Nutritional Study of Silicon in Graminaceous Crops (Part 2)

It was reported in the previous paper₁₎ that rice plants showed retardation of their vegetative growth and decrease of degree of seed setting when their silicon content was extremely low. It was concluded from these facts that silicon might most probably be essential for rice. But physiological functions or behaviours of silicon in plants should be clarified in order to solve this problem completely. Uptake and disttibution of silicon in rice plants were first investigated here, especially compared with phosphorus. A possibility of using silicon radioactive isotope (₃₁Si) for this kind of experiment was also investigated. Although tracer techniques have been greatly developed in elucidating behaviours or physiological functions of various elements in plants, radioactive silicon isotopes have been used very little in this field of science because of this very short half lives. Recently Rothbuhr and Scott ²⁾ reported having used radioactive silicon isotope for uptake experiments by plants in Harwell, England. Since a nuclear reactor (JRR-I) in the Japanese Atomic Energy Research Institute started to work in 1959, radioactive isotopes which have rather short half lives have become available for plant experiments in Japan. Methods of preparation of ³¹Si for plant experiments were examined and by using ³¹Si it was studied how silicon was absorbed and translocated by rice and wheat under some particular environmental conditions, and also how the uptake and distribution of it in rice plants were affected by various kinds of metabolic inhibitors.     

NITROGEN UPTAKE AND RECOVERY FROM N FERTILIZER AND LEGUME CROPS IN WETLAND RICE MEASURED BY 15N AND NON-ISOTOPE TECHNIQUES

Intensive rice-based cropping systems rely on nitrogenous fertilizer for optimum grain production and legume crops could be used as an alternative nitrogen (N) source for rice. We investigated the fate of N applied to dual cropping wetland rice in the form of legume residue and ¹⁵N labeled fertilizer. In 2001–2002, hairy vetch and broad bean accumulated 131 and 352 kg N ha^?1 of which 41 and 78% was derived from N₂ fixation. In 2002–2003, hairy vetch accumulated 64 kg N ha^?1 and broad bean accumulated 320 kg N ha^?1 of which 21 to 24% was derived from hairy vetch and 31 to 82% N was derived from broad bean by N difference and ¹⁵N-natural abundance method. Our results reveal that hairy vetch and broad bean can supply 50–100% of N required for intensive wetland rice and can be a viable alternative N source to enhance soil fertility.     

Effects of slag silicate fertilizer on silicon content of rice plants grown in paddy fields on the Shounai Plain,Yamagata, Japan

Abstract

Slag silicate fertilizer (SSF) is applied to paddy fields with different soil chemical properties to increase silicon (Si) concentration in rice (Oryza sativa L.) plants. However, the effects of soil chemical properties on Si availability of SSF to rice Si uptake is poorly understood. To investigate the relationships between chemical properties of soils and the effects of SSF application on the Si concentration in rice plants, a field experiment was conducted in 2007 and 2008 at 18 paddy fields on the Shounai Plain, Yamagata, Japan. Two treatments were implemented: SSF applied at 1.5 t ha^?1 and a no-SSF control. The Si concentrations of rice tissues were measured at the tillering and ripening stages. The difference in the Si concentrations of rice tissues between treatments (ΔSi concentration) was used to evaluate the effect of SSF. The Si concentrations in the shoots and aboveground parts of the rice plants were significantly increased by the SSF application in six or more of the fields at the two growth stages, whereas the Si concentrations in the panicles of the rice plants at the ripening stage were not increased significantly in most fields. Results of two-way analysis of variance evidenced a significant effect of field on the ΔSi concentrations in the shoots and aboveground parts at both growth stages. Furthermore, the ΔSi concentrations in the same rice plant tissues and at the same growth stages in the first year and in the second year were found to be positively correlated. These results indicate that the effect of SSF on the Si concentration in shoots and aboveground parts of rice plants varies from field to field. The ΔSi concentrations in shoots and aboveground plant parts at both growth stages were also found to be negatively correlated with soil chemical properties, i.e., available Si, Si adsorption capacity, contents of Si adsorbents (acid oxalate-extractable iron and manganese) and the pH under flooded soil conditions. These findings imply that those soil chemical properties of paddy fields should be taken into account for better prediction of ΔSi concentration of rice plants.     

