Effect of silicon on alleviation of manganese toxicity in pumpkin (Cucurbita moschata Duch cv. Shintosa)

A series of experiments was conducted to investigate the mechanism of Siinduced alleviation of Mn toxicity in pumpkin (Cucurbita moschata Duch cv. Shintosa) which has been used as a bloom-type stock for grafting cultivation of cucumbers. In the first experiment, the effect of Si on Mn toxicity in the Shintosa cultivar was compared with that in the Super unryu cultivar which is used as a bloomless-type stock. Without Si supply, growth depression occurred in both cultivars at the levels of 100 and 500 µM Mn in the nutrient solution. The alleviative effect of Si on the growth depression was observed more distinctly for the Shintosa cultivar than for the Super unryu cultivar, and the effect became more pronounced with increasing Si levels in the nutrient solution. Regardless of the Mn levels, addition of Si did not decrease the Mn content of the plants. The relative shoot growth was reduced to less than 80% when the molar ratio of Si/Mn in the shoots was lower than 4.5. In the second experiment, Mn and Si in the lower leaves of the Shintosa cultivar treated with different levels of Mn with and without Si supply were extracted with 10 mM Tris-HCI buffer (pH 7.4). At the levels of 100 and 250 µM Mn, the amounts of Mn in the buffer-insoluble fraction increased in the presence of Si supply and the buffer-soluble Mn accounted for less than 9% of the total Mn in the leaf. In addition, more than 84% of the total Si was found in the buffer-insoluble fraction at each Mn level. Distribution of Mn and Si on the surface of the lowerú leaves of the Shintosa cultivar treated with 250 µM Mn with and without Si supply was examined by electron probe X-ray microanalysis in the third experiment; Both Mn and Si accumulated at high concentrations around the base of the trichomes in the presence of Si supply. Without Si supply, Mn was detected around the necrotic brown lesions in addition to the base of the trichomes. These results suggested that in the Shintosa cultivar, Si alleviated the Mn toxicity through a localized accumulation of Mn with Si in a metabolically inactive form around the base of the trichomes on the leaf surface.     

Phosphorus nutrition alleviates manganese toxicity in Lolium perenne and Trifolium repens

Phosphorus (P) nutrition has been suggested to play a role in the alleviation of manganese (Mn) toxicity in some higher plant species. However, there are few reports on the role of P in regulating Mn accumulation by forage species. We studied the effect of P nutrition on Mn toxicity in Lolium perenne L. and Trifolium repens L. An increase in Mn concentration in root and shoot tissues was associated with an increase in both P supply and P tissue concentrations. Nevertheless, in both forage species, especially white clover, plant‐growth inhibition caused by Mn excess was decreased with increasing P additions. Moreover, the carboxylate exudation that had increased in response to high Mn was gradually reduced by increasing P supply. We suggest that P supply may have a beneficial effect in reducing the severity of Mn toxicity in forage species.     

