Role of iron plaque in Cd uptake by and translocation within rice (Oryza sativa L.) seedlings grown in solution culture

A hydroponics culture experiment was conducted to investigate the effect of iron plaque on Cd uptake by and translocation within rice seedlings grown under controlled growth chamber conditions. Rice seedlings were pre-cultivated for 43 days and then transferred to nutrient solution containing six levels of Fe (0, 10, 30, 50, 80 and 100 mg L⁻¹) for 6 days to induce different amounts of iron plaque on the root surfaces. Seedlings were then exposed to solution containing three levels of Cd (0, 0.1 and 1.0 mg L⁻¹) for 4 days. In order to differentiate the uptake capability of Cd by roots with or without iron plaque, root tips (white root part without iron plaque) and middle root parts (with iron plaque) of pre-cultivated seedlings treated with 0, 30 and 50 mg L⁻¹ Fe were exposed to ¹⁰⁹Cd for 24 h. Reddish iron plaque gradually became visible on the surface of rice roots but the visual symptoms of the iron plaque on the roots differed among treatments. In general, the reddish color of the iron plaque became darker with increasing Fe supply, and the iron plaque was more homogeneously distributed all along the roots. The Fe concentrations increased significantly with increasing Fe supply regardless of Cd additions. The Cd concentrations in dithionite–citrate–bicarbonate (DCB)-extracts and in shoots and roots were significantly affected by Cd and Fe supply in the nutrient solution. The Cd concentrations increased significantly with increasing Cd supply in the solution and were undetectable when no Cd was added. The Cd concentrations in DCB-extracts with Fe supplied tended to be higher than that at Fe₀ at Cd_0.1, and at Cd_1.0, DCB-Cd with Fe supplied was significantly lower. Cd concentrations in roots and shoots decreased with increasing Fe supply at both Cd additions. The proportion of Cd in DCB-extracts was significantly lower than in roots or shoots. Compared to the control seedlings without Fe supply, the radioactivity of ¹⁰⁹Cd in shoots of seedlings treated with Fe decreased when root tips were exposed to ¹⁰⁹Cd and did not change significantly when middle parts of roots were exposed. Our results suggest that root tissue rather than iron plaque on the root surface is a barrier to Cd uptake and translocation within rice plants, and the uptake and translocation of Cd appear to be related to Fe nutritional levels in the plants.     

Root phosphate exudation and pH shift in the rhizosphere are not responsible for aluminum resistance in rice

A series of hydroponic experiments and an agar culture experiment were carried out to investigate aluminum (Al) accumulation and translocation in two rice (Oryza sativa L.) cultivars (Kasalath and Koshihikari) that differ in Al resistance. Al-resistance mechanisms, including Pi exudation under Al stress and pH shifts in the rhizosphere, were also studied. Al content in rice shoots was 41 mg kg⁻¹ on average and did not differ between the two cultivars, which demonstrated that the rice cultivars were not Al accumulators. The majority of Al (95–97%) accumulated in roots. Al content in roots in the resistant cultivar (Koshihikari) was lower than that in the sensitive cultivar (Kasalath), which indicated that Al-exclusion mechanisms were mainly acting in rice. However, the rate of Pi exudation from the whole root or root tips was very low in both cultivars and was not significantly influenced by Al exposure, and thus seemed not to be the main Al-resistance mechanism. On the other hand, experiments with pH-buffered solution and color changes following culture in agar medium containing bromocresol purple revealed that the Al-induced pH increase could not explain the high Al resistance of rice. In addition, the Al content in shoots of Koshihikari was lower after the formation of iron plaque on the root surface, whereas that of Kasalath was not lower. These results suggested that rice roots cell wall components or root surfaces such as iron plaque, rather than pH changes and/or root exudates including organic acids and phosphate, play important roles in Al resistance in rice.     

