Enhanced tolerance to boron toxicity in two-rowed barley by marker-assisted introgression of favourable alleles derived from Sahara 3771

Boron (B) is an essential micronutrient for higher plant, but toxic levels can seriously diminish grain yield in cereal crops by affecting root growth, and thus restricting water extraction from the subsoil. Amelioration of high concentrations in soils is expensive and not always feasible, so breeding for B tolerance is the most viable alternative. This article reports the marker-assisted (MAS) transfer of favourable alleles from an unadapted six-rowed barley (Hordeum vulgare L.) variety, Sahara 3771, into two-rowed lines adapted to southern Australia. During the backcrossing process, the SSR marker, EBmac679, located on chromosome 4H was used to control the target region in foreground selection, but no background selection was applied. Gene introgression was confirmed with 40 BC₆F₁-derived doubled haploid lines segregating for the SSR marker EBmac679. We used a combination of molecular and conventional assays to unequivocally classify the 40 BC₆F₁-derived DH lines as B tolerant or sensitive, and then compared their means for grain yield measured over 2 years and four locations. Results showed modest improvements in grain yield of lines carrying B tolerance genes at some B toxic environments, and negative impact at others. Our results also showed that malting quality profile was not adversely affected through the introgression of the B tolerance allele from Sahara 3771, allowing the newly developed material to be used by breeding programs without risk of a penalty on malt quality.     

Mapping of chromosome regions conferring boron toxicity tolerance in barley (Hordeum vulgare L.)

 Boron toxicity has been recognised as an important problem limiting production in the low-rainfall regions of southern Australia, West Asia and North Africa. Genetic variation for boron toxicity tolerance in barley has been characterised but the mode of inheritance and the location of genes controlling tolerance were not previously known. A population of 150 doubled-haploid lines from a cross between a boron toxicity tolerant Algerian landrace, Sahara 3771, and the intolerant Australian cultivar Clipper was screened in four tolerance assays. An RFLP linkage map of the Clipper×Sahara population was used to identify chromosomal regions associated with boron tolerance in barley. Interval regression-mapping allowed the detection of four chromosomal regions involved in the boron tolerance traits measured. A region on chromosome 2H was associated with leaf-symptom expression, a region on chromosome 3H was associated with a reduction of the affect of boron toxicity on root growth suppression, a region on chromosome 6H was associated with reduced boron uptake, and a region on chromosome 4H was also associated with the control of boron uptake as well as being associated with root-length response, dry matter production and symptom expression. The benefits and potential of marker-assisted selection for boron toxicity tolerance are discussed. Received: 18 December 1997 / Accepted: 28 November 1998     

Genes mapping to boron tolerance QTL in barley identified by suppression subtractive hybridization

Boron tolerance is a quantitative trait controlled by multiple genes. Suppression subtractive hybridization was carried out on root cDNA from bulked boron tolerant and intolerant doubled haploid barley lines grown under moderate boron stress to identify genes associated with boron tolerance. One hundred and eleven clones representing known proteins were found to be up‐regulated in the tolerant bulk upon boron stress. Nine clones were genetically mapped to previously reported boron tolerance QTL. These include a clone identical to the boron transporter gene Bot1 and a clone coding for a bromo‐adjacent homology domain‐containing protein, mapping to the 6H boron tolerance locus and co‐segregating with reduced boron intake in a Clipper × Sahara‐3771 mapping population. A third clone mapping to the 2H QTL region encoding an S‐adenosylmethionine decarboxylase precursor was found to provide tolerance to high boron by heterologous expression. Yeast cells expressing Sahara SAMDC were able to grow on 15 mm boron solid media and maintained cellular boron concentrations at 13% lower than control cells expressing empty vector. The data suggest that an antioxidative response mechanism involving polyamines and the ascorbate–glutathione pathway in Sahara barley may provide an advantage in tolerating high soil concentrations of boron.     