不同柑橘砧木对锰过量胁迫的耐受性及生理响应

  【目的】  土壤pH影响土壤锰 (Mn) 有效性,酸性土壤易出现Mn过量问题,我国柑橘主要分布在南方红黄壤区,柑橘园酸性或强酸性土壤比例高,柑橘园土壤Mn过量较普遍。为此,我们研究了4种柑橘砧木对Mn过量胁迫的耐受性和生理响应,以期为Mn过量土壤上适宜砧木的选择提供依据。  【方法】  选用枳、资阳香橙、红橘和沙田柚4种常用柑橘砧木苗为材料,采用蛭石与珍珠岩1∶1的基质进行了营养液栽培试验,营养液中Mn处理包括0.01 (对照)、0.05、0.25、1.25和6.25 mmol/L 5个浓度。观察砧木苗的生长反应和中毒症状,处理60天时,测定叶绿素含量和光合参数;处理67天终止处理,测定砧木苗生长量、生物量、过氧化物酶活性、营养元素含量等生理生化指标,并用隶属函数对砧木过量锰的耐受性进行综合评价。  【结果】  柑橘砧木苗出现锰中毒的症状为叶片失绿,出现褐色坏死斑点;根量变少,呈现褐色斑点。4种砧木苗均在Mn 0.25 mmol/L处理时出现Mn中毒症状,其中枳最先出现症状且最严重,资阳香橙最迟出现症状且最轻;4种砧木在Mn 0.05 mmol/L处理时即出现Mn过量胁迫,表现为地上部和地下部鲜重和干重显著下降,根冠比升高 (红橘除外)、叶绿素含量下降、净光合速率降低、气孔开度下降、胞间CO₂浓度上升。Mn过量 (> 0.05 mmol/L) 胁迫使4种砧木叶片细胞膜受损,相对电导率和MDA含量上升;清除活性氧的SOD和POD活性上升,CAT活性下降。Mn过量胁迫影响柑橘砧木的营养元素吸收和转运,叶片和根系Mn含量上升,但随Mn胁迫浓度升高,Mn从根系到叶片的迁移率先降低后升高;Mn过量胁迫使砧木叶片K、P、Ca、Mg、Fe、Zn元素含量下降,根系K、P、Fe、Zn含量上升而Ca、Mg含量下降。  【结论】  不同砧木对Mn过量胁迫耐受性存在明显的差异,综合评价耐性强弱顺序为:资阳香橙 > 沙田柚 > 红橘 > 枳,高锰土壤的柑橘园可选用资阳香橙做砧木以减轻锰害。     

Electron paramagnetic resonance studies of manganese toxicity,tolerance, and amelioration with silicon in snapbean

Plant genotypes within species differ widely in tolerance to excess manganese (Mn) that may occur in acid soils, or in neutral or alkaline soils having poor aeration caused by imperfect drainage or compaction. However, Mn tolerance mechanisms in plants are largely unknown. Silicon (Si) is reported to detoxify Mn within plants, presumably by preventing localized accumulations of Mn associated with lesions on leaves. Because Mn is paramagnetic, electron paramagnetic resonance (EPR) spectroscopy, shows promise as a tool for characterizing toxic and non‐toxic forms of Mn in tolerant and sensitive plants. The objective of our study was to use EPR to: i) determine the chemical/ physical state of Mn in Mn‐tolerant and ‐sensitive snapbean cultivars; and ii) characterize the protective effects of Si against Mn toxicity. Manganese‐sensitive Wonder Crop 2 (WC) and Mn‐tolerant Green Lord (GL) cultivars of snapbean were grown at pH 5.0, in a greenhouse, in a modified Steinberg solution containing: Mn=0.05mg.L^‐1 (optimal); Mn=1.0mgL^‐1 (toxic); Mn=1.0 mg L^‐1 plus Si=4 mg L^‐1; and Mn=0.05 mg L^‐1 plus 4 mg Si L^‐1. All trifoliate leaf samples exhibited a 6‐line EPR signal that is characteristic of hexaaquo Mn²⁺. In both cultivars, a higher EPR Mn²⁺ signal‐intensity generally correlated with lower total leaf mass, higher total Mn concentrations and more pronounced symptoms of toxicity. Tolerance to excess Mn coincided with lower Mn²⁺ signal intensity. Silicon treatments ameliorated Mn toxicity symptoms in both genotypes, decreased total leaf Mn concentrations, and decreased EPR Mn²⁺ signal intensity. Results suggest that Mn toxicity is associated with reduced electron transport and accumulation of oxidation products in leaves. Amelioration of Mn toxicity by Si is regarded as connected with a reduction in this Mn‐induced process. Results indicated that EPR spectroscopy can be useful in investigating the biochemical basis for differential Mn tolerance in plants. The EPR observations might also help plant breeders in developing Mn‐tolerant cultivars.     

Physiology of manganese toxicity and tolerance in Vigna unguiculata (L.) Walp.