Effects of Cadmium on the Content,Accumulation, and Translocation of Nutrients in Bean Plant Cultivated in Nutritive Solution

The effects of increasing dosages of cadmium (Cd) on the content, accumulation, and translocation of macronutrients and micronutrients in different parts of bean plant (Phaseolus vulgaris L.) are reported. Plants were cultivated in Clark's nutritive solution and submitted to 0 to 0.5 mg L^?1 of Cd. Experiments were built using an entirely randomized, statistical arrangement, and measurements were performed after 60 days of exposure to Cd. Inclusion of Cd in solution decreases the contents of nutrients in bean plants, except phosphorus (P) and zinc (Zn) in fruits, sulfur (S) in root, and copper (Cu) in fruits and root. Also, accumulation of nutrients in all parts of plant was reduced, as was the the translocation index for the following nutrients: potassium, calcium, and S in fruits and Cu in shoots. Overall, this work can serve as a template study to evaluate the effects of cadmium on bean plant nutrition.     

Study of Silicon Effects on Plant Growth and Resistance to Stem Borer in Rice

This research was conducted to evaluate silicon (Si) effects on the morphological characteristics and resistance to rice stem borer under greenhouse conditions at Sari Agricultural Sciences and Natural Resources University in 2009. The experiment was conducted as factorial in a complete randomized design (two factors) with three replications. The factors included four levels of silicon (Si) (Si₀ = 0, Si₁ = 5, Si₂ = 10, Si₃ = 20 g Si kg^?1 soil) and three rice cultivars (Parto, Line 34, and Neda). The results showed Si fertilizer had a significant effect on percentage of white head, length of leaf, width of leaf, diameter of stem, and percentage of reproductive tiller. Also, increased stem Si increased the resistance to striped stem borer in the rice cultivars studied. The greatest resistance to striped stem borer was observed with the application of 20 g Si kg^?1 soil. The application of Si at the rate of 20 g Si kg^?1 soil significantly reduced the percentage of white head from 18.10% (without Si) to 0.11% (with 20 g Si kg^?1 soil) in Parto cultivar.     

Grain legumes differ in nitrogen accumulation and remobilisation during seed filling

In grain legumes, the N requirements of growing seeds are generally greater than biological nitrogen fixation (BNF) and soil N uptake during seed filling, so that the N previously accumulated in the vegetative tissues needs to be redistributed in order to provide N to the seeds. Chickpea, field bean, pea, and white lupin were harvested at flowering and maturity to compare the relative contribution of BNF, soil N uptake, and N remobilisation to seed N. From flowering to maturity, shoot dry weight increased in all crops by approximately 50%, root did not appreciably change, and nodule decreased by 18%. The amount of plant N increased in all crops, however in field bean (17?g?m^?2) it was about twice that in chickpea, pea, and lupin. The increase was entirely due to seeds, whose N content at maturity was 26?g?m^?2 in field bean and 16?g?m^?2 in chickpea, pea, and lupin. The seed N content at maturity was higher than total N accumulation during grain filling in all crops, and endogenous N previously accumulated in vegetative parts was remobilised to fulfil the N demand of filling seeds. Nitrogen remobilisation ranged from 7?g?m^?2 in chickpea to 9?g?m^?2 in field bean, and was crucial in providing N to the seeds of chickpea, pea, and lupin (half of seed N content) but it was less important in field bean (one-third). All the vegetative organs of the plants underwent N remobilisation: shoots contributed to the N supply of seeds from 58% to 85%, roots from 11% to 37%, and nodules less than 8%. Improving grain legume yield requires either reduced N remobilisation or enhanced N supply, thus, a useful strategy is to select cultivars with high post-anthesis N₂ fixation or add mineral N at flowering.     

