Longbin Huang

Physiological response of plants to low boron

This review focuses on physiological responses in higher plants to B deficiency at the whole plant and organ level. Plants respond to decreasing B supply in soil solutions by slowing down or ceasing growth. Boron deficiency inhibits root elongation through limiting cell enlargement and cell division in the growing zone of root tips. In the case of severe B deficiency, the root cap, quiescent centre and protoderm of root tips disappear and root growth ceases, leading to the death of root tips. Although vascular bundles are weakly developed in B-deficient roots, early effects of B deficiency on their initiation and differentiation is poorly understood. Inhibited leaf expansion by low B indirectly decreases the photosynthetic capacity of plants, though exact roles of B in photosynthesis remain to be explored. The early inhibition of root growth, compared to shoot growth, increases the shoot:root ratio. It is hypothesised that this may enhance the susceptibility of plants to environmental stresses such as marginally deficient supplies of other nutrients and water deficit in soil.In the field, sexual reproduction is often more sensitive to low soil B than vegetative growth, and marked seed yield reductions can occur without symptoms being expressed during prior vegetative growth. In flowers, low B reduces male fertility primarily by impairing microsporogenesis and pollen tube growth. Post-fertilisation effects include impaired embryogenesis, resulting in seed abortion or the formation of incomplete or damaged embryos, and malformed fruit. However, there is a great diversity of effects of low B on reproductive growth among species, and within the same species between sites and seasons. Much of this diversity is not explained by the current literature. Key processes in reproductive development which may be impaired under B deficiency are proposed and discussed. These include the formation of a diverse array of cell wall types, the supply of carbohydrates for growth and storage reserves, and the production of flavonols. Inflorescence architecture, floral morphology, canopy structure and prevailing weather conditions are suggested as being important for xylem B delivery into flowers because of their impact on transpiration. The extent of phloem translocation of B into reproductive organs has yet to be fully assessed. The timing of B sensitive stages in reproduction of most crop plants need defining in order to facilitate appropriate timing of corrective B treatments.As most container studies have imposed B deficiency by withholding B, much of the data on severely B-deficient plants requires re-evaluation. Further studies are warranted to understand the effects of realistically low levels of B in solution on the growth of meristematic tissues and floral organs. A B-buffered solution culture system is recommended for some of this work.

Rapid Nitric Acid Digestion of Plant Material with an Open-Vessel Microwave System

Abstract Digestion of plant materials in hot (130–140°C) concentrated nitric acid (HNO3) is a common procedure for assessing their nutrient contents. In the conventional HNO3 digestion, desired temperatures are achieved through controlled electrical heating, and digestion occurs within Pyrex test tubes. The main limitations associated with the conventional digestion method may include (1) high labor requirement for monitoring acid levels in the tubes and digest solution transfer at the end of digestion and (2) relatively high background levels, in particular, of trace elements (e.g., Cu, B, Mn, etc.) resulting from the glass matrix or/and repeated use of digestion tubes. The availability of industrial microwave technology provides opportunities for developing improved digestion systems that overcome the above constraints while routinely processing large batches of plant samples. The present article describes a simple, reliable, and rapid digestion procedure for HNO3 with hydrogen peroxide (H2O2) digestion of plant material by using an open-vessel (50 mL polypropylene tubes with caps in which a 3.2 mm diameter ventilation hole is drilled in the center), microwave-digestion system (CEM Mars 5, manufactured by CEM Corp., USA), followed by elemental quantification using an ICP-AES. The proposed method consists of two stages: (1) the predigested (overnight) sample and HNO3 mix is heated at 75°C for 10 min, followed by 109°C for 15 min; (2) after cooling for 10 min, 1 mL of H2O2 is added to each vessel through the ventilation hole and the sample mix is heated at 109°C for a further 15 min. The analytical results were statistically analyzed by using linear regression, linear correlation, and two independent means tests to determine analytical precision and accuracy of the proposed digestion method. The results have demonstrated that this method is suitable for precise and accurate determination of macronutrients calcium (Ca), potassium (K), magnesium (Mg), phosphorus (P), sulfur (S), and micronutrients boron (B), copper (Cu), manganese (Mn), and zinc (Zn) in plant materials. The analytical variability (coefficient of variation) was mostly less than 5%, apart from that of iron (Fe) (9%). There were no significant (P ≤ 0.05) differences between the measured and certified concentrations of both macro- and micronutrients in the ASPAC and NIST1515 standard reference materials (SRM), except for Fe in NIST1515 SRM. The recovery rate of Fe in the digest solution varies with plant types, for cereal samples, higher than 90%, but for dicot species (e.g., NIST apple leaves) the recovery rate was as low as 70%. One of the important advantages of this method was the consistently (across samples and different batches) low background reading (mostly under detection limits of the ICP-AES used, for example, the concentrations of B in the blank digests were consistently less than 5 µg/L). The adoption of the present digestion method may result in time saving due to short turn-around time (less than 60 min per 50 samples) and cost saving due to low labor requirement, low acid consumption, and low-cost digestion vessels.

