Zed Rengel

Agronomic approaches for improving the micronutrient density in edible portions of field crops

Abstract Increased micronutrient density in grain destined for human consumption may alleviate micronutrient deficiencies (Fe, Zn, Cu, and I) in human populations around the world. The review of literature indicates that fertilization with inorganic and organic forms of micronutrients has a potential to increase their concentrations in grain. The most effective fertilization could be via soil (for Zn and, to some extent, Cu), foliarly (for Fe) and by adding fertilizers to the irrigation water (for iodine). Care should be exercised not to overfertilize crops with micronutrients because of consequent toxicity and losses in quality and quantity of grain yield. Effectiveness of various agricultural measures in increasing micronutrient density depends on soil type, crop, cultivar, rotation, and environmental and other factors, thus necessitating development of a specific set of measures for individual regions. Agricultural measures would need to be supplemented with appropriate changes in the milling technology to ascertain that increased micronutrient concentrations in some grain parts are passed into the food chain.

Crops and genotypes differ in efficiency of potassium uptake and use

Cultivars with increased efficiency of uptake and utilization of soil nutrients are likely to have positive environmental effects through reduced usage of chemicals in agriculture. This review assesses the available literature on differential uptake and utilization efficiency of K in farming systems. Large areas of agricultural land in the world are deficient in K (e.g. 3/4 of paddy soils in China, 2/3 of the wheatbelt in Southern Australia), with export in agricultural produce (especially hay) and leaching (especially in sandy soils) contributing to lowering of K content in the soil. The capacity of a genotype to grow and yield well in soils low in available K is K efficiency. Genotypic differences in efficiency of K uptake and utilization have been reported for all major economically important plants. The K-efficient phenotype is a complex one comprising a mixture of uptake and utilization efficiency mechanisms. Differential exudation of organic compounds to facilitate release of non-exchangeable K is one of the mechanisms of differential K uptake efficiency. Genotypes efficient in K uptake may have a larger surface area of contact between roots and soil and increased uptake at the root-soil interface to maintain a larger diffusive gradient towards roots. Better translocation of K into different organs, greater capacity to maintain cytosolic K(+) concentration within optimal ranges and increased capacity to substitute Na(+) for K(+) are the main mechanisms underlying K utilization efficiency. Further breeding for increased K efficiency will be dependent on identification of suitable markers and compounding of efficiency mechanisms into locally adapted germplasm.

Isolation of culturable phosphobacteria with both phytate-mineralization and phosphate-solubilization activity from the rhizosphere of plants grown in a volcanic soil

Chilean volcanic soils contain large amounts of total and organic phosphorus, but P availability is low. Phosphobacteria [phytate-mineralizing bacteria (PMB) and phosphate-solubilizing bacteria (PSB)] were isolated from the rhizosphere of perennial ryegrass (Lolium perenne), white clover (Trifolium repens), wheat (Triticum aestivum), oat (Avena sativa), and yellow lupin (Lupinus luteus) growing in volcanic soil. Six phosphobacteria were selected, based on their capacity to utilize both Na-phytate and Ca-phosphate on agar media (denoted as PMPSB), and characterized. The capacity of selected PMPSB to release inorganic P (Pi) from Na-phytate in broth was also assayed. The results showed that from 300 colonies randomly chosen on Luria–Bertani agar, phosphobacteria represented from 44% to 54% in perennial ryegrass, white clover, oat, and wheat rhizospheres. In contrast, phosphobacteria represented only 17% of colonies chosen from yellow lupin rhizosphere. This study also revealed that pasture plants (perennial ryegrass and white clover) have predominantly PMB in their rhizosphere, whereas PSB dominated in the rhizosphere of crops (oat and wheat). Selected PMPSB were genetically characterized as Pseudomonas, Enterobacter, and Pantoea; all showed the production of phosphoric hydrolases (alkaline phosphatase, acid phosphatase, and naphthol phosphohydrolase). Assays with PMPSB resulted in a higher Pi liberation compared with uninoculated controls and revealed also that the addition of glucose influenced the Pi-liberation capacity of some of the PMPSB assayed.

Drought and salinity differentially influence activities of superoxide dismutases in narrow-leafed lupins

Abstract Effects of drought and salinity on growth and activities of superoxide dismutase (SOD) forms were studied in narrow-leafed lupins (Lupinus angustifolius L.). Shoot dry weight and the elongation rate were depressed after 3 days of drought, with leaf water potential dropping to −1.64 MPa. Activity of total SOD increased by 21%, Cu/ZnSOD by 33% and FeSOD by 50% after 2 days of withholding water; further increases were noted with an increase in severity of drought stress. Two days after resupplying water, leaf water potential and the activity of Cu/ZnSOD returned to the control level, but the FeSOD activity remained high. The activity of MnSOD was unaffected by drought. Root fresh weight and the shoot elongation rate were not affected up to 50 mM NaCl, but were reduced at 100 mM NaCl after 6 days, with leaf water and osmotic potentials being −1.4 and −1.8 MPa, respectively. Concentrations of Na+ and Cl− in leaves increased linearly with an increase in NaCl concentration in the growth medium. Salinity stress enhanced activity of Cu/ZnSOD by 145% without influencing the activity of other SOD forms. Drought and salinity differentially influence activity of SOD forms in narrow-leafed lupins, indicating that different mechanisms may be involved in oxidative stress injury caused by drought and salinity.