Interaction between iron plaque and root border cells ameliorates aluminum toxicity of Oryza sativa differing in aluminum tolerance

Root border cells (RBCs), which are generated during plant growth and surround the root cap, and iron plaque (IP), ubiquitously formed on the root surfaces of rice, are known to alleviate aluminum (Al) toxicity. To verify the interactive effects of IP and RBCs on ameliorating Al toxicity, two rice cultivars differing in Al resistance were used to compare Al tolerance between cultivars. Additionally, root elongation, Al uptake and RBCs viability were measured as indicators of the effects of Al. The amounts of DCB-extractable Fe and Al on the root surfaces were much higher in the presence of IP than the absence. IP presence significantly decreased Al-induced inhibition of root elongation and Al contents in roots and root tips. The removal of RBCs from the root tips caused a more severe inhibition of root elongation and a higher Al accumulation in rice roots and root tips. Furthermore, root growth inhibition and Al contents in roots and root tips were significantly lower in roots with a combination of IP and RBCs than in roots with IP or RBCs only. The formation of IP on the root surface maintained higher RBCs viability and depressed mucilage exudation in an Al-tolerant rice cultivar. The results suggest that both IP and RBCs ameliorate Al toxicity, and IP has a greater capacity for Al resistance. The combination of IP and RBCs exhibited a synergistic effect associated with Al resistance.     

Root iron plaque formation and characteristics under N2 flushing and its effects on translocation of Zn and Cd in paddy rice seedlings (Oryza sativa)

Background and Aims

Anoxic conditions are seldom considered in root iron plaque induction of wetland plants in hydroponic experiments, but such conditions are essential for root iron plaque formation in the field. Although ferrous ion availability and root radial oxygen loss capacity are generally taken into account, neglect of anoxic conditions in root iron plaque formation might lead to an under- or over-estimate of their functional effects, such as blocking toxic metal uptake. This study hypothesized that anoxic conditions would influence root iron plaque formation characteristics and translocation of Zn and Cd by rice seedlings.

Methods

A hydroponic culture was used to grow rice seedlings and a non-disruptive approach for blocking air exchange between the atmosphere and the induction solution matrix was applied for root iron plaque formation, namely flushing the headspace of the induction solution with N₂ during root iron plaque induction. Zn and Cd were spiked into the solution after root iron plaque formation, and translocation of both metals was determined.

Key Results

Blocking air exchange between the atmosphere and the nutrient solution by N₂ flushing increased root plaque Fe content by between 11 and 77 % (average 31 %). The N₂ flushing treatment generated root iron plaques with a smoother surface than the non-N₂ flushing treatment, as observed by scanning electron microscopy, but Fe oxyhydroxides coating the rice seedling roots were amorphous. The root iron plaques sequestrated Zn and Cd and the N₂ flushing enhanced this effect by approx. 17 % for Zn and 71 % for Cd, calculated by both single and combined additions of Zn and Cd.

Conclusions

Blocking of oxygen intrusion into the nutrient solution via N₂ flushing enhanced root iron plaque formation and increased Cd and Zn sequestration in the iron plaques of rice seedlings. This study suggests that hydroponic studies that do not consider redox potential in the induction matrices might lead to an under-estimate of metal sequestration by root iron plaques of wetland plants.     

大豆和水稻对铝胁迫响应的生理机制

通过水培方式,研究了铝处理对双子叶植物大豆和单子叶植物水稻根系生长、养分吸收、根系内含物及根分泌物的影响.结果表明,低铝 (10 μmol·L^-1)浸种刺激大豆种子萌发和根系的生长,对水稻无明显促进作用.铝处理增加了两种作物根系对P的吸收,降低K、Ca、Mg的吸收.水稻比大豆根系积累较少的Al和更多的P.铝胁迫条件下,大豆和水稻根系内源可溶性蛋白含量升高、可溶性酚含量下降、可溶性糖含量先上升后下降,且大豆根系内源柠檬酸含量下降明显.与大豆相比,水稻积累较低的柠檬酸和较高的可溶性蛋白、可溶性酚,但两者可溶性糖没有差异.铝处理增加大豆根系柠檬酸、可溶性蛋白、可溶性酚、可溶性糖的分泌量,且大豆分泌量显著高于水稻.在铝处理条件下,大豆根系具有较高的阳离子交换量(CEC),而水稻的CEC较低.这说明大豆和水稻对铝胁迫具有不同的生理反应,水稻的高耐铝性可能与其较高的磷吸收和较低的CEC有关,而与其根系分泌物的关系不大.     

