abscisic acid - Fluridone 1-methyl-3-phenyl-5-(3-[trifluoromethyl]phenyl)-4-(1H)-pyridinone - FW fresh weight - SD standard deviation     

Organ-specific changes in the content of free and conjugated polyamines in <Emphasis Type="Italic">Mesembryanthemum crystallinum</Emphasis> plants under salinity

The content of free and conjugated polyamines (PA) was studied in the leaves of secondary shoots and in roots of the facultative halophyte Mesembryanthemum crystallinum L. under salinity. Plants were grown in water culture and at the age of 10 weeks were subjected to salinity stress by a single introduction of NaCl into nutrient medium to a final concentration of 400 mM. In 0, 6, 24, and 48 h, the content of free, HClO₄-soluble, and HClO₄-insoluble conjugates of PA were measured in leaves and roots. The levels of free and conjugated PA in the roots of control plants and plants subjected to salinity stress were higher than in the leaves. In response to salinity shock, the content of all forms of spermine (Spm), particularly its HClO₄-soluble conjugates, increased in roots and leaves. In contrast, the content of Spm precursors, putrescine (Put) and spermidine (Spd), as well as their HClO₄-soluble conjugates, decreased. Salinity treatment elevated the content of free cadaverine both in roots and leaves, whereas the content of its conjugated forms decreased, which might suggest transition of conjugated forms of cadaverine into the free form. The product of oxidative degradation of Spm and Spd, 1,3-diaminopropane (Dap), was absent in leaves, whereas the content of free and conjugated forms of this diamine in roots increased under salinity conditions. The obtained data indicate organ specificity for the content of free and conjugated forms of PA, as well as their active role in adaptation of Mesembryanthemum crystallinum to salinity shock.     

Water stress‐induced abscisic acid accumulation in relation to reducing agents and sulfhydryl modifiers in maize plant

Signalling process of water stress‐induced abscisic acid (ABA) accumulation was investigated in maize (Zea mays L.) leaf and root tissues. Potent free‐radical scavengers and reducing agents, N‐acetyl cysteine (NAC) and cystein (Cys), significantly inhibited or nearly completely blocked dehydration‐induced ABA accumulation. Dithiothreitol (DTT), a reducing agent but not a free‐radical scavenger, also significantly inhibited such accumulation whereas solely free‐radical scavengers, dimethyl sulphoxide (DMSO) and melatonin (Mela), had no effects. Moreover, water stress‐induced ABA accumulation was not affected either by free radicals, such as superoxide anion and hydrogen peroxide, or by oxidants such as KIO_4. These observations suggest that the blocking of water stress‐induced ABA accumulation resulted from the reducing effect, rather than from anything associated with free radicals. The disulphide bond might be crucial to the reactivity of some signal element(s) in the signalling process of water stress‐induced ABA accumulation. To test the hypothesis, we used a sulfhydryl modifier, iodoacetamide (IOA), and found that it nearly totally blocked the water stress‐induced ABA accumulation. Furthermore, an impermeable sulfhydryl modifier, p‐chloromercuriphenylsulphonic acid (PCMBS), could also inhibit the water stress‐induced ABA accumulation in the leaf tissues. These results indicate that water stress‐perception protein(s) or receptor(s) may be located on the plasmalemma and a sulfhydryl group in the extracellular domain is critical to the reactivity of the speculated water stress receptors. Cys, DTT and IOA did not lead to a decrease of the baseline ABA level, i.e. in non‐stressed roots. Result indicates that their blocking of water stress‐induced ABA accumulation occurred upstream of the ABA biosynthesis pathway, i.e. in the signalling process that initiates such accumulation.     

