Symplastic and apoplastic uptake and root to shoot translocation of nickel in wheat as affected by exogenous amino acids

This study investigated the effect of exogenous amino acids on apoplastic and symplastic uptake and root to shoot translocation of nickel (Ni) in two wheat cultivars. Seedlings of a bread (Triticum aestivum cv. Back Cross) and a durum wheat cultivar (T. durum cv. Durum) were grown in a modified Johnson nutrient solution and exposed to two levels (50 and 100 μM) of histidine, glycine, and glutamine. Application of amino acids resulted in increasing symplastic to apoplastic Ni ratio in roots of both wheat cultivars, although glutamine and glycine were more effective than histidine under our experimental conditions. The amino acid used in the present study generally increased the relative transport of Ni from the roots to shoots in both wheat cultivars. Higher amounts of Ni were translocated to wheat shoots in the presence of histidine than the other amino acids studied, which indicated that histidine was more effective in translocation of Ni from roots to shoots. Amino acids used in the present study largely increased root symplastic Ni, but shoot Ni accumulation was much lower than the total Ni accumulation in roots, indicating a large proportion of Ni was retained or immobilized in wheat roots (either in the apoplastic or symplastic space), with only a very small fraction of Ni being translocated from the root to the shoot. According to the results, glutamine and glycine were more effective than histidine in enhancing the symplastic to apoplastic Ni ratio in the roots, while more Ni was translocated from the roots to the shoots in the presence of histidine.     

The effect of aluminium on polypeptide pattern of cell wall proteins isolated from the roots of Al-sensitive and Al-resistant barley cultivars

Accumulation of some proteins isolated from the cell wall of roots of the Al-sensitive (Alfor) and the Al-resistant (Bavaria) barley cultivars were followed during treatment with different Al³⁺ concentrations, pH changes of the root medium, and several heavy metals (Cu²⁺, Cd²⁺, Co²⁺). SDS-PAGE analysis revealed an Al-induced accumulation of polypeptides with molecular mass of 14, and 16 kDa and a group of polypeptides around 27 kDa. The accumulation pattern of Al-induced polypeptides was very similar in both cultivars but in the Al-resistant Bavaria it was induced at lower Al concentration and earlier than it was in the Al-sensitive cultivar Alfor. Changes in pH values of root medium (pH 3.5–6.5) did not show any effect on the accumulation of Al-induced cell wall polypeptides either in Al-sensitive or in Al-tolerant barley cultivar. Heavy metals (Cu, Cd, and Co) at concentration of 10 μM resulted in similar accumulation of individual polypeptides as we found after Al treatment. In comparison to Al, quantitative differences in polypeptides accumulation induced by Cu, Cd and Co were less expressed that of Al treatment. More pronounced accumulation and earlier induction of individual cell wall polypeptides in roots of Al-resistant barley cultivar than in Al-sensitive, might indicate some possible role of these polypeptides in plant resistance to Al stress.     

Al Inhibits Both Shoot Development and Root Growth in als3, an Al-Sensitive Arabidopsis Mutant

In als3, an Al-sensitive Arabidopsis mutant, shoot development and root growth are sensitive to Al. Mutant als3 seedlings grown in an Al-containing medium exhibit severely inhibited leaf expansion and root growth. In the presence of Al, unexpanded leaves accumulate callose, an indicator of Al damage in roots. The possibility that the inhibition of shoot development in als3 is due to the hyperaccumulation of Al in this tissue was examined. However, it was found that the levels of Al that accumulated in shoots of als3 are not different from the wild type. The inhibition of shoot development in als3 is not a consequence of nonspecific damage to roots, because other metals (e.g. LaCl3 or CuSO4) that strongly inhibit root growth did not block shoot development in als3 seedlings. Al did not block leaf development in excised als3 shoots grown in an Al-containing medium, demonstrating that the Al-induced damage in als3 shoots was dependent on the presence of roots. This suggests that Al inhibition of als3 shoot development may be a delocalized response to Al-induced stresses in roots following Al exposure.     