Effects of Iron and Manganese Plaques on Arsenic Uptake by Rice Seedlings (Oryza sativa L.) Grown in Solution Culture Supplied with Arsenate and Arsenite

We have shown previously that phosphorus nutrition and iron plaque on the surface of rice roots influence arsenate uptake and translocation by rice in hydroponic culture. We have now investigated the role of iron (Fe) and manganese (Mn) plaque on arsenate and arsenite uptake and translocation in rice seedlings grown hydroponically. Fe and Mn plaques were clearly visible as reddish or brown coatings on the root surface after 12 h induction, and Fe plaque was much more apparent than Mn plaque. Arsenite or arsenate supply did not decrease plant dry weights significantly. There were significant differences in shoot dry weights but little difference in root dry weights between some plaque treatments. Arsenic (As) concentrations in Fe plaque when arsenate was supplied were significantly higher than those in no plaque (control) and Mn plaque treatments, and much higher than those supplied with arsenite. This showed that Fe plaque on the rice root had higher affinity to arsenate than to arsenite. In Fe plaque treatment, the results indicated that most As was sequestered in roots when arsenite was supplied and most As concentrated in Fe plaque when arsenate was supplied. Most As was accumulated in rice roots in Mn plaque and no plaque treatments for both As species.     

Do iron plaque and genotypes affect arsenate uptake and translocation by rice seedlings (Oryza sativa L.) grown in solution culture?

The effects of Fe concentrations in the pretreatment solution on the induction of plaque and the differences between genotypes on arsenate uptake by and translocation within rice seedlings grown in nutrient solution in the greenhouse were investigated. After iron plaque on rice roots was induced in solutions containing 20, 40, 60, 80, and 100 mg Fe2+ l(-1), seedlings were transplanted into nutrient solution with 0.5 mg As l(-1). The formation of iron plaque was clearly visible as a reddish coating on the root surface after 12 h induction. Fe2+ concentrations in the pretreatment solution and 0.5 mg As l(-1) in the treatment solutions did not significantly affect rice growth. There was a significant correlation between the concentrations of Fe and As in iron plaque on the root surface for the three genotypes. About 75-89% of total As was concentrated in iron plaque (DCB-extracts). There were no significant differences in As concentrations in the roots between the three genotypes; however, As concentrations in shoots differed significantly between them. Arsenic concentrations in shoots were positively correlated with iron concentrations in the shoots. The results suggest that iron plaque may act as a 'buffer' for As in the rhizosphere.     

Iron plaque enhances phosphorus uptake by rice (Oryza sativa) growing under varying phosphorus and iron concentrations

Both solution culture and pot experiments were performed to investigate (a) the effects of external Fe (II) concentrations and forms on the formation of iron plaque on the roots of rice (Oryza sativa) and subsequent P adsorption on iron plaque and shoot P concentrations and (b) the effects of soil moisture regimes on the formation of iron plaque and P adsorption on root surfaces and P accumulation in shoots. The results showed that iron plaque was significantly increased with increasing Fe²⁺ concentrations in the solution culture. The amounts of P adsorbed on the iron plaque were increased significantly with external Fe²⁺ concentrations. Although shoot P concentration was not significantly affected by Fe²⁺ treatment after incubation for 2 days, it was significantly increased in the Fe‐treated plants compared with Fe‐deprived ones after incubation for 4 days. Soil culture experiment showed that the formation of iron plaque on root surfaces was promoted by exogenous iron, with greater amount of iron plaque being formed by addition of ferric hydroxide than of ferric oxide. Phosphorus adsorption on iron plaque also increased with the addition of iron oxides, and increasing soil P increased the amounts of P associated with the iron plaque and shoot P concentration. The amounts of iron plaque were almost sixfold higher under flooding condition than under field capacity condition. Plants pretreated under flooding condition generally had higher shoot P concentrations when they were transplanted to solutions with varying P levels, and this was most pronounced in the treatment with highest solution P concentration. The results suggest that iron plaque acts as a nutrient reservoir for phosphorus in the rhizosphere and helps enhance P acquisition by rice.     