SLAG EFFICACY AS A LIME AND SILICON SOURCE FOR RICE CROPS THROUGH THE BIOLOGICAL METHOD

There is little information about the best silicon (Si) sources for agricultural use, and yet some products have already been marketed as sources of this element. One of these products is slag, which is used as a source for Si and lime. This study evaluated the silicon supply availability and efficacy of different silicate slag types for rice crops. The experiment was conducted in a greenhouse, used Entisol Quatzipsamment soil and was set up in randomized blocks with three replications. Si source reactivity was evaluated using five metallurgic slag types and Wollastonite, which is considered a standard in Si studies. Doses of each Si source were 1000 and 2000 mg dm^?3 of Si and a control (additional treatment). Soil data [soluble Si, calcium (Ca²⁺), magnesium (Mg²⁺) and pH] and rice growth and yield were recorded. Data were evaluated by analysis of variance and contrasts were made for comparisons between each slag type and the additional treatment. Averages were compared by the Scott Knott test at 5% with the statistics program SISVAR. The efficacy of the slag types in supplying Si for the plants (ESSi) and in increasing Si availability in the soil (ESiA) was determined from the values of contrast estimates. Slag E3 and Wollastonite were effective in increasing soil silicon availability and, consequently, the efficacy of supplying silicon for the plants, while the other slag types had low efficacy.     

CALIBRATION OF SOIL AND PLANT SILICON ANALYSIS FOR RICE PRODUCTION*

Calibration of field crop response to nutrient availability is the bases for making a fertilizer recommendation from soil and tissue analyses. The purpose of this study was to evaluate and summarize results from a series of experiments on silicon (Si) fertilization of rice in the Everglades Agriculture Area. Twenty-eight rice field experiments were conducted from 1992 through 1996. The experiments consisted of 2 to 5 rates of calcium silicate applied to soils (Histosols) of varying Si soil-test values. Soil samples were taken before planting and analyzed for acetic acid (0.5 mol L^?1) extractable Si. Straw samples were collected at harvest and analyzed for total Si. Grain yield was determined. The “critical” levels for Si in the soil (point below which response to Si fertilizer is expected) calculated by the Cate & Nelson procedure was 19 mg Si L^?1 soil. The amount of silicon to correct Si deficiency in the soil and to obtain optimum rice yield was 1500, 1120 and 0 kg ha^?1 for low (<6 mg L^?1), medium (6 to 24 mg L^?1), and high (>24 mg L^?1) level of soil Si, respectively. Silicon in the straw was classified as high when Si concentration was >34 g kg^?1, medium when in between 17 and 34, and low when <17 g kg^?1 (3.4 and 1.7%, respectively).

     

Amelioration of Drought in Sorghum (Sorghum bicolor L.) by Silicon

The purpose of this study was to analyze the effects of silicon (Si) nutrition on sorghum growth under drought. The present study investigated the distribution of Si in plant parts under stress conditions and its effects on physiological and growth traits. The study was conducted during 2 years (2007–2009) at PMAS Arid Agriculture University, Rawalpindi, Pakistan. Polyethylene glycol (PEG) 6000 (–4.0, –6.0, –8.0, and –10.0 Mpa) solution was used to screen drought-tolerant (Johar1) and drought-susceptible (SPV462) sorghum (Sorghum bicolor L.) cultivars, which were replicated three times with Si sources of potassium silicate (K₂SiO₃) (Si₃₀₀: 300 ml L^?1) and control (Si₀) treatments. The results showed that drought-tolerant cultivars accumulated maximum Si under Si treatment versus Si absence, which resulted increased leaf water potential, leaf area index, Soil Plant Analysis Development (SPAD) chlorophyll, net assimilation, and relative growth rate over SPV462. Similarly, Si accumulation in leaves conserved transpiration and leaf water potential, verifying Si nutrition as a defense for plants under drought.     