Development of a boron buffered solution culture system for controlled studies of plant boron nutrition

Chelated-buffered nutrient solutions are used for studies on micronutrient metals but so far no equivalent system exists for boron nutrition studies: the present investigation was initiated with that intention. From a literature review, it was noted that a range of substances form chelates with boron including polyhydric alcohols, sugars and phenolic compounds. However, none apart from hydrofluoric acid formed chelates with formation constants comparable to those of micronutrient metal chelates like diethylenetriaminepentaacetic acid (DTPA). Moreover, most chelating substances had deleterious side effects which reduced their possible use in water culture: many of the compounds are substrates for bacterial growth, some are harmful to handle, and others are toxic to plants or humans. Borosilicate glass; was tested in a laboratory experiment but found to release boron too slowly into solution to maintain constant boron concentration in solution even when very finely ground. Current investigations centre around the use of a boron-specific resin, which strongly complexes H3BO3 on its N-methyl glucamine functional groups. The boron sorption capacity of the resin varied from 2.2 to 5.0 mg B g-1 resin. Boron saturated resin maintained an equilibrium solution boron concentration of 46 μt M when added at the rate of 2 g of resin to 1 L of boron free triple deionised water. Plants grown in complete nutrient solution with boron saturated resin added at 1 g per litre of nutrient solution grew as well as plants grown in conventional nutrient solution containing 9.2 μt M boron and their shoots contained adequate boron concentrations for growth. There was no evidence that the resin had effects on plant growth other than in releasing and equilibrating boron concentration in the nutrient solution.

Copper and zinc adsorption by softwood and hardwood biochars under elevated sulphate-induced salinity and acidic pH conditions.

Biochar adsorption may lower concentrations of soluble metals in pore water of sulphidic Cu/Pb-Zn mine tailings. Unlike soil, high levels of salinity and soluble cations are present in tailing pore water, which may affect biochar adsorption of metals from solution. In the present study, removal of soluble copper (Cu) and zinc (Zn) ions by soft- (pine) and hard-wood (jarrah) biochars pyrolysed at high temperature (about 700 °C) was evaluated under typical ranges of pH and salinity conditions resembling those in pore water of sulphidic tailings, prior to their direct application into the tailings. Surface alkalinity, cation exchange capacity, and negative surface charge of biochars affected Cu and Zn adsorption capacities. Quantitative comparisons were provided by fitting the adsorption equilibrium data with either the homogeneous or heterogeneous surface adsorption models (i.e. Langmuir and Freundlich, respectively). Accordingly, the jarrah biochar showed higher Cu and Zn adsorption capacity (Qmax=4.39 and 2.31 mg/g, respectively) than the softwood pine biochar (Qmax=1.47 and 1.00 mg/g). Copper and Zn adsorption by the biochars was favoured by high pH conditions under which they carried more negative charges and Cu and Zn ions were predicted undergoing hydrolysis and polymerization. Within the tested range, salinity had relatively weak effects on the adsorption, which perhaps influenced the surface charge and induced competition for negative charged sites between Na(+) and exchangeable Ca(2+) and/or heavy metal ions. Large amounts of waste wood/timber at many mine sites present a cost-effective opportunity to produce biochars for remediation of sulphidic tailings and seepage water.

The importance of sampling immature leaves for the diagnosis of boron deficiency in oilseed rape (Brassica napus cv. Eureka)

Plant analysis can diagnose boron (B) deficiency when the standards used have been properly developed by establishing that a close relationship exists between B concentration in a plant part and its physiological function. The purpose of the present study was to demonstrate the importance of choosing the growing immature leaves for B deficiency diagnosis and for establishing critical B concentrations for the diagnosis of B deficiency in oilseed rape (Brassica napus). In Experiment 1, the plants were subject to seven levels of B supply using programmed nutrient addition, for the estimation of critical B concentrations in plant parts for shoot growth. In Experiment 2, the plants were treated with two levels of B supply in solution: 10 (+B) and 0 (-B) μM B, for the estimation of functional B requirements for leaf elongation. The results showed that critical B concentrations varied amongst the plant parts sampled and decreased with leaf age. As B taken up by roots is largely phloem-immobile, B concentrations in mature leaves are physiologically irrelevant to plant B status at the time of sampling, giving rise to a significant over- or underestimation of the B requirement for plant growth. By contrast, a growing, immature leaf, in this case the youngest open leaf (YOL), was the most reliable plant part for B deficiency diagnosis. Critical B concentrations developed from both methods were comparable-i.e. 10–14 mg B kg−1 dry matter in the YOL at vegetative growth stages up to stem elongation.