Modelling root–soil interactions using three–dimensional models of root growth, architecture and function

BackgroundThree–dimensional root architectural models emerged in the late 1980s, providing an opportunity to conceptualise and investigate that all important part of plants that is typically hidden and difficult to measure and study. These models have progressed from representing pre–defined root architectural arrangements, to simulating root growth in response to heterogeneous soil environments. This was done through incorporating soil properties and more complete descriptions of plant function, moving into the realm of functional-structural plant modelling. Modelling studies are often designed to investigate the relationship between root architectural traits and root distribution in soil, and the spatio–temporal variability of resource supply. Modelling root systems presents an opportunity to investigate functional tradeoffs between foraging strategies (i.e. shallow vs deep rooting) for contrasting resources (immobile versus mobile resources), and their dependence on soil type, rainfall and other environmental conditions. The complexity of the interactions between root traits and environment emphasises the need for models in which traits and environmental conditions can be independently manipulated, unlike in the real world.ScopeWe provide an overview of the development of three–dimensional root architectural models from their origins, to their place today in the world of functional–structural plant modelling. The uses and capability of root architectural models to represent virtual plants and soil environment are addressed. We compare features of six current models, RootTyp, SimRoot, ROOTMAP, SPACSYS, R-SWMS, and RootBox, and discuss the future development of functional-structural root architectural modelling.ConclusionFunctional-structural root architectural models are being used to investigate numerous root–soil interactions, over a range of spatial scales. They are not only providing insights into the relationships between architecture, morphology and functional efficiency, but are also developing into tools that aid in the design of agricultural management schemes and in the selection of root traits for improving plant performance in specific environments.

Excess cation uptake, and extrusion of protons and organic acid anions by Lupinus albus under phosphorus deficiency

In symbiotically-grown legumes, rhizosphere acidification may be caused by a high cation/anion uptake ratio and the excretion of organic acids, the relative importance of the two processes depending on the phosphorus nutritional status of the plants. The present study examined the effect of P deficiency on extrusions of H(+) and organic acid anions (OA(-)) in relation to uptake of excess cations in N(2)-fixing white lupin (cv. Kiev Mutant). Plants were grown for 49 days in nutrient solutions treated with 1, 5 or 25 mmol P m(-3) Na(2)HPO(4) in a phytotron room. The increased formation of cluster roots occurred prior to a decrease in plant growth in response to P deficiency. The number of cluster roots was negatively correlated with tissue P concentrations below 2.0 g kg(-1) in shoots and 3 g kg(-1) in roots. Cluster roots generally had higher concentrations of Mg, Ca, N, Cu, Fe, and Mn but lower concentrations of K than non-cluster roots. Extrusion of protons and OA(-) (90% citrate and 10% malate) from roots was highly dependent on P supply. The amounts of H(+) extruded per unit root biomass decreased with time during the experiment. On the equimolar basis, H(+) extrusion by P-deficient plants (grown at 1 and 5 mmol P m(-3)) were, on average, 2-3-fold greater than OA(-) exudation. The excess cation content in plants was generally the highest at 1 mmol P m(-3) and decreased with increasing P supply. The ratio of H(+) release to excess cation uptake increased with decreasing P supply. The results suggest that increased exudation of OA(-) due to P deficiency is associated with H(+) extrusion but contributes only a part of total acidification.

Function of Nutrients: Micronutrients

Publisher Summary This chapter focuses on the functions of iron, manganese, copper, zinc, nickel, molybdenum, boron, and chlorine in plants and describes the effects of their deficiency and toxicity. Iron (Fe) plays a crucial role in redox systems in cells and in various enzymes. In dicotyledonous and monocotyledonous plant species, Fe deficiency is associated with the formation of rhizodermal transfer cells, which is a part of a their strategy to enhance iron uptake. Manganese (Mn) and copper (Cu) are important for redox systems, as activators of various enzymes including those involved in the detoxification of superoxide radicals, and for the synthesis of lignin. In dicotyledonous plants, intercostal chlorosis of the younger leaves is the most distinct symptom of Mn deficiency, whereas in cereals, greenish grey spots on the older leaves are the major symptoms. Stunted growth, distortion of young leaves, chlorosis/ necrosis starting at the apical meristem extending down to the leaf margins, bleaching of young leaves, and/or “summer dieback” in trees are typical visible symptoms of Cu deficiency. Zinc (Zn) plays a role in the detoxification of superoxide radicals, membrane integrity, as well as the synthesis of proteins and the phytohormone IAA. Nickel (Ni) is involved in N metabolism as a metal component of the enzyme urease, whereas molybdenum (Mo) helps in N metabolism by acting as a metal component of the nitrogenase (N2 fixation) and nitrate reductase enzymes. Boron (B) is crucial for cell wall and membrane integrity, whereas chlorine plays a role in osmoregulation and stomata movement.