CORRELATION BETWEEN TISSUE AND SUBSTRATE SILICON CONCENTRATION OF GREENHOUSE PRODUCED ORNAMENTAL SUNFLOWERS

The beneficial effects of the “nonessential” plant nutrient, silicon (Si), are well documented for several agricultural crops. Soilless growth media used in greenhouse production provides only limited amounts of available Si to container grown plants compared to plants grown in mineral-derived soils. Si supplementation is documented to increase resistance to biotic and abiotic stresses in greenhouse crops, which accumulate Si in their tissues. However, optimum Si fertilization rates and acceptable Si levels in tissues and substrate have not been established for floriculture greenhouse production. For this study ornamental sunflower (Helianthus annuus L. ‘Ring of Fire’) was used to investigate the relationship between substrate Si and accumulation of Si in the tissues of plants grown in a peat-based media. Weekly substrate drenches of potassium silicate (KSiO₃), substrate incorporation of KSiO₃ hydrous powder, or rice husk ash were used as Si supplements. Overall, leaf, stem, and flower Si concentrations of Si-supplemented plants increased compared to nonsupplemented controls. A positive correlation was observed between substrate Si concentration and leaf Si concentration for all three Si sources used in this study. Therefore, leaf tissue is the most appropriate tissue to sample in order to determine the availability of Si in a substrate and could be used to establish acceptable Si levels for soilless greenhouse floriculture.     

Effects of pretreatment and solution chemistry on solubility of rice‐straw phytoliths

Rice is a Si‐accumulator plant, whereby Si has physio‐chemical functions for plant growth. Its straw contains high shares of plant silica bodies, so‐called phytoliths, and can, when returned to the soil, be an important Si fertilizer. Release of Si from phytoliths into soil solution depends on many factors. In order to improve prognosis of availability and management of Si located in phytoliths, in this study we analyzed the effect of pretreatment of rice straw by dry and wet ashing and the soil‐solution composition on Si release. Dry ashing of rice straw was performed at 400°C, 600°C, and 800°C and wet ashing of the original straw and the sample from 400°C treatment with H₂O₂. To identify the impact of soil‐solution chemistry, Si release was measured on separated phytoliths in batch experiments at pH 2–10 and in presence of different cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, Al³⁺) and anions (Cl^–, NO$ _3^- $ , SO$ _4^{2-} $ , acetate, oxalate, citrate) in the concentration range from 0.1 to 10 mmol_c L^–1. After burning rice straw at 400°C, phytoliths and biochar were major compounds in the ash. At an electrolyte background of 0.01 mol_c L^–1, Si released at pH 6.5 was one order of magnitude higher than at pH 3, where the zeta potential (ζ) was close to zero. Higher ionic strength tended to suppress Si release. The presence of cations increased ζ, indicating the neutralization of deprotonated Si‐O^– sites. Monovalent cations suppressed Si release more strongly than bivalent ones. Neutralization of deprotonated Si‐O^– sites by cations might accelerate polymerization, leading to smaller Si release in comparison with absences of electrolytes. Addition of Al³⁺ resulted in charge reversal, indicating a very strong adsorption of Al³⁺, and it is likely that Si‐O‐Al‐O‐Si bonds are formed which decrease Si release. The negative effect of anions on Si release in comparison with deionized H₂O might be due to an increase in ionic strength. The effect was more pronounced for organic anions than for inorganic ones. Burning of rice straw at low temperatures (e.g., 400°C) appears suitable to provide silicon for rice in short term for the next growing season. High inputs of electrolytes with irrigation water and low pH with concomitant increase of Al³⁺ in soil solution should be avoided in order to keep dissolution rate of phytoliths at an appropriate level.     

