Redistribution of boron in leaves reduces boron toxicity

High soil boron (B) concentrations lead to the accumulation of B in leaves, causing the development of necrotic regions in leaf tips and margins, gradually extending back along the leaf. Plants vary considerably in their tolerance to B toxicity, and it was recently discovered that one of the tolerance mechanisms involved extrusion of B from the root. Expression of a gene encoding a root B efflux transporter was shown to be much higher in tolerant cultivars. In our current research we have shown that the same gene is also upregulated in leaves. However, unlike in the root, the increased activity of the B efflux transporter in the leaves cannot reduce the tissue B concentration. Instead, we have shown that in tolerant cultivars, these transporters redistribute B from the intracellular phase where it is toxic, into the apoplast which is much less sensitive to B. These results provide an explanation of why different cultivars with the same leaf B concentrations can show markedly different toxicity symptoms. We have also shown that rain can remove a large proportion of leaf B, leading to significant improvements of growth of both leaves and roots.Key words: Bor genes, boron tolerance, boron toxicity, efflux pumping, leaf necrosis, membrane transportB-toxic soils are widespread throughout agricultural areas of the world where they cause significant and often substantial reductions in crop quality and yield. The mechanism by which B is toxic to plants is not well understood¹ but toxicity symptoms include reduced root growth which affects uptake of water and nutrients, and the development of necrotic patches on leaves which impairs photosynthesis. Tolerance to B toxicity has been recognized in a number of crops, notably in cereals. In most cases, tolerance is achieved by reduced uptake of B into the root, which then leads to reduced uptake into the shoot. Genetic studies established that in barley, a locus associated with reduced tissue B occurred on chromosome 4 and that this locus could be transferred to other barley cultivars with desirable agronomic traits.²Hayes and Reid³ made a careful study of the characteristics of B uptake in a highly tolerant landrace barley cultivar Sahara, and found that although B was highly permeable, the root B concentration in this cultivar could be maintained at only half that in the external medium, whereas in sensitive cultivars, B was the same in both intracellular and extracellular phases. It was concluded that tolerant cultivars must have a membrane active transporter that exports B from the root. A B exporter, AtBor1 had previously been discovered in Arabidopsis where it was involved in B loading into the xylem⁴ but it was later found to be degraded under high B conditions⁵ and therefore would not be useful in B tolerance.However, other Bor1 homologues were subsequently discovered in Arabidopsis and in rice. Based on homology with rice, Reid⁶ cloned genes from barley and from wheat (HvBor2 and Tabor2 respectively) which were shown to be strongly upregulated in roots of tolerant cultivars, and virtually undetectable in sensitive cultivars. Thus, a simple mechanism to explain tolerance was established; efflux of B from the root reduced the intracellular concentration of B in the root cells, thereby reducing toxicity and improving root growth. At the same time, the lower root content meant that less B was transferred to the shoot, resulting in lower shoot toxicity.Yet there remained several unanswered questions regarding B toxicity. Firstly, it was commonly observed that toxicity symptoms were not reliably correlated with leaf B concentration, and that often after rain, toxicity symptoms became less severe. Nable et al.⁷ had investigated the effect of rain on shoot B concentrations and concluded that although rain did reduce the B concentration in leaves, it did not affect growth and yield. Secondly, field trials with cultivars in which the B tolerance traits were expressed, did not show the improvements in growth and yield that could be observed in glasshouse trials.