Boron toxicity in barley (Hordeum vulgare L.) seedlings in relation to soil temperature

Boron (B) is an essential micronutrient in crop growth but its sufficiency range is narrow. Boron toxicity is a widespread problem in arid and semi-arid areas with cold weather. We investigated the effect of soil temperature (5, 10 and 15°C) on development of symptoms of B toxicity, plant growth and plant development, and on content and concentration of B in tissue of seedlings of four barley lines grown in soil with high level of available B (12 mg kg^–1). Visual symptoms of toxicity were first observed in the high B soil concentration treatment at 5 °C at 12 days after emergence. Concentration of B in tissue decreased with increasing soil-temperatures. There was no effect of soil temperature on B content or B concentration in plant tissue at the final sample (17 days after emergence). High soil B reduced seedling and leaf emergence rates, although the final seedling emergence and number of leaves were unaffected. Barley lines differed in concentration of B in tissues and visual toxicity symptom development. Adaptation to high B was either through maintaining low tissue B concentration or through tolerance to high tissue B concentration. While the investigated range of temperature does influence B toxicity in barley seedlings, it remains to be determined whether it affects crop yield.     

The mechanism of boron tolerance for maintenance of root growth in barley (Hordeum vulgare L.)

Cultivar differences in root elongation under B toxic conditions were observed in barley (Hordeum vulgare L.). A significant increase in the length and width of the root meristematic zone (RMZ) was observed in Sahara 3771 (B tolerant) when it was grown under excessive B concentration, compared to when grown at adequate B supply. This coincided with an increase in cell width and cell numbers in the meristematic zone (MZ), whereas a significant decrease in the length and no significant effect on the width of the MZ was observed in Clipper (B intolerant) when it was grown under excessive B supply. This was accompanied by a decrease in cell numbers, but an increase in the length and width of individual cells present along the MZ. Excessive B concentrations led to a significantly lower osmotic potential within the cell sap of the root tip in SloopVic (B tolerant) and Sahara 3771, while the opposite was observed in Clipper. Enhanced sugar levels in the root tips of SloopVic were observed between 48 and 96 h after excess B was applied. This coincided with an increase in the root elongation rate and with a 2.7-fold increase in sucrose level within mature leaf tissue. A significant decrease in reducing sugar levels was observed in the root tips of Clipper under excessive B concentrations. This coincided with significantly lower root elongation rates and lower sucrose levels in leaf tissues. Results indicate a B tolerance mechanism associated with a complex control of sucrose levels between leaf and root tip that assist in maintaining root growth under B toxicity.     

Molecular marker linked to a chromosome region regulating seed Zn accumulation in barley

Zinc deficiency is a critical nutritional problem in soils, restricting yield and nutritional quality of barley (Hordeum vulgare L.). Some genotypes (Zn-efficient) can produce greater yield and accumulate more Zn in seed under Zn deficiency than standard (Zn-inefficient) genotypes. However, there is little information regarding the genetics of Zn uptake/accumulation and location of genes conferring Zn efficiency in barley. Selection through molecular markers for seed Zn accumulation might be an efficient complementary breeding tool in barley. With the aim of developing molecular markers for increased accumulation of Zn in seed, a population of 150 DH lines derived from a cross between Clipper (low-Zn-accumulator) and Sahara 3771 (high-Zn-accumulator) was screened in the field and glasshouse for seed Zn concentration and content. One dominant DNA polymorphism was detected using the microsatellite-anchored fragment length polymorphism (MFLP) technique. The candidate MFLP marker was isolated from the MFLP gel, re-amplified by PCR, cloned, sequenced, and converted into simple sequence-specific and PCR-based marker. This marker, located on the short arm of chromosome 2H, might be useful for the improvement of barley nutritional quality and productivity programs in Zn-deficient environments. However, high seed Zn alone can not replace the need for Zn fertilization.     