The effect of iron plaque on lead translocation in soil-Carex cinerascens kukenth. system

A pot experiment was conducted to investigate the effect of iron plaque on Pb uptake by and translocation in Carex cinerascens Kukenth. grown under open-air conditions. Using Scanning Electron Microscopy and Energy Dispersive X-Ray Spectrometry, iron plaque was present as an amorphous coating on root surfaces with uneven distribution. The amount of iron plaque increased significantly with increasing Fe additions regardless of Pb additions. The presence of iron plaque on the root surface of Carex cinerascens Kukenth. increased the concentrations of Pb adsorbed by iron plaque. The Pb percentage in whole roots increased by 14.52% at 500 mg kg^?1 Fe treatment than at 0 mg kg^?1 Fe, and the distribution coefficient (DC) of Pb and translocation factor (TF) root increased with Fe additions, but translocation factor (TF) shoot decreased with Fe additions. The results suggested that iron plaque could promote the translocation of Pb from soil to roots to some extent, and it played a role to reduce heavy metals pollution of Poyang Lake wetland.     

Effect of iron plaque outside roots on nutrient uptake by rice (Oryza sativa L.): Phosphorus uptake

Under anaerobic conditions, ferric hydroxide deposits on the surface of rice roots have been shown to affect the uptake of some nutrients. In the present experiment, different amount of this iron plaque were induced on the roots of rice (Oryza sativa L. cv. TZ88-145) by supplying different Fe(OH)₃ concentrations in nutrient solutions, and the effect of the iron plaque on phosphorus uptake was investigated. Results showed that 1) iron plaque adsorbed phosphorus from the growth medium, and that the amount of phosphorus adsorbed by the plaque was correlated with the amount of plaque; 2) the phosphorus concentration in the shoot increased by up to 72% after 72 h at concentration of Fe(OH)₃ in the nutrient solution from 0 to 30 mg Fe/L, corresponding with amounts of iron plaque from 0.2 to 24.5 mg g^-1 (root d. wt); 3) the phosphorus concentration in the shoots of rice with iron plaque was higher than that without iron plaque though the concentration in the shoot decreased when Fe(OH)₃ was added at 50 mg Fe/L producing 28.3 mg g^-1 (root d. wt) of plaque; and 4) the phosphorus concentrations in Fe-deficient and Fe-sufficient rice plants with iron plaque were the same, although phytosiderophores were released from the Fe-deficient roots. The phytosiderophores evidently did not mobilise phosphorus adsorbed on plaque. The results suggest that iron plaque on rice plant roots might be considered a phosphorus reservoir. This revised version was published online in June 2006 with corrections to the Cover Date.     

OsFRDL1 Is a Citrate Transporter Required for Efficient Translocation of Iron in Rice