Changes in nitrogen and phosphorus concentrations of silicon‐fertilized rice grown on organic soil 1

Rice grown on the organic soils of the Everglades is routinely fertilized with silicon (Si). The objective of this research was to investigate changes in nitrogen (N) and phosphorus (P) concentration in various plant parts in response to Si fertilization. Two cultivars were grown in lysimeters filled with low‐Si soil. Half the lysimeters were fertilized with calcium silicate to provide 2Mg Si ha^‐1 and the other lysimeters remained unfertilized as a control. Nitrogen concentration decreased in all plant parts with Si fertilization. Phosphorus concentration increased with Si. Maturity was earlier in the Si fertilized rice.     

Phytolith‐rich biochar increases cotton biomass and silicon‐mineralomass in a highly weathered soil

Non‐essential silicon (Si) is beneficial to plants. It increases the biomass of Si‐accumulator plants by improving photosynthetic activity and alleviating stresses. Desilication, however, takes place because of natural soil weathering and removal of harvested biomass. Pyrolysis transforms Si‐rich biomass into biochar that can be used to supply bioavailable Si. Here, we applied two biochar materials differing in Si content on soils differing in weathering stage: a young Cambisol and a highly weathered Nitisol. We studied the impact of biochar supply on the bioavailability of Si, cotton biomass, and Si mineralomass. The biochar materials derived from, respectively: Miscanthus × giganteus (Mi; 34.6 g Si kg^?1 in biochar) and soft woody material (SW; 0.9 g Si kg^?1 in biochar). They were compared to conventional Si fertilizer wollastonite (Wo; CaSiO₃). Amendments were incorporated in soils at the rate of 3% (w/w). The content of bioavailable Si in soil was determined through 0.01 M CaCl₂ extraction. In the Cambisol, the proportion (CaCl₂ extractable Si: total Si content) was significantly smaller for Mi (0.9%) than for Wo (5.2%). In the Nitisol, this proportion was much larger for Mi (1.4%) than for Wo (0.7%). Mi‐biochar significantly increased Si‐mineralomass relatively to SW‐biochar in both soils. This increase was, however, much larger in the Nitisol (5.9‐fold) than in the Cambisol (2.2‐fold). Mi biochar is thus an alternative Si fertilizer to Wo to supply bioavailable Si, increase plant biomass, and promote the biological cycle of Si in the soil‐plant system in the Nitisol. Besides, it increased soil fertility and soil organic carbon content.     

Arsenic uptake,distribution, and accumulation in bean plants: Effect of Arsenite and salinity on plant growth and yield

The response of bean plants (Phaseolus vulgaris L.) to different levels of arsenic (As) and salinity was investigated, including the processes of uptake, distribution, and accumulation of As and the effect of arsenite and salinity on plant growth and fruit production. The experiment was performed in soilless culture at two levels of As: 2 and 5 mg As L^‐1 [added as sodium arsenite (NaAsO₂)], and three saline levels [only sodium chloride (NaCl) was added]: 1,000,2,000, and 4,000 μS#lbcM^‐1. Arsenic uptake and concentration in root increased upon increased NaAsO₂ concentration in the nutrient solution. However, the increase in the As root content was not proportional to the As level in the nutrient solution. High levels of salinity in solution decreased As uptake and the concentration of As in root, stem, and leaf. Upon uptake, As was readily translocated to the aerial organs and approximately half of the absorbed As was transported to the upper parts of the bean plants. The As concentration in fruit always remained below the recommended limit for As content in fruit and edible vegetal products. While salinity did not significantly affect plant growth, arsenite was found to be phytotoxic to the bean plants.     

Some significant functions of silicon to higher plants

Silicon may be regarded as an essential element to cereals plant from an agronomic viewpoint. It is implicated as a factor influencing the degree of susceptibility of cereals to fungal attack. Vegetation in the tropics contains much more silicon for the protection. Once the silicon dioxide has been taken up by plants, it is rapidly accumulated in insoluble form and remains in the tissues. Yield response over the control will not be obtained if available silicon exceeds 11 mg SiO₂/100 g in the soil. The addition of silicon to the culture solution, at the rate of 75 ppm Si, decreased the accumulation of Mn, Cu, Fe, Zn, N, P and transpiration rate, but increased Ca, Mg, Si and carbohydrate contents. It is concluded that addition of silicon is particularly effective when combined with a heavy rate of nitrogen and magnesium.     