^8,9Our recent work¹⁰ has provided new insights into these phenomena. Sensitive and tolerant cultivars of both wheat and barley were grown in varying levels of B. Then, ignoring the level of B in the growth solution, leaves of the different cultivars that displayed the same degree of leaf necrosis were selected. This revealed that in the tolerant cultivars, necrosis began to appear at leaf B levels that were two-to five-fold higher than in sensitive cultivars. Since no internal tolerance mechanism had been reported, it was hypothesised that in the tolerant cultivars, internal toxicity was reduced by pumping B from the cytoplasm into the cell wall where B is much less toxic. To prove this hypothesis three types of experiment were conducted. Firstly protoplasts were isolated from leaves of tolerant and sensitive cultivars of barley, and it was shown that when incubated in the same concentration of B, the tolerant cultivar was able to reduce the intracellular B concentration to approximately half that of the sensitive cultivar. Secondly, it was reasoned that if more B was accumulated in the apoplast of the tolerant cultivar, then it should be more quickly released by washing of the leaf; this was confirmed. Thirdly, it was shown that the same efflux transporters that were responsible for B export from the root were also highly expressed in leaves of tolerant cultivars of wheat and barley. The combination of these three experiments provided compelling evidence that redistribution of B in the leaf was a significant factor in B tolerance.The elution experiment also highlighted the fact that because B is highly soluble and has high membrane permeability, it can easily be washed from leaves. Obviously in the field B could be removed from leaves by rain, but no positive effect of this on growth had been quantified. In our experiments, we simulated the average rainfall during the early growing season in a high B region of Southern Australia by spraying plants with calibrated amounts of water for 16 d. At high B concentrations, rain reduced leaf B by around 50% while simultaneously improving growth of shoots by up to 90%. Rather surprisingly, the rain treatment, which had no significant effect on root B concentrations, caused a two-fold increase in root growth, presumably by improving the supply of photosynthate from the shoot.This study has enabled an evaluation of the importance of three main factors in determining the severity of B toxicity; two genetically determined processes, efflux pumping of B in roots and leaves, coupled with the variable leaching of B from leaves by rain (Fig. 1). The results also provide an explanation for the poor correlations observed between toxicity and shoot B concentrations in cereals.^7,11Open in a separate windowFigure 1Summary of processes contributing to reduced B toxicity in wheat and barley. The intensity of shading indicates the level of B in different regions of the plant. Boron (B) enters the leaf via the xylem and continues to accumulate as the leaf grows. When plants are grown in high concentrations of B, the older parts of the leaf become necrotic first while the younger basal tissues continue to expand. In tolerant cultivars, B efflux transporters in leaves pump B from the cytoplasm where it is toxic into the cell walls where it can be tolerated at high concentrations. Sensitive cultivars have a very low capacity for B efflux and therefore retain much higher concentrations inside the cell than in tolerant cultivars. rain can remove large amounts of B from leaves, thereby alleviating toxicity. In roots of tolerant cultivars, the same B efflux transporters that occur in leaves are used to pump B from the cells into the external medium. This reduces the toxicity to roots and limits the amount of B entering the xylem and reaching the leaves.     