Genetic and molecular analyses of resistance to a variant of Puccinia striiformis in barley

Seedlings of 62 Australian barley cultivars and two exotic barley genotypes were assessed for resistance to a variant of Puccinia striiformis, referred to as “Barley Grass Stripe Rust” (BGYR), first detected in Australia in 1998, which is capable of infecting wild Hordeum species and some genotypes of cultivated barley. Fifty-three out of 62 cultivated barley cultivars tested were resistant to the pathogen. Genetic analyses of seedling resistance to BGYR in six Australian barley cultivars and one Algerian barley landrace indicated that they carried either one or two major resistance genes to the pathogen. A single recessive seedling resistance gene, rpsSa3771, identified in Sahara 3771, was located on the long arm of chromosome 1 (7 H), flanked by the restriction fragment length polymorphism (RFLP) markers Xwg420 and Xcdo347 at genetic distances of 12.8 and 21.9 cM, respectively. Mapping resistance to BGYR at adult plant growth stages using the doubled haploid (DH) population Clipper × Sahara 3771 identified two major quantitative trait loci (QTL), one on the long arm of chromosome 3 (3 H) and the second on the long arm of chromosome 1 (7 H), accounting for 26 % and 18 % of the total phenotypic variation, respectively. The QTL located on chromosome 7HL corresponded to seedling resistance gene rpsSa3771 and the second QTL was concluded to correspond to a single APR gene, designated rpsCl, contributed by cultivar Clipper.     

Boron tolerance in barley is mediated by efflux of boron from the roots

Many plants are known to reduce the toxic effects of high soil boron (B) by reducing uptake of B, but no mechanism for limiting uptake has previously been identified. The B-tolerant cultivar of barley (Hordeum vulgare L.), Sahara, was shown to be able to maintain root B concentrations up to 50% lower than in the B-sensitive cultivar, Schooner. This translated into xylem concentrations that were approximately 64% lower and leaf concentrations 73% lower in the tolerant cultivar. In both cultivars, B accumulation was rapid and reached a steady-state concentration in roots within 3 h. In Schooner, this concentration was similar to the external medium, whereas in Sahara, the root concentration was maintained at a lower concentration. For this to occur, B must be actively extruded from the root in Sahara, and this is presumed to be the basis for B tolerance in barley. The extrusion mechanism was inhibited by sodium azide but not by treatment at low temperature. Several anion channel inhibitors were also effective in limiting extrusion, but it was not clear whether they acted directly or via metabolic inhibition. The ability of Sahara to maintain lower root B concentrations was constitutive and occurred across a wide range of B concentrations. This ability was lost at high pH, and both Schooner and Sahara then had similar root B concentrations. A predictive model that is consistent with the empirical results and explains the tolerance mechanism based on the presence of a borate anion efflux transporter in Sahara is presented.     

An investigation of boron toxicity in barley using metabolomics

Boron (B) is an essential micronutrient that affects plant growth at either deficient or toxic concentrations in soil. The aim of this work was to investigate the adaptation of barley (Hordeum vulgare) plants to toxic B levels and to increase our understanding of B toxicity tolerance mechanisms. We used a metabolomics approach to compare metabolite profiles in root and leaf tissues of an intolerant, commercial cultivar (cv Clipper) and a B-tolerant Algerian landrace (cv Sahara). After exposure to elevated B (200 and 1,000 microM), the number and amplitude of metabolite changes in roots was greater in Clipper than in Sahara. In contrast, leaf metabolites of both cultivars only responded following 1,000 microM treatment, at which B toxicity symptoms (necrosis) were visible. In addition, metabolite levels were dramatically altered in the tips of leaves of the sensitive cultivar Clipper after growth in 1,000 microM B compared to those of Sahara. This correlates with a gradual accumulation of B from leaf base to tip in B-intolerant cultivars. Overall, there were always greater differences between tissue types (roots and leaves) than between the two cultivars. This work has provided insights into metabolic differences of two genetically distinct barley cultivars and information about how they respond metabolically to increasing B levels.     