EFFECT OF THE APPLICATION OF SILICON HYDROXIDE ON YIELD AND QUALITY OF CHERRY TOMATO

Silicon (Si) plays an important role in the structural rigidity of cell walls. When plants have a passive or selective assimilation or they are poor accumulators as solanaceae, the percentage of silicon absorbed and present in the plants is lower than 1%, but its presence can provide significant benefits to the plant before it undergoes biotic and abiotic stresses. The objective of this work was to assess the effect of fertilization with monosilicic acid on yield and quality of cherry tomato crops (Lycopersicon esculentum var. cerasiforme cv. ‘Salomee’) grown on rockwool in a greenhouse. Two types of treatments were investigated: control test (conventional fertilization) and fertilization with silicic acid [Si(OH)₄] [seven applications of 250 mL of Si(OH)₄·ha^?1 for each crop cycle]. Significant differences were observed, including a higher number of fruits (fruits/plant) and a larger yield (kg m^?2) in the plots that were fertilized with silicon.     

A contemporary overview of silicon availability in agricultural soils

Our current understanding of silicon (Si) availability in agricultural soils is reviewed and knowledge gaps are highlighted. Silicon is a beneficial rather than essential plant nutrient and yield responses to its application have been frequently demonstrated in Si‐accumulator crops such as rice and sugarcane. These crops are typically grown on highly weathered (desilicated) soils where soil solution Si concentrations are low. Increased yields are the result of simultaneous increases in plant tolerance to a wide range of biotic (plant pathogens, insect pests) and abiotic (water shortage, excess salts, metal toxicities) stresses. Traditionally, soil solution Si is viewed as being supplied by dissolution of primary and secondary minerals and buffered by adsorption/desorption of silicate onto Al and Fe hydrous oxide surfaces. In recent years it has become recognized that phytogenic cycling of Si [uptake of Si by plants, formation of phytogenic silica (SiO₂ · nH₂O) mainly in leaves and subsequent return of this silica to soils in plant litter] is the main determinant of soil solution Si concentrations in natural forests and grasslands. Considerable diminution of the phytogenic Si pool in agricultural soils is likely due to regular removal of Si in harvested products. A range of extractants (unbuffered salts, acetate‐based solutions, and acids) can provide valuable information on the Si status of soils and the likelihood of a yield response in rice and sugarcane. The most common Si fertilizers used are industrial byproducts (e.g., blast furnace slag, steel slag, ferromanganous slag, Ca slag). Since agriculture promotes soil desilication and Si is presently being promoted as a broad spectrum plant prophylactic, the future use of Si in agriculture is likely to increase. Aspects that require future research include the role of specific adsorption of silicate onto hydrous oxides, the significance of phytogenic Si in agricultural soils, the extent of loss of phytogenic Si due to crop harvest, the role of hydroxyaluminosilicate formation in fertilized soils, and the effect of soil pH on Si availability.     

Calibration and Categorization of Plant Available Silicon in Rice Soils of South India

ABSTRACT

Calibration of field crop responses to nutrient availability acts as a basis for making fertilizer recommendations from soil and tissue analysis. The purpose of this study was to evaluate and summarize silicon (Si) fertilization of rice in different soils of south India. The experiment consists of four levels of calcium silicate as Si with three replications. Initially, soils were analyzed using eleven different extractants. The grain and straw yield were recorded and analyzed for Si content. The critical levels for plant available Si in the soil ranged from 14 mg kg^?1 (distilled water-1) to 207 mg kg^?1 [0.005 M sulfuric acid (H₂SO₄)]. There was a wide variation in low, medium, and high categories of plant available Si for different extractants calculated based on percent relative yield. The critical level of Si in straw and grain were 2.9 and 1.2%, respectively.     