In cowpea (Vigna unguiculata (L.) Walp.) tolerance of manganese (Mn) excess depends on genotype, silicon (Si) nutrition, form of nitrogen (N) supply, and leaf age. The physiological mechanisms for improved Mn leaf-tissue tolerance are still poorly understood. On the basis of the density of brown spots per unit of leaf area and the callose content which are sensitive indicators of Mn toxicity, it was confirmed that cultivar (cv.) TVu 1987 was more Mn-tolerant than cv. TVu 91, young leaves were more Mn-tolerant, Si improved Mn tolerance, and NO₃^—-grown plants were more Mn-tolerant than NH₄⁺-grown plants. A close positive relationship existed between the bulk-leaf Mn content and the vacuolar Mn concentration from the same leaves. Since no clear and consistent differences existed between leaf tissues differing in Mn tolerance, the results suggest that accumulation of Mn in the vacuoles and its complexation by organic anions do not play a role in Mn leaf-tissue tolerance in cowpea. A near linear relationship was found between leaf Mn contents and concentrations of free (H₂O-soluble) and exchangeable-bound (BaCl₂-extractable) Mn in the apoplastic washing fluid (AWF) extracted from whole leaves by an infiltration and centrifugation technique. There were no differences in apoplastic Mn concentrations owing to genotype and form of nitrogen nutrition. However, Si decreased the Mn concentration in the AWF. With increasing bulk-leaf Mn contents, concentrations of organic anions in the AWF also increased. The results suggest that complexation of Mn by organic anions in the leaf apoplast contribute to Mn tolerance due to genotype and more clearly due to NO₃-N nutrition. Cell wall-bound peroxidase activity increased with leaf age and was higher in the Mn-sensitive cv. TVu 91 than in cv. TVu 1987. This was in agreement with a higher H₂O₂ production rate in cv. TVu 91. Also, a lower ratio of reduced to oxidized ascorbic acid in the AWF revealed that in Mn-sensitive leaf tissue, the apoplastic reduction capacity was lower than in Mn-tolerant leaf tissue when genotypes and leaves of different age were compared. We interpret our results as strong circumstantial evidence that Mn tolerance depends on the control of the free Mn²⁺concentration and of Mn²⁺-mediated oxidation/reduction reactions in the leaf apoplast.     

Silicon modulates the metabolism and utilization of phenolic compounds in cucumber (Cucumis sativus L.) grown at excess manganese

Cucumber plants (Cucumis sativus L. cv. Chinese long) were grown in nutrient solution with increasing manganese (Mn) concentrations (0.5, 50, and 100 µM) with (+Si) or without silicon (–Si) supplied as silicic acid at 1.5 mM. High external Mn supply induced both growth inhibition of the whole plant and the appearance of Mn‐toxicity symptoms in the leaves. The application of Si alleviated Mn toxicity by increasing the biomass production. Although the total Mn concentration in the leaves did not differ significantly between +Si and –Si plants, symptoms of Mn toxicity were not observed in Si‐treated plants. The concentrations of phenolic compounds, particularly in the leaf extracts of cucumber plants grown at high external Mn concentrations, differed from those of plants grown without Si. The increased tissue concentrations of phenols (e.g., coniferyl alcohol, coumaric and ferulic acids) were in agreement with enhanced enzymes activities, i.e., peroxidases (PODs) and polyphenol oxidases (PPO) in the tissues of –Si plants. The activities of both enzymes were kept at a lower level in the tissue extracts of +Si plants grown at high external Mn concentrations. These results suggest that Si nutrition modulates the metabolism and utilization of phenolic compounds mainly at the leaf level, most probably as a consequence of the formation of Si‐polyphenol complexes.     

Manganese Supply and pH Influence Growth,Carboxylate Exudation and Peroxidase Activity of Ryegrass and White Clover

ABSTRACT

The effects of differential manganese (Mn) supply (0 to 355 μ M) and pH (4.8 and 6.0) on dry weight (DW), tissue concentrations of Mn, exudation of carboxylates, and the peroxidase activity were studied in ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) grown in nutrient solution. In both plant species, the increase in Mn supply caused a significant reduction in DW due to severe Mn toxicity, especially at pH 4.8. The critical toxicity concentration of Mn in shoots was 421 mg kg^{? 1} for ryegrass and 283 mg kg^{? 1} for white clover. For both plant species, an increase in Mn supply levels stimulated the exudation of carboxylates and the activity of peroxidase, which was related to stress conditions. The highest amount of carboxylates was exuded at pH 4.8. There was no clear effect of carboxylates on the complexation of Mn^{2 +}.     