Effects of boron and limestone on cereal yields and on B and N concentration of plant tissue

Summary Rates of added B up to 1.8 kg/ha had no effect on grain yield, but at rates of 2.24 kg/ha and higher it decreased yield of both barley and wheat. Lime-stone applications up to 4000 kg/ha did not induce B deficiency at low levels of B or alleviate B toxicity at high B levels in the two crops. Liming increased barley yields where soil pH was less than 5.8 but gave no yield response when soil pH exceeded 5.8. In general, liming did not decrease the B concentration of boot stage tissue (b.s.t.) except in barley at 1.8 kg B/ha on Location 1, where liming to pH 6.3 and 6.6 decreased B in b.s.t. from 16.4 to 12.2 and 11.4 ppm. The applications of B generally increased the N concentration of wheat grain where yields decreased due to B toxicity. Contribution no. 360, Research Branch, Research Station, Charlottetown, Prince Edward Island, Canada, C1A 7M8. Contribution no. 360, Research Branch, Research Station, Charlottetown, Prince Edward Island, Canada, C1A 7M8.     

Effect of boron application on barley in a sierozem sandy soil

Summary In a pot culture experiment on a sierozem sandy soil (pH 8.2) rates of added B at 3 ppm although decreased root yield significantly but shoot and grain yield was unaffected even at 6 ppm added B, even though shoot B concentration was as high as 360 ppm and Ca/B ratio as low as 11. At 6 ppm applied B, shoot yield was increased by 18.5 per cent, whereas grain yield was at par with control. The results suggested that Ca/B ratio in barley straw was not a reliable index for determing the magnitude of B problem in the soil.     

Identification of boron transporter genes likely to be responsible for tolerance to boron toxicity in wheat and barley

Tolerance to boron (B) toxicity in cereals is known to be associated with reduced tissue accumulation of B. Genes from roots of B-tolerant cultivars of wheat and barley with high similarities to previously reported B efflux transporters from Arabidopsis and rice were cloned. Expression of these genes was strongly correlated with the ability of tolerant genotypes to lower the concentration of B in roots. The gene from barley located to chromosome 4. Backcross lines containing a B tolerance locus on chromosome 4 showed tolerance in proportion to the level of expression of the transporter gene, whereas those lacking the locus were sensitive to B and had very low levels of gene expression. The results are consistent with a widespread mechanism of tolerance to high B based on efflux of B from root cells.     

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.     

Development of gene-specific markers for acid soil/aluminium tolerance in barley (Hordeum vulgare L.)

Acid soil/aluminium toxicity is one of the major constraints on barley production around the world. Genetic improvement is the best solution and molecular-marker-assisted selection has proved to be an efficient tool for developing barley cultivars with acid soil/aluminium tolerance. In this study, barley variety Svanhals—introduced from CYMMIT (International Maize and Wheat Improvement Center)—was identified as acid soil/aluminium tolerant and the tolerance was mapped to chromosome 4H in 119 doubled haploid (DH) lines from a cross of Hamelin/Svanhals. The HvMATE gene, encoding an aluminium-activated citrate transporter, was selected as a candidate gene and gene-specific molecular markers were developed to detect acid soil/aluminium tolerance based on the polymerase chain reaction. Sequence analysis of the HvMATE gene identified a 21-bp indel (insertion–deletion) between the tolerant and sensitive cultivars. The new marker was further mapped to the QTL (quantitative trait loci) region on chromosome 4H for acid soil tolerance and accounted for 66.9 % of phenotypic variation in the DH population. Furthermore, the polymorphism was confirmed in other tolerant varieties which have been widely used as a source of acid soil tolerance in Australian barley breeding programs. The new gene-specific molecular marker provides an effective and simple molecular tool for selecting the acid soil tolerance gene from multiple tolerance sources.     