Multidrug and toxic compound extrusion (MATE) transporters represent a large family in plants, but their functions are poorly understood. Here, we report the function of a rice (Oryza sativa) MATE gene (Os03g0216700, OsFRDL1), the closest homolog of barley (Hordeum vulgare) HvAACT1 (aluminum [Al]-activated citrate transporter 1), in terms of metal stress (iron [Fe] deficiency and Al toxicity). This gene was mainly expressed in the roots and the expression level was not affected by either Fe deficiency or Al toxicity. Knockout of this gene resulted in leaf chlorosis, lower leaf Fe concentration, higher accumulation of zinc and manganese concentration in the leaves, and precipitation of Fe in the root's stele. The concentration of citrate and ferric iron in the xylem sap was lower in the knockout line compared to the wild-type rice. Heterologous expression of OsFRDL1 in Xenopus oocytes showed transport activity for citrate. Immunostaining showed that OsFRDL1 was localized at the pericycle cells of the roots. On the other hand, there was no difference in the Al-induced secretion of citrate from the roots between the knockout line and the wild-type rice. Taken together, our results indicate that OsFRDL1 is a citrate transporter localized at the pericycle cells, which is necessary for efficient translocation of Fe to the shoot as a Fe-citrate complex.     

An aluminum-activated citrate transporter in barley

Soluble ionic aluminum (Al) inhibits root growth and reduces crop production on acid soils. Al-resistant cultivars of barley (Hordeum vulgare L.) detoxify Al by secreting citrate from the roots, but the responsible gene has not been identified yet. Here, we identified a gene (HvAACT1) responsible for the Al-activated citrate secretion by fine mapping combined with microarray analysis, using an Al-resistant cultivar, Murasakimochi, and an Al-sensitive cultivar, Morex. This gene belongs to the multidrug and toxic compound extrusion (MATE) family and was constitutively expressed mainly in the roots of the Al-resistant barley cultivar. Heterologous expression of HvAACT1 in Xenopus oocytes showed efflux activity for (14)C-labeled citrate, but not for malate. Two-electrode voltage clamp analysis also showed transport activity of citrate in the HvAACT1-expressing oocytes in the presence of Al. Overexpression of this gene in tobacco enhanced citrate secretion and Al resistance compared with the wild-type plants. Transiently expressed green fluorescent protein-tagged HvAACT1 was localized at the plasma membrane of the onion epidermal cells, and immunostaining showed that HvAACT1 was localized in the epidermal cells of the barley root tips. A good correlation was found between the expression of HvAACT1 and citrate secretion in 10 barley cultivars differing in Al resistance. Taken together, our results demonstrate that HvAACT1 is an Al-activated citrate transporter responsible for Al resistance in barley.     

Magnesium ameliorates aluminum rhizotoxicity in soybean by increasing citric acid production and exudation by roots

Superior effectiveness of Mg over Ca in alleviating Al rhizotoxicity cannot be accounted for by predicted changes in plasma membrane Al3+ activity. The influence of Ca and Mg on the production and secretion of citrate and malate, and on Al accumulation by roots was investigated with soybean genotypes Young and PI 416937 which differ in Al tolerance. In the presence of a solution Al3+ activity of 4.6 microM, citrate and malate concentrations of tap root tips of both genotypes increased with additions of either Ca up to 3 mM or Mg up to 50 microM. Citrate efflux rate from roots exposed to Al was only enhanced with Mg additions and exceeded malate efflux rates by as much as 50-fold. Maximum citrate release occurred within 12 h after adding Mg to solution treatments. Adding 50 microM Mg to 0.8 mM CaSO4 solutions containing Al3+ activities up to 4.6 microM increased citrate concentration of tap root tips by 3- to 5-fold and root exudation of citrate by 6- to 9-fold. Plants treated with either 50 microM Mg or 3 mM Ca had similar reductions in Al accumulation at tap root tips, which coincided with the respective ability of these ions to relieve Al rhizotoxicity. Amelioration of Al inhibition of soybean root elongation by low concentrations of Mg in solution involved Mg-stimulated production and efflux of citrate by roots.     

Non-destructive estimation of root mass using electrical capacitance on ten herbaceous species

Background and Aims

Characteristically baseline levels of Sb in the environment are low, but problematic local elevation trends arise from anthropogenic activities such as mining and incineration. Arsenic (analog of Sb) accumulation by rice can be reduced by iron (Fe) plaque. A hydroponic experiment was conducted to investigate whether Fe plaque could reduce the uptake and translocation of different Sb species in different rice cultivars.