Relation of solute and sorbed boron to the boron hazard in irrigation water

Summary The current criteria for evaluating the boron (B) hazard of irrigation water for specified crops are based on the concentration of B in the irrigation water without consideration of soil properties or the leaching fraction. Experiments were conducted to determine the influence of B sorption capacity on plant uptake of B at rates of 0.1, 2.5, 5.0 and 10.0 ppm B in the irrigation water with a leaching fraction of 0.5. A relatively B sensitive crop, oats (Avena sativa), was grown on four arid-region soils of varying B sorption capacities. The results show that B in solution rather than sorbed B influenced B toxicity. Contribution from the Department of Soils, Water and Engineering, The University of Arizona, Tucson, Arizona 85721. Arizona Agricultural Experiment Station No. 2508. Research Associates and Associate Professor, respectively. The senior author is currently at the Department of Soils and Irrigation, American University of Beirut Beirut, Lebanon.     

Distribution of boron in the environment

The findings of a study to identify and quantify the orders of magnitude for major reservoirs and flows of boron (B) in the environment are outlined. The orders of magnitude for B reservoirs and flows arising through natural processes, such as the hydrological cycle and volcanism, are compared with those arising from anthropogenic activities, such as coal combustion and the extraction and use of borates for commercial purposes. The major stores and reservoirs for B have been identified, in order of magnitude, as the continental and oceanic crusts (10¹⁸ kg B), the oceans (10¹⁵ kg B), groundwater (10¹¹ kg B), ice (10¹¹ kg B), coal deposits (10¹⁰ kg B), commercial borate deposits (10¹⁰ kg B), biomass (10¹⁰ kg B), and surface waters (10⁸ kg B). The largest flows of B in the environment arise from the movement of B into the atmosphere from oceans, at between 1.3 * 10⁹ kg and 4.5 * 10⁹ kg B per annum. Other hydrological flows are also important. Drainage from soil systems into groundwaters and surface waters accounts for between 4.3 * 10⁸ kg and 1.3 * 10⁹ kg B per annum. B mining and volcanic eruptions represent the next most significant B flows, accounting for approx 4.0 * 10⁸ kg and 3.0 * 10⁸ kg B, respectively.     

Enhanced boron transport into the ear of wheat as a mechanism for boron efficiency

Genotypic variation in boron (B) efficiency in wheat (Triticum aestivum L.) is expressed as large differences in grain set and pollen fertility under low soil B, but the mechanisms responsible for such differences are unknown. This paper aims to determine whether differences in B transport and retranslocation can explain cultivar differences in B efficiency between B-efficient (Fang 60) and B-inefficient (SW41) wheat cultivars. Plants were grown with adequate ¹¹B (10 μM), until the premeiotic interphase stage in anther development, then transferred into ¹⁰B at 0.1 or 10 μM. After five days, ending at the young microspore stage, plants were returned to adequate ¹¹B. Plants were harvested at 0, 1 and 5 days after transferring into ¹⁰B, and at anthesis when fresh pollen was examined for viability. After 5 days in 0.1 μM B, pollen viability in SW41 was depressed by 47%, but pollen of Fang 60 was not affected. When B supply was low, the proportion of plant B partitioned into the ear of Fang 60 was almost twice as high as that in SW 41, enabling Fang 60 to maintain B concentration in the ear at 6.8 mg kg^?1 dry weight (DW), whereas it dropped to 3.8 mg kg^?1 DW in SW 41. Boron accumulation in the ear, when external supply was restricted, did not come from the ¹¹B previously taken up by the plant. The greater ¹⁰B accumulation in ears of Fang 60 compared to SW 41, with limited external B supply, indicated that B efficiency was associated with xylem transport of B. The greater increase of ¹⁰B:¹¹B ratio in the ear of Fang 60 compared to SW 41, over the 5 days of B interruption further indicated that greater B efficiency was associated with a stronger capability for long distance transport of B from the rooting medium into the ear via the xylem rather with than retranslocation of B from vegetative parts.     

Effect of water stress on the boron response of wheat genotypes under low boron field conditions

Sterility has emerged as a widespread problem for wheat (Triticum aestivum) production in South and South-east Asia. Whilst boron (B) deficiency is commonly associated with sterility in wheat, the expression of sterility is complicated in rainfed conditions by a number of environmental factors including water stress. A field experiment was conducted to examine the effect of water stress on B response of wheat genotypes on a low B soil (0.087 mg B kg-1soil) at Chiang Mai, Thailand (18°45 N, 99° E) during the dry season from November to March. The experiment consisted of three factors arranged in a split-split plot design with two levels of irrigation: water stress (I–) and full irrigation (I+) in main plots, two levels of B: 0 kg (B0) and 1 kg B ha-1 (B+) as borax in sub plots and four wheat genotypes: SW 41, BL 1022, UP 262 and Sonora 64 in sub–sub plots. Water stress was applied by discontinuing irrigation in I– treatments after the double ridge stage. In all genotypes, above ground biomass was decreased by I–, but not by B deficiency. Significant B×genotype interactions were detected for reproductive growth. SW 41 and BL 1022 strongly responded to added B with relief from B deficiency symptoms at anthesis and improved grain set index (GSI), grains ear-1, ears with grain and grain yield at maturity. By contrast, Sonora 64 could set grain well at B0 and did not show any response to added B with respect to these parameters. Grains ear-1 of SW 41 and BL 1022 was not affected by full irrigation at B0, but were significantly increased when fully irrigated with added B. In all genotypes, B concentration of the flag leaf and the ear at booting and at anthesis was significantly higher in B+, but was not affected by irrigation. Boron × irrigation interactions detected in this study indicate the possibility of the influence of water stress on the severity of wheat sterility in South and South-east Asia.     