Toward a new paradigm for tailings phytostabilization – nature of the substrates, amendment options and anthropogenic pedogenesis

Base metal tailings (BMTs) are normally sulfidic and contain high abundance of residue metals. Their adverse impacts on the environment can last for decades to centuries if without appropriate stabilization. While in situ phytostabilization has been thought to be a promising approach to stabilize surface tailings, few studies have reported success in constructing a sustainable plant community in BMTs so far, implying that a new paradigm involving a sophisticated understanding of the nature of BMTs is needed for BMTs phytostabilization. Using a property database of BMTs worldwide built in this study as a backdrop, this review explores how BMTs are different from normal soils and how these differences influence the strategies of BMTs phytostabilization. It is found that BMTs are mineralogically and chemically different from natural soils, which endows BMTs with unstable geochemistry and inherent extreme toxicity. Studies have documented that amendment options and soil development in BMTs phytostabilization are largely constrained by these abiotic factors. From a viewpoint of pedogenesis, BMTs can be seen as novel parent materials rather than soil. Accordingly, we propose that in BMTs phytostabilization, extensive engineering efforts are required to increase the biocapacity of tailings (i.e., anthropogenic pedogenesis) rather than focus on the selection and establishment of plants.

Iron-fortified parboiled rice – A novel solution to high iron density in rice-based diets

The present study pioneered an investigation of a novel and cost-effective approach to fortify Fe in rice and to greatly improve Fe nutrition in rice-based diets through parboiling, though it remains at its preliminary phase. Rice grains of seven cultivars were parboiled in deionised water containing different levels of Fe chelate made by mixing different proportions of Fe sulfate (FeSO4) with ethylenediaminetetra-acetic acid disodium salt (Na2EDTA). Adding Fe to the parboiling water resulted in an increased Fe concentration in the most grain, effectively where FeSO4 and Na2EDTA were mixed at 2:1 molar ratio (11.16g Fe per 100g raw paddy grain). This treatment resulted in Fe concentrations in white rice milled for 60s and 120s, which were 20-50 times higher than those in the unfortified milled raw rice grains. The Fe concentrations in milled rice grains were 50-150mg Fe kg(-1) in 60s milled grains with a slight reduction in 120s milled grains. Perls Prussian blue staining of the cross section of Fe-fortified parboiled rice grains suggested inward movement of added Fe into the endosperm through the apoplastic pathway in the dorsal region of the rice grain. The retention rates of fortified Fe varied among the different cultivars, possibly due to different physical-chemical properties of the grains. The percentages of soluble fraction of the total Fe were higher than 50% in all cultivars tested, indicating its high bioavailability potential, though it remains to be evaluated. The present findings provided a preliminary basis for further investigation of this innovative technique, before its adoption by parboiled rice industry, such as optimising the levels of Fe addition and industrial process and Fe bioavailability in Fe-fortified-parboiled rice.

Boron in Plant Reproduction

In the period since the first B symposium in 1997 (Bell and Rerkasem 1997; Dell et al., 1997), progress in defining either structural or metabolic roles for B in reproductive parts of seed plants has been disappointing. However, better control of B in solution (Huang et al., 1999) has allowed B effects on anther and pollen development in wheat to be more fully documented (Huang et al., 2000, Huang et al., 2001). This work on wheat is briefly reviewed to highlight principles that are emerging about the function of B in reproductive development.

Prognosis of boron deficiency in oilseed rape (Brassica napus) by plant analysis

Reliable prediction of boron (B) supply before planting can enable growers to avoid yield losses by applying fertiliser if required. Several reports indicate the successful use of soil analysis to evaluate soil B levels and predict their effects on growth and yield of crops (e.g. Morrill et al., 1977; Howeler et al., 1978; see also Bell 1997, Bell 1999). Pre-planting soil analysis should be more effective than the present approach to B nutrient management in southeast China (Wang et al., 1999) which relies on foliar spraying of oilseed rape when symptoms appear on crops: such treatment is often too late to fully correct the yield loss from B deficiency (Z. Q. Ye and Y. Yang, personal communication). Studies also show that B concentrations in specific plant parts were well correlated with oilseed rape yield (Wei et al., 1998, also see Bell, 1997 for other examples). Whereas plant analysis can reflect B nutrient status in crops directly, there may not be sufficient time left in the season to make effective use of the information about a possible or actual deficiency. In such cases, soil analysis of samples taken before sowing is the most practical means of determining if B fertiliser is required.

From lithotroph- to organotroph-dominant: directional shift of microbial community in sulphidic tailings during phytostabilization

Engineering microbial diversity to enhance soil functions may improve the success of direct revegetation in sulphidic mine tailings. Therefore, it is essential to explore how remediation and initial plant establishment can alter microbial communities, and, which edaphic factors control these changes under field conditions. A long-term revegetation trial was established at a Pb-Zn-Cu tailings impoundment in northwest Queensland. The control and amended and/or revegetated treatments were sampled from the 3-year-old trial. In total, 24 samples were examined using pyrosequencing of 16S rRNA genes and various chemical properties. The results showed that the microbial diversity was positively controlled by soil soluble Si and negatively controlled by soluble S, total Fe and total As, implying that pyrite weathering posed a substantial stress on microbial development in the tailings. All treatments were dominated by typical extremophiles and lithotrophs, typically Truepera, Thiobacillus, Rubrobacter; significant increases in microbial diversity, biomass and frequency of organotrophic genera (typically Nocardioides and Altererythrobacter) were detected in the revegetated and amended treatment. We concluded that appropriate phytostabilization options have the potential to drive the microbial diversity and community structure in the tailings toward those of natural soils, however, inherent environmental stressors may limit such changes.