Effect of pH on Mn absorption by barley genotypes in a chelate-buffered nutrient solution

A new chelate-buffering technique was used to investigate the effect of pH (6.00, 6.85 and 7.70) on manganese (Mn) absorption from nutrient solution by three genotypes of barley plants differeing in Mn efficiency. The nutrient composition was adjusted such that the calculated activities of Mn²⁺, Zn²⁺, Cu²⁺ and Ni²⁺ were similar in each pH, thus eliminating any effect of the pH treatment on Mn²⁺ supply. Increasing pH from 6.00 to 7.70 increased the rate of Mn absorption and decreased the external Mn requirement for optimal growth rate. With increasing pH, Mn concentrations in roots rose markedly, and were higher than those in shoots at pH 7.70. Genotypic differences in Mn concentration of roots appeared only at higher pH. We suggest that higher Mn concentration in roots of inefficient plants may be related to Mn immobilisation in roots, and this may be a factor in the mechanism of Mn efficiency.     

Differential responses of antioxidant enzymes to aluminum toxicity in two rice (Oryza sativa L.) cultivars with marked presence and elevated activity of Fe SOD and enhanced activities of Mn SOD and catalase in aluminum tolerant cultivar

Seedlings of two Indica rice (Oryza sativa L.) cvs. HUR-105 and Vandana, differing in Al-tolerance were used to identify the key mechanisms involved in their differential behaviour towards Al toxicity. Cv. HUR-105 appeared to be Al sensitive by showing significant reduction (p ≤ 0.01) in root/shoot length, fresh weight, dry weight and water content in presence of 421 μM Al³⁺ in growth medium whereas cv. Vandana appeared to be fairly Al³⁺ tolerant. A conspicuous and significant reduction in dry weight of root and shoot was observed in Al sensitive cv. HUR-105 with 178 μM Al³⁺ treatment for 3 days. Al was readily taken up by the roots and transported to shoots in both the rice cultivars. Localization of absorbed Al was always greater in roots than in shoots. Our results of the production of reactive oxygen species (ROS) H₂O₂ and O₂ ^.? and activities of major antioxidant enzymes such as total superoxide dismutase (SOD), Cu/Zn SOD, Mn SOD, Fe SOD, catalase (CAT) and guaiacol peroxidase revealed Al induced higher oxidative stress, greater production of ROS and lesser capacity to scavenge ROS in cv. HUR-105 than Vandana. With Al treatment, higher oxidative stress was noted in shoots than in roots. Greatly enhanced activities of SOD (especially Fe and Mn SOD) and CAT in Al treated seedlings of cv. Vandana suggest the role of these enzymes in Al tolerance. Furthermore, a marked presence of Fe SOD in roots and shoots of the seedlings of Al tolerant cv. Vandana and its significant (p ≤ 0.01) increase in activity due to Al-treatment, appears to be the unique feature of this cultivar and indicates a vital role of Fe SOD in Al-tolerance in rice.     

Effect of P and Mn on growth response and uptake of Fe,Mn and P by sorghum

Summary The effects of P and Mn on growth response and uptake of Fe, Mn and P by grain sorghum were investigated using nutrient culture. High P and Mn concentrations in solution (greater than 40 and 1 mg/l for P and Mn, respectively) markedly reduced plant height and shoot and root dry weight of 4-week-old sorghum plants. High Mn concentrations in solution increased the concentrations of Mn and P in shoot tissue and uptake of Mn, but depressed the uptake of P. High levels of P enhanced Mn uptake by sorghum and accentuated Mn toxicity at low Mn levels. The tissue Fe and total uptake of Fe were both reduced markedly by the high levels of P and Mn concentrations in solution. The increases of P, Mn and Fe concentrations in root tissue with a concomitant decrease of Fe in shoots suggested that the translocation of Fe from roots to shoots was hindered under high P and Mn conditions. Since coating occurred on root surfaces and intensified with increasing Mn concentrations in the substrate, part of the reduction of Fe in shoots could be attributed to the formation of high valent manganese oxides on the root surfaces which may retain Fe and reduce its absorption by sorghum.Contribution from the Department of Agronomy and Range Sci., University of California, Davis, CA.     