Effect of high boron application on boron content and growth of melons

Synthetic chelates, such as ethylene diamine tetraacetic acid (EDTA), have been shown to enhance phytoextraction of Pb from contaminated soil but also cause leaching of heavy metal-chelate complexes, posing a groundwater contamination threat. In a soil column study, we examined the effect of EDTA and a biodegradable chelate [S,S] isomere of ethylene diamine disuccinate ([S,S]-EDDS), newly introduced in phytoextraction research, on the uptake of Pb by the Chinese cabbage (Brassica rapa) and Pb leaching through the soil profile. Soil water sorption characteristics were modified by acrylamide hydrogel. The addition of 0.1 and 0.2% (w/w) of hydrogel amendments increased soil field water capacity from initial 24.6% to 28.5% and 31.3%, respectively. The additions of 2.5, 5 and 10 mmol EDTA kg^–1 soil were more effective in enhancing Pb plant uptake than comparable [S,S]-EDDS treatments, but caused (as also 10 mmol kg^–1 [S,S]-EDDS additions) unacceptably high Pb leaching in treatments with any soil water sorption conditions tested. The most efficient level of EDTA (10 mmol kg^–1) enhanced plant Pb uptake by 97 times compared to the control. Shoots Pb concentrations reached 500 mg kg^–1 of dry biomass. However, in this treatment 36.2% of total initial Pb was leached from the soil during the first four weeks after chelate addition. Hydrogel soil amendments were more effective in treatments with [S,S]-EDDS than with EDTA. In treatments with 10 mmol kg^–1[S,S]-EDDS hydrogel amended soils, plant Pb uptake was significantly reduced and Pb leach was as high as 44.2% of total initial soil Pb. At lower [S,S]-EDDS concentrations, the effect of hydrogel soil amendment on Pb leaching was the opposite. The addition of 5 mmol kg^–1 [S,S]-EDDS soil to the soil amended with 0.2% hydrogel increased Pb uptake by 18 times while only 0.2% of total initial Pb was leached. In all treatments, the concentrations of Pb in dry plant biomass were far from concentrations required for efficient soil remediation within a reasonable time span.     

Effects of liming and boron fertilization on boron uptake of Picea abies

The effects of liming on concentrations of boron and other elements in Norway spruce [Picea abies (L) Karst.] needles and in the mor humus layer were studied in long-term field experiments with and without B fertilizer on podzolic soils in Finland. Liming (2000+4000 kg ha^-1 last applied 12 years before sampling) decreased needle B concentrations in the four youngest needle age classes from 6–10 mg kg^-1 to 5 mg kg^-1. In boron fertilized plots the corresponding concentrations were 23–35 mg kg^-1 in control plots and 21–29 mg kg^-1 in limed plots. Both liming and B fertilizer decreased the Mn concentrations of needles. In the humus layer, total B concentration was increased by both lime and B fertilizer, and Ca and Mg concentrations and pH were still considerably higher in the limed plots than controls. Liming decreased the organic matter concentration in humus layer, whilst B fertilizer increased it.The results about B uptake were confirmed in a pot experiment, in which additionally the roles of increased soil pH and increased soil Ca concentration were separated by means of comparing the effects of CaCO₃ and CaSO₄. Two-year-old bare-rooted Norway spruce seedlings were grown in mor humus during the extension growth of the new shoot. The two doses of lime increased the pH of soil from 4.1 to 5.6 to 6.1, and correspondingly decreased the B concentrations in new needles from 22 to 12 to 9 mg kg^-1. However, CaSO₄ did not affect the pH of the soil or needle B concentrations. Hence the liming effect on boron availability in these soils appeared to be caused by the increased pH rather than increased calcium concentration.     