Methods

After Fe plaque on rice roots was induced in solution containing 0, 0.2, 0.4, 0.7, 1.2, 2.0?mM Fe²⁺ for 24?h, seedlings were transferred into nutrient solution with 20?μM Sb(V) or Sb(III) for 3?d.

Results

About 60–80% (Sb(III) treatment) and 40–60% (Sb(V) treatment) of the total Sb accumulated in Fe plaque. There was a significant correlation between the concentrations of Sb and Fe on the root surface. A similar relationship was observed in roots and shoots. Cultivar (Jiahua 1) formed the most Fe plaque, had the highest Fe associated Sb sequestration but the lowest Sb concentration in the root interior.

Conclusions

Fe plaque may act as a ‘buffer’ for Sb(V) and Sb(III) in the rhizosphere, and cultivars played an important role in the different species Sb uptake and translocation.     

Form of aluminium for uptake and translocation in buckwheat (Fagopyrum esculentum Moench)

 The forms of Al for uptake by the roots and translocation from the root to the shoot were investigated in a buckwheat (Fagopyrum esculentum Moench, cv. Jianxi) that accumulates Al in its leaves. The Al concentration in the xylem sap was 15-fold higher in the plants exposed to AlCl₃ than in those exposed to an Al-oxalate (1:3) complex, suggesting that the roots take up Al in the ionic form. The Al concentration in the xylem sap was 4-fold higher than that in the external solution after a 1-h exposure to AlCl₃ solution and 10-fold higher after a 2-h exposure. The Al concentration in the xylem sap increased with increasing Al concentration in the external solution. The Al uptake was not affected by a respiratory inhibitor, hydroxylamine, but significantly inhibited by the addition of La. These results suggest that Al uptake by the root is a passive process, and La³⁺ competes for the binding sites for Al³⁺ on the plasma membrane. The form of Al in the xylem sap was identified by ²⁷Al-nuclear magnetic resonance analysis. The chemical shift of ²⁷Al in the xylem sap was around 10.9 ppm, which is consistent with that of the Al-citrate complex. Furthermore, the dominant organic acid in the xylem sap was citric acid, indicating that Al was translocated in the form of Al-citrate complex. Because Al is present as Al-oxalate (1:3) in the root, the present data show that ligand exchange from oxalate to citrate occurs before Al is released to xylem. Received: 10 December 1999 / Accepted: 3 February 2000     

Iron plaque decreases cadmium accumulation in Oryza sativa L. and serves as a source of iron

• Cadmium (Cd) contamination occurs in paddy soils; hence it is necessary to reduce Cd content of rice. Application and mode of action of ferrous sulphate in minimizing Cd in rice was monitored in the present study.
• Pot culture with Indian rice variety Swarna (MTU 7029) was maintained in Cd‐spiked soil containing ferrous sulphates, which is expected to reduce Cd accumulation in rice. Responses in rhizosphere pH, root surface, metal accumulation in plant and molecular physiological processes were monitored.
• Iron plaque was induced on root surfaces after FeSO4 application and the amount of Fe in plaque reduced with increases in Cd in the soil. Rhizosphere pH decreased during plaque formation and became more acidic due to secretion of organic acids from the roots under Cd treatment. Moreover, iron chelate reductase activity increased with Cd treatment, but in the absence of Cd, activity of this enzyme increased in plaque‐induced plants. Cd treatment caused expression of OsYSL18, whereas OsYSL15 was expressed only in roots without iron plaque. Fe content of plants increased during plaque formation, which protected plants from Cd‐induced Fe deficiency and metal toxicity. This was corroborated with increased biomass, chlorophyll content and quantum efficiency of photo‐synthesis among plaque‐induced plants.
• We conclude that ferrous sulphate‐induced iron plaque prevents Cd accumulation and Fe deficiency in rice. Iron released from plaque via organic acid mediated dissolution during Cd stress.