Physiological and biochemical effects of boron toxicity in mustard during the seedling stage

A study was conducted to determine the physiological and biochemical effects of boron in seedlings of mustard (Brassica juncea L. var Varuna). For this seeds were sown in petridishes with varying concentrations of boron (0, 0.33, 3.3, 33, 330 mM) in seed germinator. Seed germination and vigor index was found to be decreased and percentage phytotoxicity was increased in seeds with increase in the concentration of boron in germinating solution. There was accumulation of sugars and decreased starch concentration in cotyledons and embryonic axes of growing seedling with increasing boron stress. For the estimation of oxidative damage in cotyledons and embryonic axes of mustard seedlings, hydrogen peroxide (H₂O₂), thiobarbituric acid reactive substances (TBARS), phenols and activities of antioxidative enzymes- polyphenoloxydase (PPO), superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) were determined. Boron toxicity caused accumulation of H₂O₂, TBARS and phenols and affected the antioxidative enzyme activity in growing seedling components.     

Visual symptoms and tissue manganese concentrations associated with manganese toxicity in wheat

Abstract

Wheat (Triticum aestivum L.) was grown in nutrient solution in the greenhouse. Mn concentrations in the nutrient solutions used ranged from 0.0025 to 50,000 mg/1. Visual symptoms associated with high tissue Mn content were stunting, general chlorosis, necrotic leaf spots, white flecking, purpling, and leaf tip burn. Mn tissue concentrations of 380 mg/kg were found to reduce dry matter production by 10%.     

Silicon‐induced cadmium resistance in rice (Oryza sativa)

Silicon (Si)‐induced cadmium (Cd) tolerance in rice (Oryza sativa L.) was investigated by analyzing Cd uptake, growth, and physiological parameters. Silicon treatments (0.0, 0.2, or 0.6 mM) were added to 6 d–old seedlings, and Cd treatments (0.0 or 5.0 μM) were added to 20 d–old seedlings. Parameters determined included: maximum net CO₂ assimilation (A_max), stomatal conductance (g_smax), and transpiration (E_max) rates at varying intercellular CO₂ concentrations (C_i). Also measured were chlorophyll fluorescence, growth, and Cd‐uptake parameters. Results showed a Si‐induced inhibition of Cd uptake. However, 0.2 mM or 0.6 mM Si treatment concentrations did not differentially inhibit Cd uptake or differentially alleviate Cd‐induced growth inhibition, despite a significant increase in tissue Si concentration due to 0.6 mM Si treatment compared to 0.2 mM Si treatment. Additionally, photosynthesis and chlorophyll‐fluorescence analysis showed that treatment with Cd significantly inhibited photosynthetic efficiency. Interestingly, the addition of 0.2 mM Si, more so than the addition of 0.6 mM Si, significantly alleviated the inhibitory effects of Cd toxicity on photosynthesis and chlorophyll‐fluorescence parameters. Our results suggest that 0.2 mM Si could be close to an optimum Si‐dose requirement for the alleviation of toxicity symptoms mediated by moderate (5 μM) Cd exposure.     

Evaluating quantitative screening methods for manganese toxicity in cotton genotypes 1

Identification of cotton genotypes more tolerant of toxic concentrations of soil solution manganese (Mn²⁺) would integrate well with soil ameliorations of that problem. Several quantitative and semi‐quantitative methods to determine the amount of Mn toxicity were evaluated on three genotypes of Gossypium hirsutum (LaDSIS 12513, LaDASS 5175, and Coker gl 79–501) and one genotype of Gossypium barbadense (Pitnas S‐5). Specific leaf weight (SLW) and the semi‐quantitative, ‘percentage of leaves that were damaged’ (PLD) correlated the least with other methods of Mn toxicity determination. Neither SLW or PLD provided more separation between genotypes than area/leaf (AL), peroxidase (POD) activity, and indole‐3‐acetic acid oxidase (IAAO) activity. Similar genotype separations occurred for AL, POD, and IAAO at 10 mg/L Mn in solution, but POD and IAAO produced more genotype separations than AL at 5 mg/L of Mn. There were differences in enzyme activity between genotypes at control (0.25 mg/L) Mn solution concentration, making assessment difficult, especially between species. Barring this caveat, the relatively fast POD activity assay was considered to be the best method since it paralleled activity of IAAO, the functional enzyme of Mn toxicity, which had a relatively slow assay method.     