Additive effects of Na+ and Cl- ions on barley growth under salinity stress

Soil salinity affects large areas of the world's cultivated land, causing significant reductions in crop yield. Despite the fact that most plants accumulate both sodium (Na(+)) and chloride (Cl(-)) ions in high concentrations in their shoot tissues when grown in saline soils, most research on salt tolerance in annual plants has focused on the toxic effects of Na(+) accumulation. It has previously been suggested that Cl(-) toxicity may also be an important cause of growth reduction in barley plants. Here, the extent to which specific ion toxicities of Na(+) and Cl(-) reduce the growth of barley grown in saline soils is shown under varying salinity treatments using four barley genotypes differing in their salt tolerance in solution and soil-based systems. High Na(+), Cl(-), and NaCl separately reduced the growth of barley, however, the reductions in growth and photosynthesis were greatest under NaCl stress and were mainly additive of the effects of Na(+) and Cl(-) stress. The results demonstrated that Na(+) and Cl(-) exclusion among barley genotypes are independent mechanisms and different genotypes expressed different combinations of the two mechanisms. High concentrations of Na(+) reduced K(+) and Ca(2+) uptake and reduced photosynthesis mainly by reducing stomatal conductance. By comparison, high Cl(-) concentration reduced photosynthetic capacity due to non-stomatal effects: there was chlorophyll degradation, and a reduction in the actual quantum yield of PSII electron transport which was associated with both photochemical quenching and the efficiency of excitation energy capture. The results also showed that there are fundamental differences in salinity responses between soil and solution culture, and that the importance of the different mechanisms of salt damage varies according to the system under which the plants were grown.     

Expression of the Arabidopsis vacuolar H+‐pyrophosphatase gene (AVP1) improves the shoot biomass of transgenic barley and increases grain yield in a saline field

Cereal varieties with improved salinity tolerance are needed to achieve profitable grain yields in saline soils. The expression of AVP1, an Arabidopsis gene encoding a vacuolar proton pumping pyrophosphatase (H⁺‐PPase), has been shown to improve the salinity tolerance of transgenic plants in greenhouse conditions. However, the potential for this gene to improve the grain yield of cereal crops in a saline field has yet to be evaluated. Recent advances in high‐throughput nondestructive phenotyping technologies also offer an opportunity to quantitatively evaluate the growth of transgenic plants under abiotic stress through time. In this study, the growth of transgenic barley expressing AVP1 was evaluated under saline conditions in a pot experiment using nondestructive plant imaging and in a saline field trial. Greenhouse‐grown transgenic barley expressing AVP1 produced a larger shoot biomass compared to null segregants, as determined by an increase in projected shoot area, when grown in soil with 150 mm NaCl. This increase in shoot biomass of transgenic AVP1 barley occurred from an early growth stage and also in nonsaline conditions. In a saline field, the transgenic barley expressing AVP1 also showed an increase in shoot biomass and, importantly, produced a greater grain yield per plant compared to wild‐type plants. Interestingly, the expression of AVP1 did not alter barley leaf sodium concentrations in either greenhouse‐ or field‐grown plants. This study validates our greenhouse‐based experiments and indicates that transgenic barley expressing AVP1 is a promising option for increasing cereal crop productivity in saline fields.     

Genotypic variation in response of barley to boron deficiency

Responses of a range of barley (Hordeum vulgare L.) genotypes to boron (B) deficiency were studied in two experiments carried out in sand culture and in the field at Chiang Mai, Thailand. In experiment 1, two barley genotypes, Stirling (two-row) and BRB 2 (six-row) and one wheat (Triticum aestivum L.) genotype, SW 41, were evaluated in sand culture with three levels of applied B (0, 0.1 and 1.0 μM B) to the nutrient solution. It was found that B deficiency depressed flag leaf B concentration at booting, grain number and grain yield of all genotypes. In barley Stirling, B deficiency also depressed number of spikes plant^-1, spikelets spike^-1 and straw yield. However, no significant difference between genotypes in flag leaf B concentration was found under low B treatments. Flag leaf B concentration below 4 mg kg^-1 was associated with grain set reduction and could, therefore, be used as a general indicator for B status in barley. In experiment 2, nine barley and two wheat genotypes were evaluated in the field on a low B soil with three levels of B. Boron levels were varied by applying either 2 t of lime ha^-1 (BL), no B (B0) or 10 kg Borax ha^-1 (B+) to the soil prior to sowing. Genotypes differed in their B response for grain spike^-1, grain spikelet^-1 and grain set index (GSI). The GSI of the B efficient wheat, Fang 60, exceeded 90% in all B treatments. The B inefficient wheat SW 41 and most of the barley genotypes set grain normally (GSI >80%) only at the B+. In B0 GSI of the barley genotypes ranged from 23% to 84%, and in BL from 19% to 65%. Three of the barley with severely depressed GSI in B0 and BL also had a decreased number of spikelets spike^-1. In experiment 3, 21 advanced barley lines from the Barley Thailand Yield Nursery 1997/98 (BTYN 1997/98) were screened for B response in sand culture with no added B. Grain Set Index of the Fang 60 and SW 41 checks were 98 and 65%, respectively, and GSI of barley lines ranged between 5 and 90%. One advanced line was identified as B efficient and two as moderately B efficient. The remaining lines ranked between moderately inefficient to inefficient. These experiments have established that there is a range of responses to B in barley genotypes. This variation in the B response was observed in vegetative as well as reproductive growth. Boron efficiency should be considered in breeding and selection of barley in low B soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Phosphorus efficiencies and responses of barley (<Emphasis Type="Italic">Hordeum vulgare</Emphasis> L.) to arbuscular mycorrhizal fungi grown in highly calcareous soil