     

The beneficial effect of aluminium and the role of citrate in Al accumulation in Melastoma malabathricum

* Here we investigated the beneficial effect of aluminium (Al) on the development of the Al accumulating plant Melastoma malabathricum. * Seedlings of M. malabathricum were cultivated in a nutrient solution containing 0.5 mM Al and compared with barley (Hordeum vulgare). In addition, roots of M. malabathricum were divided into one part growing in a nutrient solution, and the other part growing in a calcium solution. Al (0.5 mM) was applied to either solution. * Al-induced improvements of the root activity contributed to a growth enhancement in M. malabathricum. Al exposure without nutrients did not increase root growth and Al accumulation in the leaves. The beneficial effect, however, was induced by the combination of Al and nutrients. * We suggest that without nutrients roots are not able to synthesize an adequate amount of citrate that is required for transporting Al to the leaves. High Al levels in the plant tissues and/or an interaction of Al with particular nutrient elements in the apoplast of root cells appear to be essential to exert the beneficial effect of Al.     

Effect of iron plaque outside roots on nutrient uptake by rice (Oryza sativa L.). Zinc uptake by Fe-deficient rice

This solution culture study examined the effect of the deposition of iron plaque on zinc uptake by Fe-deficient rice plants. Different amounts of iron plaque were induced by adding Fe(OH)₃ at 0, 10, 20, 30, and 50 mg Fe/L in the nutrient solution. After 24 h of growth, the amount of iron plaque was correlated positively with the Fe(OH)₃ addition to the nutrient solution. Increasing iron plaque up to 12.1 g/kg root dry weight increased zinc concentration in shoots by 42% compared to that at 0.16 g/kg root dry weight. Increasing the amount of iron plaque further decreased zinc concentration. When the amounts of iron plaque reached 24.9 g/kg root dry weight, zinc concentration in shoots was lower than that in shoots without iron plaque, implying that the plaque became a barrier for zinc uptake. While rice plants were pre-cultured in –Fe and +Fe nutrient solution in order to produce the Fe-deficient and Fe-sufficient plants and then Fe(OH)₃ was added at 20, 30, and 50 mg Fe/L in nutrient solution, zinc concentrations in shoots of Fe-deficient plants were 54, 48, and 43 mg/kg, respectively, in contrast to 32, 35, and 40 mg/kg zinc in shoots of Fe-sufficient rice plants. Furthermore, Fe(OH)₃ addition at 20 mg Fe/L and increasing zinc concentration from 0.065 to 0.65 mg Zn/L in nutrient solution increased zinc uptake more in Fe-deficient plants than in Fe-sufficient plant. The results suggested that root exudates of Fe-deficient plants, especially phytosiderophores, could enhance zinc uptake by rice plants with iron plaque up to a particular amount of Fe.     

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。     

Rhythmic iron stress reactions in sunflower at suboptimal iron supply

Uptake and translocation of labelled iron were studied in sunflower ( Helianthus annuus L. cv. Sobrid) grown in nutrient solution with low FeEDDHA concentrations during preculture. In contrast to conditions for plants adequately supplied with iron, suboptimal iron supply leads to temporary Fe stress with rhythmic rates of uptake and translocation of iron (period 2–4 days). This rhythmic behaviour of iron uptake is associated with corresponding changes in morphology (thickening of root tips) and physiology (increase in reducing capacity) of the roots. Iron stress is alleviated within less than one day if sufficient iron is available. This is indicated by normalisation of root morphology, reducing capacity and rate of iron uptake and translocation. This rhythm in iron uptake stresses the importance of rhythmic patterns of biochemical behaviour in complex biological systems. It is suggested that phytohormones are involved in the transformation of the iron nutritional status of the shoot apex into a "signal" for the uptake sites of iron in the roots. Preliminary experiments with sunflower in calcareous soil indicate an ecological importance of this fine regulation mechanism for plants on soil with a low iron availability, manifested in rhythmic iron stress reactions.