Effect of manganese on the resistance of tomato to Pseudocercospora fuligena

Black leaf mold caused by Pseudocercospora fuligena is an important fungal disease of tomato in Southeast Asia. The objective of this study was to evaluate the control of this disease using high manganese (Mn) applied to the root substrate and to evaluate the role of the leaf apoplast in plant response to fungal infection. In a nethouse experiment in Thailand, Mn above the optimum for plant growth but below toxicity increased resistance of tomato plants to black leaf mold. Enhanced resistance caused by Mn was also obtained when tomato plants were grown under controlled conditions in a mist chamber and artificially inoculated with the fungus. Manganese significantly increased plant peroxidases in the leaf apoplast. The highest peroxidase activity was measured when plants were inoculated with Pseudocercospora fuligena. Defense‐related proteins in the leaf apoplast increased when plants were inoculated with Pseudocercospora fuligena but not when treated with high Mn. It is concluded that Mn above the optimum level for plant growth can contribute to the control of Pseudocercospora fuligena in tomato. The Mn effect on disease resistance is associated with the activation of plant peroxidases in the leaf apoplast. A systemic response, possibly mediated by NADH peroxidase activity, also seems to trigger disease resistance in leaves with low Mn concentrations.     

硅对大麦铝毒的消除和缓解作用研究

在温室和实验室进行了施硅对消除或缓解大麦酸害铝毒的土培和溶液培养试验。结果表明,施硅后大麦幼苗的地上部茎、叶和地下部根的生物量均比不施硅明显增加。施硅能有效地促使植株吸收的铝在根部积累,抑制铝向地上部分运转;施硅还能调节根吸收的磷向地上部分运移,以减轻因伴随铝毒而产生的缺磷症状。施硅消除或缓解酸害铝毒的可能机理是:铝与硅形成无毒的铝硅酸复合离子(HAS),降低活性铝的浓度,及硅能调节大麦幼苗地上部和根内铝和磷的再分配。     

Manganese toxicity in bush bean as affected by concentration of manganese and iron in the nutrient solution

An experiment was conducted to clarify the relationship between Mn toxicity and Fe deficiency in bush snap bean (Phaseolus vulgaris L. cv. ‘Wonder Crop No. 2'). Seedlings were grown in full strength Hoagland No. 2 solution at pH 6.0 for ten days. Six concentrations of Mn as MnCl₂.4H₂O were used in combination with three concentrations of Fe as FeEDTA.

Toxicity symptoms, induced by low levels of Mn (0.1 ppm and above), included: small brown necrotic spots and veinal necrosis on primary leaves; necrosis on primary leaf petioles; interveinal chlorosis, with or without brown necrotic spots, on trifoliate leaves; and brown necrotic spots on stipules. Manganese toxicity symptoms were alleviated or prevented by increasing Fe concentration in the nutrient solution.

Manganese concentration in the leaves increased with increasing Mn and decreased with increasing Fe concentration in the nutrient solution, Iron concentration in the roots increased with increasing Fe concentration in the nutrient solution; however, Fe concentration in the leaves was not significantly affected by increasing Mn concentration in the solution culture. Manganese toxicity symptoms developed when Mn concentration in the leaves reached about 120 ppm.

A decrease in the Fe/Mn ratio in the nutrient solution resulted in a proportionate decrease in that of the leaves. Manganese toxicity symptoms occurred when the Fe/Mn ratio in the solution was 10.0 and below, or when the ratio in the leaves was less than 1.5. The ratio of Fe/Mn in the solution required for optimum growth of ‘Wonder Crop No. 2’ bean, without Mn toxicity symptoms, was in the range of 20.0 to 25.0.

Results indicate that the chlorosis on bush bean leaves induced by excessive Mn in the nutrient solution was due to excessive accumulation of Mn and not to Fe deficiency.     