Requirement of ginkgo pollen-derived tissue cultures for boron and effects of boron deficiency

Ginkgo biloba L. pollen-derived tissue, which is made up of small, friable masses of homogeneous parenchymatous cells, was shown to require boron in the culture medium. If no boron is supplied, growth soon stops. Growth responses to additions of boron were observed up to an optimum level of 0.1 mg of boron per liter.

Histological examination and chemical analyses showed 2 general effects of boron deficiency: (1) a reduced rate of cell division, with no significant effect on cell size, and (2) some alteration in composition of the cell walls. With the exception of a reduction in fructose, the concentration of soluble and of readily hydrolyzable carbohydrates, and the concentration of protein in the tissue, were not affected by boron deficiency.

     

Translocation and effectiveness of foliar-fertilized boron in broccoli plants of varying boron status

The translocation and effectiveness of foliar-fertiilized boron (B) was investigated in broccoli plants supplied via the root system with luxury, sufficient or deficient levels of B. ¹⁰B-enriched boric acid was applied three times to lower leaves, beginning one week prior to inflorescence emergence, and the shoot and floret yields, as well as the ¹⁰B and ¹¹B contents or concentrations of xylem sap, phloem exudate and various plant parts, were determined three weeks after inflorescence emergence. The amount of ¹⁰B translocated in phloem from fed leaves to the remainder of the shoot did not exceed 0.5%, of that supplied, but it was inversely related to plant-B status. The partitioning of translocated ¹⁰B to florets (16–30%) and the degree of enhancement in floret yield (28–75%) was also inversely related to plant-B status. It is concluded that foliar-B fertilization may be more effective for preventing B deficiency than soil-derived B in leaves.Abbreviations ¹⁰B/¹¹B mass isotopes of boron - DM dry matter - FF foliar fertilization - RF root fertilization     

A review of boron effects in the environment

Boron (B) is a naturally occurring element that is found in the form of borates in the oceans, sedimentary rocks, coal, shale, and in some soils. Borates are released naturally into the atmosphere and aquatic environment from oceans, geothermal steams, and weathering of clay-rich sedimentary rocks. B is also released to a lesser extent from anthropogenic sources. B concentrations in air range from <0.5 to 80 ng/m³ with an average of 20 ng/m³, and in soils from 10 to 300 mg/kg with an average of 30 mg/kg. Concentrations of B in surface freshwaters are typically < 0.1–0.5 mg/L; much higher concentrations are measured in a few areas, depending on the geochemical nature of the drainage catchment. B accumulates in both aquatic and terrestrial plants, but it does not appear to be biomagnified through the food chain. No observed effect concentrations (NOECs) for aquatic invertebrates tend to be in the range of 6–10 mg B/L with lower values of 1–2 mg/L for community studies. No effect concentrations for fish in natural waters are around 1 mg/L, although lower values have been recorded in reconstituted water. Comparing no effect concentrations with the general ambient environmental levels indicates that the risk to aquatic ecosystems from B is low. In a few B-rich areas, natural levels will be higher; however, there is some indication that organisms may be Actapted to the local conditions. B is an essential micronutrient for higher plants with interspecies differences in the levels required for optimum growth. In general, there is a small concentration range between deficiency and toxicity; however, toxicity owing to excess B is much less common in the environment than B deficiency. Levels of B in aquatic plants growing in areas receiving B-rich runoff from irrigated fields are higher than dietary concentrations, which cause effects on the growth of young birds in the laboratory; however, the bioavailability in the field of such plant-accumulated B is uncertain.     