Effect of Cadmium and Iron on Rice (Oryza Sativa L.) Plant in Chelator-Buffered Nutrient Solution

ABSTRACT

To better understand the mechanisms responsible for differences in uptake and distribution of cadmium (Cd), nutrient-solution experiments were conducted with different varieties of rice (Oryza sativa), ‘Khitish’ and ‘CNRH3’. The plants were grown in a complete nutrient solution with different levels of pCd (-log free Cd⁺² activity) and pFe [-log free iron (Fe⁺²) activity]. The required concentrations of chelating agent and metals were determined using a computerized chemical equilibrium model such as Geochem-PC. Experimental treatments included a combination of four pCd activity levels (0, 7.9, 8.2, and 8.5) applied as Cd (NO₃)₂ 4H₂O, and two pFe activity levels (17.0 and 17.8) applied as FeCl₃. The application of both Cd and Fe in solution culture significantly affected plant growth, yield, and Cd accumulation in plant tissue. In general, yield of rice was decreased by an increase in amount of solution Cd; however, yield response varied among the cultivars. At the 7.9 pCd level, yields of rice cultivars ‘Khitish’ and ‘CNRH3’ were reduced to 69% and 65%, respectively, compared with control plants. Root Cd concentrations ranged from 2.6 mg kg^?1 (control plants) to 505.7 mg kg^?1 and were directly related to solution Cd concentrations. In rice plants, Cd toxicity symptoms resembled Fe chlorosis. Differential tolerance of varieties to phytotoxicity was not readily visible, but a significant interaction of substrate Cd and variety was obtained from dry-matter yields. Significant interactions indicated that response of tissue Cd concentration, plant Cd uptake, and translocation of Cd to the aerial parts were dependent on variety as well as substrate Cd. Uptake of Cd by roots was significantly higher than by shoots. Higher Cd uptake by rice plants decreased the uptake of other beneficial metals.

The effect of Cd and Fe on the rate of phytometallophore release was also studied in the nutrient solution. Among the rice genotypes, ‘Khitish’ was the most sensitive to Cd toxicity. In both genotypes, with the onset of visual Cd-toxicity symptoms, the release of phytometallophore (PM) was enhanced. Among the rice varieties, ‘Khitish’ had the highest rate of PM release. Treatments with the metal ions studied produced a decrease in chlorophyll and enzyme activity. A decrease in concentrations of chlorophyll pigments in the third leaf was observed due to the highest activity level of Cd (pCd 7.9). Activities of enzymes such as peroxidase (POD) and superoxide dismutase (SOD) are altered by toxic amounts of Cd. Changes in enzyme activities occurred at the lowest activity of Cd (pCd 8.5) in solution. Peroxidase activity increased in the third leaf. Results showed that in contrast with growth parameters, the measurements of enzyme activities may be included as early biomarkers in a plant bioassay to assess the phytotoxicity of Cd-contaminated solution on rice plants. Evidence that Cd uptake and translocation are genetically controlled warrants the selection of varieties that assimilate the least Cd and that translocate the least metal to the plant part to be used for human and animal consumption.     

Effects of Lead on the Content,Accumulation, and Translocation of Nutrients in Bean Plant Cultivated in Nutritive Solution

Increasing contents of lead (Pb; from 0 to 10 mg L^?1) as contaminant were added in Clark's nutritive solution to evaluate the effects on content, accumulation, and translocation of macronutrients [phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S)] and micronutrients [copper (Cu), manganese (Mn), zinc (Zn), and iron (Fe)] in different parts of the bean plant (Phaseolus vulgaris L.). Experiments were built using an entirely randomized statistical arrangement and measurements were performed after 60 days of exposure to Pb. The contents of almost all nutrients decreased when Pb was added, except for P in root, Mg in fruits, and S in root and fruits. Reduction in accumulation of all nutrients in the three parts of plants was determined, and an increase in the translocation index was observed for the following nutrients: Mg and Zn in fruits and S in fruits and leaves. For the remaining nutrients, the translocation index diminished.