The Effects of Partially Drying Part of the Root System of Helianthus annuus on the Abscisic Acid Content of the Roots, Xylem Sap and Leaves

Plants of Helianthus annuus were grown in soil in pots suchthat approximately 30% of the root system protruded throughthe base of the pot. After 7 d further growth in aerated nutrientsolution, the attached, protruding roots were air-dried for10–15 min and thereafter surrounded with moist still air,in the dark, for 49 h, whilst the soil was kept at field capacity.The roots of the control plants remained in the nutrient solutionthroughout the experiment. This treatment rapidly reduced the water content of protrudingroots from 20.5 to 17.8 g g^–1 dry mass (DM), which remainedless than that of the control roots for the rest of the experiment.This treatment also reduced root turgor and water potential.The abscisic acid (ABA) concentrations in the protruding roots,xylem sap and leaves of the treated plants increased significantly,compared to values recorded for control plants. In treated roots, the ABA concentration was significantly increased4 h after treatment, with a maximum of 4.4+0.1 nmol g^–1(DM) after 25 h. The ABA concentration in the xylem sap of thetreated plants was significantly greater than in the controls25 h, 30 h, and 49 h after the partial drying of the roots,with a maximum concentration of approximately 970 pmol ABA cm-3at 49 h. Initially, the ABA concentration in the leaves was0.45 nmol g^–1 (DM) which increased significantly to 1.1±0.1 nmol g^–1 at 25 h, to 1.7±0.3 nmol g^–1at 49 h. Leaf conductance was significantly less in plants with air-driedroots than in the controls 8 h after the start of the treatmentand thereafter. The water relations of the leaves of the treatedplants did not differ from those of the control plants. These results confirm previous reports that ABA is rapidly generatedin partially-dried and attached root systems and demonstratesa concomitant large increase in the ABA content of the xylemsap. It is suggested that partial dehydration of some of theroots of Helianthus annuus, increases ABA concentration in thetranspiration stream and decreases leaf conductance in the absenceof changes in leaf water status. As these responses were initiatedin free-growing roots the stimulus is independent of any increasesin soil shear strength that are associated with soil drying. Key words: Soil drying, roots, ABA, leaf conductance, water relations     

Etude comparee des lipides et de la fixation passive du calcium dans les racines et les fractions subcellulaires du Lupinus luteus et de la Vicia faba

In both lupin and broad bean, the root lipids contain paraffins, triglycerides, diglycerides, free fatty acids and polar lipids (phospholipids and galactolipids). The polar lipids and the triglycerides are the more abundant classes. The root galactolipids are mono- and di-galactosyldiglycerides; two steryl glycosides are also present. The phospholipids in both species are: phosphatidylinositol, phosphatidylcholine, phosphatidylglycerol, phosphatidylserine, phosphatidylethanolamine, diphosphatidylglycerol and phosphatidic acid. This last phospholipid represents 8·3% of total lipid phosphorus in Lupinus against 2·3% in Vicia. The other acidic phospholipids represent 30·4% in Lupinus against 20·9% in Vicia. The lipids of Lupinus are rich in linolenic acid whereas those found in Vicia are richer in linoleic acid. The various subcellular fractions prepared from the roots of both species have an homogeneous lipid composition, reflecting exactly that of entire cells. The calcium passive fixation capacity in microsomes and mitochondria of Lupinus roots is more important than that in the same organelles of Vicia faba roots. Thus a relationship is suggested between the amount of phospholipids in membranes and the passive fixation of calcium.     

Der basipetale [2-14C] Abscisinsäuretransport in Wurzeln intakter Bohnenkeimlinge und seine Bedeutung für den Wurzelgeotropismus

Summary One h and 3 h after point source application of [2-¹⁴C] ABA to the upper side of geotropically stimulated root tips of intact seedlings of Phaseolus coccineus L. a weak downward lateral transport of radioactivity was observed. A clear upward lateral translocation, however, occurred when ABA was applied to the lower side of the root tip. Thus a uniform distribution of radioactivity was established in horizontal roots regardless of the site of the application point, while in upright roots the distribution was asymmetric. Six h after ABA injection radioactivity was uniformly distributed in both the vertical and horizontal root tips.