Investigations of boron uptake at the cellular level

The efficiency of recovery of P by iron oxide-impregnated filter paper, as used in the new P_i test for soil phosphorus, was found to depend on the method used for impregnating the paper with iron oxide and could range from as little as 28% to more than 98%. The greatest efficiency of recovery was obtained with filter papers which had been washed with deionised water following iron oxide-impregnation. These filter papers were also found to give the most reproducible results. ei]{gnB E}{fnClothier}     

Characteristics of root boron nutrition confer high boron efficiency in Brassica napus cultivars

Background and aims

Brassica napus has high boron (B) demand, but significant genotype differences exist with respect to B deficiency. The aim of this research was to elucidate the relationship between the different sensitivities of Brassica napus cultivars to low B stress and the characteristics of B uptake and transport to characterise the regulation of B efficiency in Brassica napus.

Methods

B-efficient and B-inefficient Brassica napus cultivars were used to compare the uptake and transport of B using the stable isotope ¹⁰B tracer and grafting experiments, as well as expression of B transporters by RT-PCR.

Results

B-efficient cultivars have significant advantages with regard to B limitation. The B-efficient cultivar HZ showed less severe B deficiency symptoms and higher dry biomass than the B-inefficient cultivars LW and LB. Both the amount of total B and the ¹⁰B concentration and accumulation in the shoots and roots of B-efficient HZ were higher than those of B-inefficient cultivars. In B-inefficient LW, the amount of total B and the ¹⁰B that was transported into shoots was less than in the other three cultivars and the content and accumulation of total B and ¹⁰B in the roots of B-inefficient LB were the lowest among all of the cultivars. When the roots of B-efficient HZ were used as stocks, the grafted plants showed B-efficient characteristics, such as mild B deficiency symptoms, and higher dry biomass and B accumulation, regardless of whether they originated from B-efficient or B-inefficient cultivars. In contrast, the grafted plants with B-inefficient LW used as stocks were B-inefficient. The expressions of BnBOR1;1c, BnBOR1;2a and BnNIP5;1 were up-regulated in roots under low B stress compared with the normal B condition. However, there was no obvious difference in the expressions of the three genes or of four other BnBOR1s between B-efficient and B-inefficient cultivars in low or normal B environments.

Conclusions

These results indicate that the B efficiency of Brassica napus is controlled primarily by roots, which allow more uptake and accumulation of B in B-efficient cultivars than B-inefficient cultivars in a low B environment. However the molecular mechanism regulating B efficiency in Brassica napus remains to be determined.     

Analysis of boron distribution in vivo for boron neutron capture therapy using two different boron compounds by secondary ion mass spectrometry

The efficiency of boron neutron capture therapy (BNCT) for malignant gliomas depends on the selective and absolute accumulation of (10)B atoms in tumor tissues. Only two boron compounds, BPA and BSH, currently can be used clinically. However, the detailed distributions of these compounds have not been determined. Here we used secondary ion mass spectrometry (SIMS) to determine the histological distribution of (10)B atoms derived from the boron compounds BSH and BPA. C6 tumor-bearing rats were given 500 mg/kg of BPA or 100 mg/kg of BSH intraperitoneally; 2.5 h later, their brains were sectioned and subjected to SIMS. In the main tumor mass, BPA accumulated heterogeneously, while BSH accumulated homogeneously. In the peritumoral area, both BPA and BSH accumulated measurably. Interestingly, in this area, BSH accumulated distinctively in a diffuse manner even 800 microm distant from the interface between the main tumor and normal brain. In the contralateral brain, BPA accumulated measurably, while BSH did not. In conclusion, both BPA and BSH each have advantages and disadvantages. These compounds are considered to be essential as boron delivery agents independently for clinical BNCT. There is some rationale for the simultaneous use of both compounds in clinical BNCT for malignant gliomas.     