Nitric oxide protects sour pummelo (Citrus grandis) seedlings against aluminum-induced inhibition of growth and photosynthesis

Limited data are available on the amelioration of nitric oxide (NO) on aluminum (Al)-toxicity. Sour pummelo (Citrus grandis) seedlings were irrigated for 18 weeks with nutrient solution containing 0 and 1.2 mM AlCl₃·6H₂O × 0 and 10 μM sodium nitroprusside (SNP, an NO donor). Under Al stress, SNP increased root phosphorus (P) and Al, but decreased shoot Al. Al decreased photosynthesis, maximum quantum yield of primary photochemistry (F_v/F_m) and total performance index (PI_tot,sbs), but increased inactivation of oxygen-evolving complex (OEC), K-band and relative variable fluorescence at I-steps (V_I). SNP alleviated Al-induced changes for all these parameters. SNP stimulated Al-induced secretion of malate and citrate by excised roots from Al-treated seedlings, while Al did not increase their contents in roots. Antioxidant system in leaves and roots was up- and down-regulated by Al, respectively. SNP prevented Al-induced accumulation of malondialdehyde (MDA) in roots and leaves. In conclusion, SNP alleviates Al-induced inhibition of growth and impairment of the whole photosynthetic electron transport chain. This occurs through increasing Al-immobilization and P level in roots and Al-induced secretion of malate and citrate from roots, and decreasing Al accumulation in shoots. Thus, the decrease of photosynthesis is prevented. Increased P level and Al-immobilization in roots through SNP may be effected through enhanced secretion of malate and citrate.     

Cd accumulation in roots and shoots of durum wheat: the roles of transpiration rate and apoplastic bypass

Cadmium is readily taken up from soils by plants, depending on soil chemistry, and variably among species and cultivars; altered transpiration and xylem transport and/or translocation in the phloem could cause this variation in Cd accumulation, some degree of which is heritable. Using Triticum turgidum var. durum cvs Kyle and Arcola (high and low grain Cd accumulating, respectively), the objectives of this study were to determine if low-concentration Cd exposure alters transpiration, to relate transpiration to accumulation of Cd in roots and shoots at several life stages, and to evaluate the role of apoplastic bypass in the accumulation of Cd in shoots. The low abundance isotope (106)Cd was used to probe Cd translocation in plants which had been exposed to elemental Cd or were Cd-na?ve; apoplastic bypass was monitored using the fluorescent dye PTS (8-hydroxy-1,3,6-pyrenetrisulphonate). Differential accumulation of Cd by 'Kyle' and 'Arcola' could be partially attributed to the effect of Cd on transpiration, as exposure to low concentrations of Cd increased mass flow and concomitant Cd accumulation in 'Kyle'. Distinct from this, exposure of 'Arcola' to low concentrations of Cd reduced translocation of Cd from roots to shoots relative to root accumulation of Cd. It is possible, but not tested here, that sequestration mechanisms (such as phytochelatin production, as demonstrated by others) are the genetically controlled difference between these two cultivars that results in differential Cd accumulation. These results also suggest that apoplastic bypass was not a major pathway of Cd transport from the root to the shoot in these plants, and that most of the shoot Cd resulted from uptake into the stele of the root via the symplastic pathway.     

Influence of phosphorus on the growth and ergot alkaloid content of Neotyphodium coenophialum-infected tall fescue (Festuca arundinacea Schreb.)