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Abkürzungen ABA Abscisinsäure - dpm disintegrations perminute - FG Frischgewicht - GA Gibberellinsäure     

Sensitivity of chickpea and faba bean to root‐zone hypoxia,elevated ethylene,and carbon dioxide

During soil waterlogging, plants experience O₂ deficits, elevated ethylene, and high CO₂ in the root‐zone. The effects on chickpea (Cicer arietinum L.) and faba bean (Vicia faba L.) of ethylene (2 μL L^?1), CO₂ (2–20% v/v) or deoxygenated stagnant solution were evaluated. Ethylene and high CO₂ reduced root growth of both species, but O₂ deficiency had the most damaging effect and especially so for chickpea. Chickpea suffered root tip death when in deoxygenated stagnant solution. High CO₂ inhibited root respiration and reduced growth, whereas sugars accumulated in root tips, of both species. Gas‐filled porosity of the basal portion of the primary root of faba bean (23%, v/v) was greater than for chickpea (10%), and internal O₂ movement was more prominent in faba bean when in an O₂‐free medium. Ethylene treatment increased the porosity of roots. The damaging effects of low O₂, such as death of root tips, resulted in poor recovery of root growth upon reaeration. In conclusion, ethylene and high CO₂ partially inhibited root extension in both species, but low O₂ in deoxygenated stagnant solution had the most damaging effect, even causing death of root tips in chickpea, which was more sensitive to the low O₂ condition than faba bean.     

Abscisic acid and auxin accumulation in Catasetum fimbriatum roots growing in vitro with high sucrose and mannitol content

Endogenous contents of indolyl-3-acetic acid (IAA) and abscisic acid (ABA) were quantified in excised roots of Catasetum fimbriatum (Orchidaceae) cultured in vitro on solidified Vacin and Went medium with 1, 2, 4, 6, 8 and 10 % sucrose, as well as 2 % sucrose plus mannitol. Maximum root growth was observed in media with 4 % sucrose and 2 % sucrose plus 2.2 % mannitol, suggesting that a moderate water or osmotic stress promotes orchid root growth. Contents of both ABA and IAA increased in parallel to increasing sucrose concentration and a correlation between root elongation and the ABA/IAA ratio was observed. Incubating isolated C. fimbriatum roots with radiolabeled tryptophan, we showed an accumulation of IAA and its conjugates.     

Effect of abscisic acid on endogenous IAA, auxin protector levels and peroxidase activity during adventitious root initiation in Vigna radiata cuttings

Endogenous levels of free and conjugated IAA, auxin protectors (Prs) and peroxidase (PER) activity and their relation to adventitious root initiation (ARI) were investigated at the potential sites of adventitious rooting in relation to exogenous application of 250 μM ABA during the first 120 h after treatment. Cuttings from 7-day-old mung bean [Vigna radiata (L.) Wilcz.] seedlings were treated with 125, 250, and 500 μM ABA for 24 h. ABA significantly stimulated ARI but extremely inhibited epicotyl growth as compared to control. Free and conjugated IAA were measured by reversed-phase high performance liquid chromatography while Prs and PER activities were measured spectrophotometrically. The present results also indicate that endogenous free IAA levels peaked later in ABA-treated cuttings than that in control, suggesting that ABA extended the length of the induction phase of rooting process in treated cuttings and that might explain the significant delay of the appearance of roots at the treated cuttings. Higher level of IAA conjugates was found in ABA-treated cuttings than that in untreated ones. Pr level also peaked later in ABA-treated cuttings than that in control, indicating that ABA extended the period of Pr activity. An initial temporary decrease of PER activity was found in associating with high levels of free IAA and Prs during most of the primary events, while the opposite occurred during the secondary events of adventitious rooting process in both treated and untreated cuttings. Thus, ABA may stimulate ARI in mung bean Vigna radiata cuttings by regulating the concentration and /or activities of endogenous IAA, Prs, and PER activity in favor of inducing a large number of adventitious roots at their potential sites of adventitious rooting.     