Effects of salinity and varying boron concentrations on boron uptake and growth of wheat

Summary Two sandculture experiments were conducted with wheat (Triticum aestivum) to determine the effects of (1) osmotic potential (Ψ_π) and (2) fluctuating boron (B) concentrations on B availability (toxicity), shoot growth and leaf concentrations of B of wheat. The first experiment consisted of growing wheat to the spike emergence stage in sandcultures irrigated with a complete nutrient solution containing 1.0, 7.5, and 15.0 mg Bl⁻¹ and having Ψ_π values of −0.02, −0.07, −0.12, and −0.17 MPa produced by CaCl₂−NaCl additions. Statistically, shoot weight was independently influenced by the B and Ψ_π treatments but not by their interaction. Only the B treatment had a significant effect on leaf boron concentrations; the B x Ψ_π interaction was nonsignificant with respect to leaf B concentrations. The second experiment was designed to determine if growth and B uptake of wheat responds to the time integrated mean (TIM) concentration of B. This experiment consisted of four fixed-B concentrations and four fluctuating-B concentrations designed to produce two TIM concentrations (3.9 and 7.4 mg Bl⁻¹) approached low to high and vice versa. With respect to shoot weight, there was no statistical difference among treatments having the same TIM concentration during the 10 week experiment. However, shoot B concentrations differed greatly; they were higher when the B concentration was progressively increased over the 10 week period. Leaf B concentrations (Y leaf at flowering), while not as high as the shoot B concentrations, were also higher under the treatment of increasing B concentration, indicating B uptake rates are higher for mature plants than for seedlings.     

Reactions of boron with soils

Boron is an essential micronutrient for plants, but the range between deficient and toxic B concentration is smaller than for any other nutrient element. Plants respond directly to the activity of B in soil solution and only indirectly to B adsorbed on soil constituents. Soil factors affecting availability of B to plants are: pH, texture, moisture, temperature, organic matter and clay mineralogy. Boron adsorbing surfaces in soils are: aluminium and iron oxides, magnesium hydroxide, clay minerals, calcium carbonate, and organic matter. Boron adsorption reactions can be described empirically using the Langmuir adsorption isotherm equation, the Freundlich adsorption isotherm equation, and the phenomenological Keren model. Chemical models such as the constant capacitance model, the triple layer model, and the Stern VSC-VSP model can describe B adsorption over changing conditions of solution pH and B concentration. Boron desorption reactions often exhibit hysteresis. The rate of B desorption can be described using the first order rate equation, the Elovich reaction rate equation, and the power function equation.     

Specificities of boron disubstituted sumanenes

In this article we focused on computational research of sumanenes disubstituted by boron where the two carbon atoms are substituted by two boron atoms. Disubstitution of rim carbon atoms with boron atoms significantly affected the geometry of the bowl. The main stability factors were used to determine the stability of isomers. The most stable, the shallowest and the deepest isomers were subjected to further study of NMR parameters, chemical shielding and NICS, aromaticity, bowl to bowl inversion barrier and NBO/NPA analysis. The introduction of boron atoms significantly affected the above parameters, changing the aromatic nature of rings, reducing bowl to bowl inversion barrier and produced charge transfer. The NICS are correlated with bowl depth having the result that the function of the fourth degree of bowl depth does not only correlate well to the bowl to bowl inversion barrier with bowl depth, but also finely correlates the change of the NICS and NICSzz with bowl depth.     

Considerations in the determination of boron at low concentrations

Experimental evidence now supports the nutritional essentiality of boron (B) in some biological systems, and accordingly, the need for reliable analytical B data is increasing. However, the accurate determination of B in biological materials is a formidable challenge at low concentrations (<1 mg B/kg). Recent studies still show significant analytical discrepancies in the analysis of animal tissues and fluids, despite the development of instrumental techniques such as TIMS, ICP-MS, ICP-ES, ICAP, SIMS, NA-MS, PGAA, NRA, and so forth, which have demonstrated detection limits approaching or exceeding (μgB/kg concentrations. Since boric acid is both volatile and ubiquitous in nature, the chemical and physical pathways for B contamination and its loss are manifold, especially during sample preparation. An added obstacle is the inadequacy of biological reference materials certified for B below mg B/kg. With an emphasis toward sample preparation and ICP-MS analysis, examples are provided in this article to help the analyst avoid common problems associated with the analysis of B from biological sources. Topics that are discussed include contamination from Teflon vessels during microwave digestion, losses owing to freeze-drying, B isotopic variations, standards preparation, reagent backgrounds, and instrumental interferences.