Effect of manganese on endomycorrhizal sugar maple seedlings

Abstract

Manganese (Mn) toxicity may play an important role in the poor survival of seedlings in declining sugar maple (Acer saccharum Marsh.) stands in northern Pennsylvania. To determine the effect of Mn on the growth of sugar maple seedlings, 1‐year‐old seedlings inoculated with vesicular‐arbuscular mycorrhizal (VAM) fungi and growing in sand‐vermiculite‐peat moss medium were irrigated for 7 weeks with nutrient solution (pH 5) containing 0.1 (control), 1, 2, 4, 8, or 16 mg L^?1 Mn. Total seedling dry weight was negatively correlated with Mn, becoming significantly different than the control at 2 mg L^?1 Mn. Stem and root dry weight were reduced by lower Mn levels than leaf dry weight. Manganese had no effect on the root/shoot ratio. The concentration of Mn in roots and leaves increased as the level of Mn in the nutrient solution increased, with the concentration in the leaves 2.2‐ to 3.7‐fold greater than the concentration in the roots. Except for a reduction of P in the roots, Mn had little effect on the concentration of nutrient elements in the roots or leaves. Colonization of the roots by VAM fungi was increased by Mn, with a maximum percentage at 4 mg L^?1 Mn. Manganese toxicity symptoms in the leaves, small discrete chlorotic spots, began to appear at 1 mg L^?1 Mn. The sensitivity of sugar maple seedlings to Mn found in this study supports the hypothesis that Mn may affect regeneration in declining sugar maple stands. However, evaluation of the effects of Mn on seedlings in native soils under field conditions will be necessary before the role of Mn in sugar maple regeneration can be understood.     

Silicon Increases Tolerance to Boron Toxicity and Reduces Oxidative Damage in Barley

ABSTRACT

Silicon (Si) protects plants from multiple abiotic and biotic stresses The effect of exogenous Si levels (50, 75, and 100 mg kg^?1) on the growth, boron (B) and Si uptake, lipid peroxidation (MDA), lipoxygenase activity (LOX; EC 1.13.11.12), proline, and H₂O₂ accumulation, non-enzymatic antioxidant activity (AA) and the activities of major antioxidant enzymes (superoxide dismutase, SOD, EC 1.15.1.1; catalase, CAT, EC 1.11.1.6 and ascorbate peroxidase, APX, EC 1.11.1.11) of barley (Hordeum vulgare L.) were investigated under glasshouse conditions. Increasing levels of Si supplied to the soil with 20 mg kg^?1 B counteracted the deleterious effects of B on shoot growth. Application of B significantly increased the B concentration in barley plants. However, Si application decreased B concentrations. Increasing application of Si increased the Si concentration in barley plants. The concentration of H₂O₂ was increased by B toxicity but decreased by Si supply. Boron toxicity decreased proline concentrations and increased lipid peroxidation (MDA content) and LOX activity of barley. Compared with control plants, the activities of AA, SOD, CAT, and APX in B stressed plants grown without Si decreased, and application of Si increased their activities under toxic B conditions. The LOX activity was decreased by Si. Based on the present work, it can be concluded that Si alleviates B toxicity by possibly preventing oxidative membrane damage, both through lowering the uptake of B and by increasing tolerance to excess B within the tissues.     

Effect of silicon on the alleviation of boron toxicity in wheat growth,boron accumulation,photosynthesis activities,and oxidative responses

Little is known about the alleviation of boron (B) toxicity in wheat induced by silicon (Si), especially on the photosynthesis properties and antioxidative responses. The purpose of this research was to evaluate the effect of Si on the toxicity of B in wheat and the related mechanisms. A greenhouse pot experiment was conducted using wheat (Triticum aestivum Linn.) at the early seedling stage. Boric acid was added to soil to create three B concentrations. Each B treatment consisted of two Si treatment including control and Si application. Our results show that Si has an alleviative effect on B toxicity in wheat plant. Si showed significant alleviative effect on wheat growth at 150 mg B kg^?1 whereas did not show significant alleviative effect at 300 mg B kg^?1. Under B stress, plant dry weight of wheat was reduced and the reduction was alleviated by Si. However, plant tissue B accumulation was not reduced by the application of Si. Net photosynthetic rates of wheat were not influenced significantly by B or Si. The oxidative damages in wheat that were caused by excess B were not significantly alleviated by Si. These results suggest that the effect of Si on B toxicity in wheat is still controversial and more studies need to be conducted.     