Role of magnesium in alleviation of aluminium toxicity in plants

Magnesium is pivotal for activating a large number of enzymes; hence, magnesium plays an important role in numerous physiological and biochemical processes affecting plant growth and development. Magnesium can also ameliorate aluminium phytotoxicity, but literature reports on the dynamics of magnesium homeostasis upon exposure to aluminium are rare. Herein existing knowledge on the magnesium transport mechanisms and homeostasis maintenance in plant cells is critically reviewed. Even though overexpression of magnesium transporters can alleviate aluminium toxicity in plants, the mechanisms governing such alleviation remain obscure. Possible magnesium-dependent mechanisms include (i) better carbon partitioning from shoots to roots; (ii) increased synthesis and exudation of organic acid anions; (iii) enhanced acid phosphatase activity; (iv) maintenance of proton-ATPase activity and cytoplasmic pH regulation; (v) protection against an aluminium-induced cytosolic calcium increase; and (vi) protection against reactive oxygen species. Future research should concentrate on assessing aluminium toxicity and tolerance in plants with overexpressed or antisense magnesium transporters to increase understanding of the aluminium-magnesium interaction.

Salicylic acid improves salinity tolerance in Arabidopsis by restoring membrane potential and preventing salt-induced K+ loss via a GORK channel

Despite numerous reports implicating salicylic acid (SA) in plant salinity responses, the specific ionic mechanisms of SA-mediated adaptation to salt stress remain elusive. To address this issue, a non-invasive microelectrode ion flux estimation technique was used to study kinetics of NaCl-induced net ion fluxes in Arabidopsis thaliana in response to various SA concentrations and incubation times. NaCl-induced K+ efflux and H+ influx from the mature root zone were both significantly decreased in roots pretreated with 10–500 μM SA, with strongest effect being observed in the 10–50 μM SA range. Considering temporal dynamics (0–8-h SA pretreatment), the 1-h pretreatment was most effective in enhancing K+ retention in the cytosol. The pharmacological, membrane potential, and shoot K+ and Na+ accumulation data were all consistent with the model in which the SA pretreatment enhanced activity of H+-ATPase, decreased NaCl-induced membrane depolarization, and minimized NaCl-induced K+ leakage from the cell within the first hour of salt stress. In long-term treatments, SA increased shoot K+ and decreased shoot Na+ accumulation. The short-term NaCl-induced K+ efflux was smallest in the gork1-1 mutant, followed by the rbohD mutant, and was highest in the wild type. Most significantly, the SA pretreatment decreased the NaCl-induced K+ efflux from rbohD and the wild type to the level of gork1-1, whereas no effect was observed in gork1-1. These data provide the first direct evidence that the SA pretreatment ameliorates salinity stress by counteracting NaCl-induced membrane depolarization and by decreasing K+ efflux via GORK channels.

Arsenic speciation in terrestrial plant material using microwave-assisted extraction, ion chromatography and inductively coupled plasma mass spectrometry

Determining arsenic speciation in terrestrial plants is necessary to understand how plants transform and metabolise arsenic. Ion chromatography in conjunction with inductively coupled plasma mass spectrometry can be used to measure arsenic speciation in plant material. However, the arsenic species need to be extracted quantitatively from the plant matrix before measurement. A method is therefore presented for the extraction of arsenic species from freeze-dried plant material. The method was optimised by using a variety of extracts (water, a modified protein extracting solution, and 10% (v/v) tetramethylammonium hydroxide solution), different microwave heating temperatures (50, 70 or 90 °C) and different heating times (5, 10 or 20 min). Microwave-heating the samples at 90 °C during 20 min in a modified protein extracting solution provided good extraction efficiency (98 ± 1%). The individual arsenic species remained intact during the extraction procedure, with fortification recoveries between 97 and 107% for all species (arsenite, arsenate, dimethylarsinic acid and monomethylarsonic acid) (added as 0.333 mg As kg−1). The method was used to extract arsenic species from shoots and roots of canola (Brassica napus), velvet grass (Holcus lanatus), red brome grass (Bromus rubens), Arabidopsis thaliana and Senna planitiicola containing various amounts of total arsenic (0.2–2434 mg As kg−1 dry weight). Extraction efficiency across all these diverse plant materials was 104 ± 16%. Most of the plant samples contained mainly arsenate and arsenite, but low levels (1 to 2%) of dimethylarsinic and monomethylarsonic acids were measured in shoots and roots of Holcus lanatus and Arabidopsis thaliana.