Tall fescue (Festuca arundinacea Schreb.) plants infected by the fungal endophyte Neotyphodium coenophialum (Morgan-Jones & Gams) (Glenn et al., 1996) often perform better than noninfected plants, especially in marginal resource environments. There is a lack of information about endophyte related effects on the rhizosphere of grasses. In a greenhouse experiment, four endophyte-infected (E+) tall fescue clones (DN2, DN4, DN7, DN11) and their endophyte-free (E–) forms were grown in limed (pH 6.3) Porter soil (low fertility, acidic, high aluminum and low phosphorus content, coarse-loamy mixed mesic Umbric Dystrochrept) at three soil P levels (17, 50, and 96 mg P kg^-1 soil) for five months. Excluding the genotype effect, endophyte infection significantly increased cumulative herbage DM yield by 8% at 17 mg P kg^-1 soil but reduced cumulative herbage DM yield by 12% at 96 mg P kg^-1 soil. With increased P availability in the soil, shoot and root DM, and root/shoot ratio in E+ plants were significantly less when compared to E– plants. Endophyte infection increased specific root length at 17 and 50 mg P kg^-1soil. At soil P level of 17 mg P kg^-1soil, E+ plants had significantly higher P concentrations both in roots and shoots. Similar relationships were found for Mg and Ca. E+ plants had significantly higher Zn, Fe, and Al concentration in roots, and lower Mn and Al concentration in shoots when compared to E– plants. Ergot alkaloid concentration and content in shoot of E+ plants increased with increasing P availability in the soil from 17 to 50 mg P kg^-1 but declined again at 96 mg P kg^-1 soil. Ergot alkaloid accumulation in roots increased linearly with P availability in the soil. Results suggest that endophyte infection affects uptake of phosphorus and other mineral nutrients and may benefit tall fescue grown on P-deficient soils. Phosphorus seems also to be involved in ergot alkaloid accumulation in endophyte-infected tall fescue.     

Effects of suboptimal root zone temperatures and shoot demand on net translocation of micronutrients from the roots to the shoot of maize

The effects of suboptimal root zone temperatures (RZTs) on net translocation rates from the roots to the shoots and the concentrations of Fe, Mn, Zn, and Cu were examined in maize grown in nutrient solution or soil. Plants were grown at 12 °C, 18 °C and 24 °C RZT. At each RZT, the growth-related shoot demand for nutrients was varied by independently modifying the temperature of the shoot base (SBT) including the apical shoot meristem. The net translocation rates of Mn and Zn from the roots to the shoots were reduced at low RZTs, irrespective of the SBT and of the substrate (soil or nutrient solution). Obviously, the net translocation rates of Mn and Zn at low RZT were mainly regulated by temperature effects on the roots and not by the chemical nutrient availability in the rhizosphere or by shoot growth rate as controlled by SBTs. When both RZT and SBT were reduced, the decrease in net translocation rates of Mn and Zn was similar to the decline in the shoot growth rate and concentrations of Mn and Zn in the shoot fresh matter were not greatly affected or were even increased by low RZT. However, at high SBT and low RZT in nutrient solution, the depressed net translocation rates of Mn and Zn combined with the increased shoot growth resulted in significantly decreased concentrations of Mn and Zn in the shoot, indicating that Mn and Zn may become deficient even at high chemical availability. By contrast to Mn and Zn, the net translocation rates of Fe and Cu at all RZTs were markedly enhanced by increased SBTs. Accordingly, the concentrations of Fe and Cu in the shoot fresh matter were not greatly affected by RZTs, irrespective of the SBTs. These results indicate that the ability of roots to supply Fe and Cu to the shoot was internally regulated by the growth related shoot demand per unit of roots. Deceased 21 September 1996 Deceased 21 September 1996     

Nitric oxide exacerbates Al-induced inhibition of root elongation in rice bean by affecting cell wall and plasma membrane properties

Aluminum (Al) toxicity is one of the most widespread problems for crop production on acid soils, and nitric oxide (NO) is a key signaling molecule involved in the mediation of various biotic and abiotic stresses in plants. Here we found that exogenous application of the NO donor sodium nitroprusside (SNP) exacerbated the inhibition of Al-induced root growth in rice bean [Vigna umbellata (Thunb.) Ohwi & Ohashi ‘Jiangnan’, Fabaceae]. This was accompanied by an increased accumulation of Al in the root apex. However, Al treatments had no effect on endogenous NO concentrations in root apices. These results indicate that a change in NO concentration is not the cause of Al-induced root growth inhibition and the adverse effect of SNP on Al-induced root growth inhibition should result from increased Al accumulation. Al could significantly induce citrate efflux but SNP had no effects on citrate efflux either in the absence or presence of Al. On the other hand, SNP pretreatment significantly increased Al-induced malondialdehyde accumulation and Evans Blue staining, indicating an intensification of the disruption of plasma membrane integrity. Furthermore, SNP pretreatment also caused greater induction of pectin methylesterase activity by Al, which could be the cause of the increased Al accumulation. Taken together, it is concluded that NO exacerbates Al-induced root growth inhibition by affecting cell wall and plasma membrane properties.     