Abscisic acid‐mediated modifications of radial apoplastic transport pathway play a key role in cadmium uptake in hyperaccumulator Sedum alfredii

Abscisic acid (ABA) is a key phytohormone underlying plant resistance to toxic metals. However, regulatory effects of ABA on apoplastic transport in roots and consequences for uptake of metal ions are poorly understood. Here, we demonstrate how ABA regulates development of apoplastic barriers in roots of two ecotypes of Sedum alfredii and assess effects on cadmium (Cd) uptake. Under Cd treatment, increased endogenous ABA level was detected in roots of nonhyperaccumulating ecotype (NHE) due to up‐regulated expressions of ABA biosynthesis genes (SaABA2, SaNCED), but no change was observed in hyperaccumulating ecotype (HE). Simultaneously, endodermal Casparian strips (CSs) and suberin lamellae (SL) were deposited closer to root tips of NHE compared with HE. Interestingly, the vessel‐to‐CSs overlap was identified as an ABA‐driven anatomical trait. Results of correlation analyses and exogenous applications of ABA/Abamine indicate that ABA regulates development of both types of apoplastic barriers through promoting activities of phenylalanine ammonialyase, peroxidase, and expressions of suberin‐related genes (SaCYP86A1, SaGPAT5, and SaKCS20). Using scanning ion‐selected electrode technique and PTS tracer confirmed that ABA‐promoted deposition of CSs and SL significantly reduced Cd entrance into root stele. Therefore, maintenance of low ABA levels in HE minimized deposition of apoplastic barriers and allowed maximization of Cd uptake via apoplastic pathway.     

Changes in abscisic acid and its glucose ester in Phaseolus vulgaris L. during chilling and water stress

Levels of free and conjugated abscisic acid (ABA) were determined in leaves and roots of intact bean (Phaseolus vulgaris L., cv. Mondragone) seedlings under chilling (3C) and drought as well as during recovery from stress. Abscisic acid-glucose ester (ABAGE) was the only conjugate releasing free ABA after alkaline hydrolysis of the crude aqueous extracts. During the first 20–30 h chilled plants rapidly dehydrated and wilted without any change in ABA and ABAGE levels. Subsequently, leaf and root ABA levels increased and plants regained turgor. ABAGE concentration showed a slight increase in leaves but not in roots. Upon recovery from chilling a transient, but significant, rise in leaf ABA content was observed, while no appreciable change in ABAGE was found. Drought triggered ABA accumulation in leaves and roots, while a rise in ABAGE content was detected only in leaf tissues. Recovery from stress caused a drop in ABA levels without a correspondent increase in ABAGE concentration. We conclude that ABAGE is not a source of free ABA during either chilling or water stress and that only a small proportion of the ABA produced under stress is metabolised to ABAGE during recovery.Abbreviations ABA = abscisic acid - ABAGE = abscisic acid-glucose ester - DW = dry weight - FW = fresh weight - RIA = radioimmunoassay - RWC = relative water content - w = water potential - o = osmotic potential - p = turgor potential     

Root adaptations to cadmium-induced oxidative stress contribute to Cd tolerance in the hyperaccumulator Sedum alfredii

Short-term responses of Sedum alfredii roots to Cd exposure was compared in Cd hyperaccumulator (HE) and nonhyperaccumulating ecotype (NHE). Cadmium exposure significantly inhibited root elongation and induced loss of plasma membrane integrity and lipid peroxidation of roots tips in the NHE, whereas these effects were much less pronounced in the HE plants. A strong accumulation of reactive oxygen species with increasing Cd concentration was noted in the NHE root tips, but not in HE. After Cd exposure, a dose-dependent decrease in oxidized glutathione and marked increase in reduced glutathione and non-protein thiols were observed in root tips of HE, but were not seen in the NHE plants. These results suggest that the HE tolerates high Cd in the environment through the differential adaptations against Cd-induced oxidative stress.     

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.