Two experiments were carried out to investigate phosphorus efficiencies and mycorrhizal responsiveness in an improved cultivar (Clipper) and a landrace (Sahara) of barley (Hordeum vulgare L.). In experiment 1, two pot sizes were used to evaluate the effect of soil volume on P uptake and mycorrhizal responsiveness. In experiment 2, a compartmented ("cross-pot") system was used to monitor (32)P delivery by external hyphae of arbuscular mycorrhizal fungi (AMF) to the host plant. Results showed that, irrespective of growth conditions, Sahara had much larger root biomass than Clipper and consequently substantially more P was allocated to roots in Sahara than in Clipper. Specific root length in Clipper was much longer than in Sahara. Increase in soil volume enhanced percentage root length colonised by AMF, plant growth and P uptake, and Sahara was more sensitive to changes in soil volume than Clipper. Pot size (soil volume) used to assess responsiveness to AMF by different plant species or genotypes with different root/shoot ratios might be a confounding factor. Clipper was more responsive to AMF than Sahara in terms of tissue P concentrations, which is partly related to their differences in root/shoot ratios. However, increases in SPU [specific P uptake, mg P (g root biomass)(-1)] caused by AMF were bigger in Clipper, suggesting that AMF played a larger role in P uptake. In accordance with the larger increase in SPU, Clipper took up more (32)P via AMF hyphae than Sahara. The compartmented system using radioactive P might be an alternative approach to directly investigate mycorrhizal responsiveness of different plant species or varieties than conventional pot experiments, provided that the same AM fungus is used.     

Interactions Between Nitrogen and Manganese Nutrition of Barley Genotypes Differing in Manganese Efficiency

Ammonium-fed plants may acidify the rhizosphere and thus increaseavailability of Mn in calcareous alkaline soils. The importanceof N nutrition in the differential expression of tolerance toMn deficiency among cereal genotypes is not yet clear. Two factorialexperiments testing effects of the NH₄-N/NO₃-N ratio and Mnfertilization on growth of barley genotypes differing in toleranceto Mn deficiency were conducted in two calcareous alkaline soilsin pots in a controlled environment. In the soil containing80% CaCO₃at pH 8.5, better root and shoot growth and highershoot Mn concentrations were achieved with nitrate supply, especiallyat lower rates of Mn fertilization. The Mn-efficient genotypeWeeah (tolerant of Mn deficiency) achieved better root and shootgrowth than Mn-inefficient Galleon barley (sensitive to Mn deficiency)regardless of experimental treatment. Fertilization with Mndid not influence total N concentration in barley roots andshoots. In the soil containing 5% CaCO₃at pH 7.8, ammonium-fedplants had better root and shoot growth and, at shoot Mn concentrationsabove the critical level, Mn-inefficient Galleon performed betterthan Mn-efficient Weeah barley. It appears that differentialexpression of Mn efficiency among barley genotypes is not associatedwith differences in Mn availability expected to be producedby differential rhizosphere acidification as a response to differentforms of N supply. There is an apparent preference of locallyselected barley genotypes for nitrate nutrition when grown onthe highly calcareous alkaline soils of southern Australia. Ammonium; calcareous soil; Hordeum vulgare ; manganese; nitrate; nitrogen form; nutrient efficiency; rhizosphere