边缘细胞对荞麦根尖铝毒的防护效应和对细胞壁多糖的影响

以2个荞麦(Fygopyrum esculentum Moench)基因型‘江西荞麦’(耐性)和‘内蒙荞麦’(敏感)为材料,采用悬空培养(保持边缘细胞附着于根尖和去除根尖边缘细胞),研究边缘细胞对根尖铝毒的防护效应以及对细胞壁多糖组分的影响。结果表明,铝毒抑制荞麦根系伸长,导致根尖Al积累。去除边缘细胞的根伸长抑制率和根尖Al含量高于保留边缘细胞的根。去除边缘细胞使江西荞麦和内蒙荞麦根尖的酸性磷酸酶(APA)活性显著升高,前者在铝毒下增幅更大。同时,铝毒胁迫下去除边缘细胞的根尖果胶甲酯酶(PME)活性和细胞壁果胶、半纤维素1、半纤维素2含量显著高于保留边缘细胞的酶活性和细胞壁多糖含量。表明边缘细胞对荞麦根尖的防护效应,与其阻止Al的吸收,降低根尖细胞壁多糖含量及提高酸性磷酸酶活性有关,以此缓解Al对根伸长的抑制。     

Effects of aluminium and mineral nutrition on growth and chemical composition of hydroponically grown seedlings of five different forest tree species

Forest die-back and impaired tree vitality have frequently been ascribed to Al-toxicity and Al-induced nutritional disorders due to increased acidification of forest soils. Therefore, in this experiment effects of Al were studied on growth and nutrient uptake with seedlings of five different forest tree species. During growth in culture solutions with and without Al all five species proved to be very Al-tolerant, despite high accumulation of Al in roots. In the coniferous evergreens Douglas-fir and Scots pine shoot as well as root Al concentrations were significantly higher than in the deciduous broad-leaved species oak and birch. Larch showed intermediate Al levels. In none of the five species did Al reduce nutrient concentrations or the Ca/Al ratio to values below the critical level. Besides differences in Al accumulation, coniferous and broad-leaved species also differed with respect to uptake and assimilation of nitrogen. Due to extra NH ₄ ⁺ uptake, oak and birch showed a much higher N uptake and higher NH ₄ ⁺ preference than the coniferous species. Especially with oak this high NH ₄ ⁺ preference in combination with a low specific root surface area resulted in a high root proton efflux density. In comparison to both broad-leaved trees and Scots pine the NO ₃ ⁻ reduction capacity of larch and Douglas-fir was extremely low. This may have important consequences for both species if grown in NO ₃ ⁻ -rich soils.     

Al-Fe interactions and growth enhancement in Melastoma malabathricum and Miscanthus sinensis dominating acid sulphate soils

Plants growing in acid sulphate soils are subject to high levels of Al availability, which may have effects on the growth and distribution of these species. Although Fe availability is also high in acid sulphate soils, little is known about the effect of Fe on the growth of native plants in these soils. Two species dominating this soil type in Asia, viz. Melastoma malabathricum and Miscanthus sinensis were grown hydroponically in a nutrient solution with different concentrations of Al and Fe. Melastoma malabathricum is found to be sensitive to Fe (40 and 100 microm). Application of 500 microm Al, however, completely ameliorates Fe toxicity and is associated with a decrease of Fe concentration in shoots and roots. The primary reason for the Al-induced growth enhancement of M. malabathricum is considered to be the Al-induced reduction of toxic Fe accumulation in roots and shoots. Therefore, Al is nearly essential for M. malabathricum when growing in acid sulphate soils. In contrast, application of both Fe and Al does not reduce the growth of M. sinensis, and Al application does not result in lower shoot concentrations of Fe, suggesting that this grass species has developed different mechanisms for adaptation to acid sulphate soils.     