Interaction of silicon,iron, and manganese in rice (oryza sativa L.) rhizosphere at low ph in relation to rice growth

Rice (Oryza Sativa L.) nutrition is influenced by the interactions of (Iron) Fe, (Manganese) Mn, and (Silicon) Si in the rhizosphere. A greenhouse experiment was carried out with rice grown in four low‐pH soils (a granitic lateritic red earth, a paddy soil from the red earth, a basaltic latosol, and a paddy soil from the latosol). Rice was grown in pots with the roots confined in rhizobags and the rhizosphere soil and nonrhizosphere soil were analyzed separately for active Si, Fe, and Mn by Tamm's solution. Silicon and Mn concentrations were lower in the rhizosphere soil indicating a depletion which was higher for the basaltic soils and for the paddy soils. Iron concentrations were higher in the rhizosphere soil indicating an accumulation that was higher for granitic soils and for the upland soils. Plant growth response was due mostly to Mn with the basaltic soils supplying toxic amounts and the granitic soils being deficient. Iron accumulation in the rhizosphere caused lower plant uptake of Si, phosphorus (P), and calcium (Ca) and higher Fe and aluminum (Al) absorption leading to the conclusion that Fe deposition on plant roots and in rhizosphere may block the uptake of other nutrients.     

Nickel toxicity in two durum wheat cultivars differing in drought sensitivity

The effects of nickel (Ni) on growth, leaf water status, and mineral nutrient concentration were studied in two wheat (Triticum durum Desf.) cultivars with different sensitivity to water stress: ‘Adamello’ [drought sensitive (DS)] and ‘Ofanto’ [drought tolerant, (DT)]. The DT cultivar showed a higher Ni absorption capacity: ‘Ofanto’ seedlings grown in the presence of 35 μM Ni had a 3.5 times greater concentration of Ni in roots than did ‘Adamello’. Despite the greater Ni tissue content, the DT cultivar exhibited better growth and nutritional status when compared to the DS cultivar. In the DS cultivar the concentration of chlorophyll a and b was reduced by Ni treatment. Chlorophyll a concentration decreased in the DT cultivar, but to a lesser extent than in the DS cultivar; chlorophyll b was not altered by Ni level in the DT cultivar. Nickel caused a decrease in the water potential (ψ_w) and relative water content (RWC) in both cultivars, but these decreases were greater in the DS cultivar. The antioxidative defense enzymes, guaiacol peroxidase, ascorbate peroxidase, and glutathione reductase, showed increased activity in Ni‐treated DS seedlings; this increase in activity was not observed in the DT seedlings. These data suggest that different wheat genotypes may markedly differ in Ni uptake and sensitivity and that a enhanced capacity to counteract Ni stress may be associated with drought resistance.     

Silicon effects on aluminum toxicity to mungbean seedling growth

The effects of silicon (Si) on the toxicity of aluminum (Al) to mungbean (Phaseolus aureus Roxb.) seedlings were studied in a growth chamber. Mungbean seedlings were grown in a nutrient solution with combinations of three concentrations of Si (0,1, and 10 mM) and three concentrations of Al (0, 2, and 5 mM) in randomized completely block design experiments for 16 days. Silicon at 1 mM in the solution decreased root length, fresh and dry weights, and chlorophyll content, and showed no significant effect on epicotyl length and seedling height, and protein contents of shoots or roots in mungbean seedling under no Al stress. But, Si at 10 mM showed marked toxic effects on mungbean seedling growth and increased protein contents of the shoots or roots. In contrast, under 2 mM Al stress, Si addition at 1 mM had significant increasing effect on root length, fresh and dry weights, and chlorophyll content. It also had decreasing effect on protein contents of the shoots or roots, and had no effect on epicotyl length and seedling height. Silicon addition at 10 mM showed no effect on morphological and physiological measurements of mungbean seedling. However, Si at 1 mM added to solution only increased seedling height, epicotyl length, fresh weight, and chlorophyll content, but decreased dry weight and protein content of the roots under 5 mM Al stress, significantly. Silicon addition at 10 mM showed similar toxic effects on mungbean seedling growth under 5 mM Al stress to that under no Al stress.