Apoplastic binding of aluminum is involved in silicon-induced amelioration of aluminum toxicity in maize

The alleviating effect of silicon (Si) supply on aluminum (Al) toxicity was suggested to be based on ex or in planta mechanisms. In our experiments with the Al-sensitive maize (Zea mays) cultivar Lixis, Si treatment but not Si pretreatment ameliorated Al-induced root injury as revealed by less root-growth inhibition and callose formation. Si treatment did not affect monomeric Al concentrations in the nutrient solution, suggesting an in planta effect of Si on Al resistance. A fractionated analysis of Si and Al in the 1-cm root apices revealed that more than 85% of the root-tip Al was bound in the cell wall. Al contents in the apoplastic sap, the symplastic sap, and the cell wall did not differ between -Si and +Si plants. Si did not affect the Al-induced exudation of organic acid anions and phenols from the root apices. However, Al treatment greatly enhanced Si accumulation in the cell wall fraction, reducing the mobility of apoplastic Al. From our data we conclude that Si treatment leads to the formation of hydroxyaluminumsilicates in the apoplast of the root apex, thus detoxifying Al.     

Quantitative Changes in Maize Membrane Proteins Induced by Aluminium

Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) was used for monitoring Al-induced changes in polypeptide composition of membrane proteins isolated from 3-d-old maize seedlings subjected to aluminium stress. Analysis of peripheral membrane proteins isolated from maize root showed an Al-induced increase in accumulation of 14 polypeptides with apparent molecular mass from 10 to 135 kDa. Qualitative differences were found between peripheral membrane proteins isolated from root tip (increased accumulation of 4 polypeptides with M_r 42 000 – 135 000) and from root base (increased accumulation of 10 polypeptides with M_r 10 000 – 59 000). On the other hand, no Al-induced changes were observed in peripheral membrane proteins isolated from maize coleoptile and integral membrane proteins isolated either from root or coleoptile. These results indicate that peripheral membrane proteins undergo considerable changes during 24-h Al treatment while integral membrane proteins pattern is stable.     

Silicon modifies root anatomy, and uptake and subcellular distribution of cadmium in young maize plants

Background and Aims

Silicon (Si) has been shown to ameliorate the negative influence of cadmium (Cd) on plant growth and development. However, the mechanism of this phenomenon is not fully understood. Here we describe the effect of Si on growth, and uptake and subcellular distribution of Cd in maize plants in relation to the development of root tissues.

Methods

Young maize plants (Zea mays) were cultivated for 10 d hydroponically with 5 or 50 µm Cd and/or 5 mm Si. Growth parameters and the concentrations of Cd and Si were determined in root and shoot by atomic absorption spectrometry or inductively coupled plasma mass spectroscopy. The development of apoplasmic barriers (Casparian bands and suberin lamellae) and vascular tissues in roots were analysed, and the influence of Si on apoplasmic and symplasmic distribution of ¹⁰⁹Cd applied at 34 nm was investigated between root and shoot.

Key Results

Si stimulated the growth of young maize plants exposed to Cd and influenced the development of Casparian bands and suberin lamellae as well as vascular tissues in root. Si did not affect the distribution of apoplasmic and symplasmic Cd in maize roots, but considerably decreased symplasmic and increased apoplasmic concentration of Cd in maize shoots.

Conclusions

Differences in Cd uptake of roots and shoots are probably related to the development of apoplasmic barriers and maturation of vascular tissues in roots. Alleviation of Cd toxicity by Si might be attributed to enhanced binding of Cd to the apoplasmic fraction in maize shoots.     

Aluminium-induced changes in root epidermal cell patterning, a distinctive feature of hyperresistance to Al in Brachiaria decumbens

Brachiaria, a genus of forage grasses of African origin, is gaining considerable importance because of both its nutritional value and its high stress resistance. An extraordinary resistance to Al toxicity has been reported in B. decumbens. The mechanisms of this hyperresistance are still unknown. This study explores the localization of Al in two contrasting Brachiaria species, the hyperresistant B. decumbens and the less resistant B. brizantha. Scanning Electron Microscope/Energy Dispersive Spectrometry, confocal fluorescence microscopy and optical microscopy of lumogallion or morin-stained roots was performed. Species differences in Al resistance were evident at 32 μM Al³⁺ activity in low ionic strength full nutrient solution containing Si. Roots of B. decumbens accumulated less Al than those of B. brizantha. Moreover, location and Al form seemed different. In B. decumbens Al accumulation was localized in hot spots of high Al concentrations. These sites with high Al accumulation mainly correspond to root hairs. B. brizantha exhibited a more even distribution of Al in cell walls of the root tip. Analysis of soluble phenolic substances in roots revealed species differences in response to Al. An Al-induced increase of chlorogenic acid concentrations was found in B. decumbens but not in B. brizantha. Taken together the results suggest a possible role for chlorogenic acid as a primer for changes in root epidermal cell patterning that may contribute to the Al hyperresistance in B. decumbens.     

Using a chelate-buffered nutrient solution to establish the critical solution activity of Mn2+ required by barley (Hordeum vulgare L.)

Relatively little is known about the responses of plants to micronutrients when these nutrients are maintained at the very low levels found in soils of low fertility. We have determined the requirement of barley (Hordeum vulgare L. cv Herta) for ionic Mn²⁺ in plant culture solutions using the chelating agent HEDTA as a buffer for micronutrient metal ions. The chemical activity of Mn²⁺ was varied approximately 10,000-fold from log(Mn²⁺)=–10.8 to –6.8 (pMn 10.8 to pMn 6.8), while holding constant the activities of the other micronutrient cations. Growth, appearance, and composition of Herta barley indicated that log(Mn²⁺) of approximately –8.3 would permit optimal dry matter production and normal plant development. The specific accumulation rate of Mn by 15 to 23 day old seedlings was a linear function of the Mn²⁺ activity in solution. At log(Mn²⁺) of about –9.8 or below, barley seedlings were unable to accumulate significant amounts of Mn, and at some harvests, suffered a net loss of Mn to solution. Seedlings younger than 11 days old were ineffective accumulators of several cations, including Mn, Fe, Zn, Cu, Mg, and Ca. Differences in Mn availability did not influence uptake of other cations, except that Cu uptake by roots increased with increasing Mn uptake.Abbreviations MES 2-(N-morpholino)-ethanesulphonic acid - HEDTA N-(2-hydroxyethyl)ethylene-dinitrilotriacetic acid - DTPA diethylenetrinitrilopentaacetic acid     

Al uptake kinetics in roots of Melastoma malabathricum L. – an Al accumulator plant

The mechanism of Al uptake in melastoma (Melastoma malabathricum L.), which accumulates Al in excess of 10 000 mg kg^–1 in its leaves and roots, was investigated. Al uptake kinetics in excised melastoma roots showed a biphasic pattern, with an initial rapid phase followed by a slow phase. It was indicated that Al uptake in the excised roots occurs mostly through passive accumulation in the apoplast. On the other hand, Al uptake rate in roots of whole melastoma plant was almost double that in excised roots. The difference of Al uptake rate between excised roots and whole plant seems to be due to transpiration-depended Al uptake. Results from a long-term experiment showed that different characteristics of Al accumulation between melastoma and barley was caused by the difference in capacity to retain Al in root symplast, rather than by the difference in uptake rate into symplast. Concentrations of oxalate in root symplastic and apoplastic fractions, and total oxalate in shoots and roots, did not change greatly with time of Al exposure compared to Al concentration, although oxalate is considered as a main Al ligand in tissue of melastoma. On the other hand, oxalate exudation to root apoplast was induced within 24 h of Al exposure; the role of such exudation was discussed.