Screening for boron tolerance in wheat (T. aestivum) by solution culture in filter paper

A new screening technique for tolerance to high concentrations of boron, namely a filter paper technique, and a soil experiment were compared to investigate the response of wheat genotypes known to differ in tolerance to high concentrations of boron.Under high boron concentrations in filter papers, the more tolerant genotypes had significantly longer roots than those of the more sensitive genotypes. There was no significant correlation between the root lengths at the control treatment and the other three boron treatments (50, 100, 150 mg B L^-1). Thus, the differences in root lengths at the high boron treatments could not be attributed to inherent differences in root growth but to the genetic variation in response to high boron concentrations among varieties.Root lengths at the three boron treatments in filter papers were highly significantly correlated with the three characters determined for plants grown in soil containing high levels of boron, namely the concentrations of boron in the shoots, plant dry weight and plant symptoms, indicating that root length could be used as a selection criterion in a genetic study or breeding program for boron tolerance.Department of Plant Science, Roseworthy Campus, University of Adelaide     

Physiological and genetic control of the tolerance of wheat to high concentrations of boron and implications for plant breeding

Physiological and genetic studies have been undertaken to further the understanding of genetic variation in response to high concentrations of B in the soil and so facilitate the breeding of tolerant varieties for cultivation in high B regions. Genetic variation in response to high concentrations of B has been identified for a number of crop and pasture species of southern Australia, including wheat, barley, oats, field peas and annual pasture medics. The wheat variety Halberd, which was the most widely grown variety in Australia during the 1970s and early 1980s, is the most tolerant of the current Australian wheat varieties. The mechanism of tolerance for all species studied is reduced accumulation of B by tolerant genotypes in both roots and shoots. Results from experiments of uptake kinetics indicate that control of B uptake is a non-metabolic process. The response of wheat to high B supply is under the control of several major additive genes, one of which has been located to chromosome 4A.     

Genetic aspects of aluminum tolerance in sorghum

To obtain crops tolerant to aluminum (Al) toxicity in acid soils, several methods have been used to screen different plant species and genotypes to this production constraint. Little is known about the effect of the method on genetic analyses and breeding method suggested. Three genetic studies were conducted to examine evaluation method on inheritance and gene action of sorghum [Sorghum bicolor (L.) Moench] to Al toxicity as measured by seedling dry matter production.Results of acid soil and solution culture studies indicated that tolerance to Al toxicity was inherited as a dominant character. Narrow-sense heritability estimates in the greenhouse acid-soil study were low for shoot and root dry matter production. Six of the same hybrids tested in solution culture produced high additive-genetic variance and had narrow-sense heritability estimates of 72% for shoot and 65% for root DM yields. Griffing's diallel analysis showed that seven of nine restorer lines had substantially higher specific than general combining ability variances for both root and shoot dry matter yields.Inconsistencies between the acid soil and solution culture techniques showed that different genetic responses to the treatments were being measured. The solution culture study indicated to the plant breeder that genes conditioning Al tolerance could be incorporated into pure lines while the greenhouse acid-soil study would predict that this would not be possible. The diallel study of plants grown in solution culture showed that developing both Al-tolerant varieties and hybrids would be possible depending upon Al-tolerant germplasm used. Acid-soil field studies of actual genetic gain for Al to lerance are needed.     

Mapping and validation of chromosome regions conferring boron toxicity tolerance in wheat (Triticum aestivum)

Boron is an essential plant micro-nutrient which can be phytotoxic to plants if present in soils in high concentration. Boron toxicity has been recognised as an important problem limiting production in the low rainfall areas of southern Australia, West Asia and North Africa. Genetic variation for boron toxicity tolerance in wheat has been well-characterised. The efficiency of breeding for boron toxicity tolerance could be greatly enhanced by the development of molecular markers associated with QTLs for tolerance in wheat. A population of 161 doubled haploids from a cross between the tolerant cultivar Halberd and the moderately sensitive cultivar Cranbrook was used to identify chromosomal regions involved in boron tolerance. A combined RFLP and AFLP linkage map of the Cranbrook x Halberd population was used to identify chromosomal regions involved in the boron tolerance traits measured. Regions on chromosome 7B and 7D were associated with leaf symptom expression. The region on chromosome 7B was also associated with the control of boron uptake and with a reduction in the effect of boron toxicity on root-growth suppression. RFLP markers at the chromosome 7B and 7D loci were shown to be effective in selecting for improved boron tolerance in an alternative genetic background. Halberd alleles at the chromosome 7B locus were associated with the concentration of boron in whole shoots and grain. The concentration of boron in whole shoots and in grain were both related to grain yield in a field trial conducted on soil containing toxic levels of boron. Implications relating to marker-assisted selection for boron toxicity tolerance in wheat are discussed. Received: 3 September 1999 / Accepted: 12 February 2000     

The combined treatment of Mn and Al alleviates the toxicity of Al or Mn stress alone in barley

Aluminum (Al) and manganese (Mn) toxicity commonly coexists in acid soil, so the crop cultivars suitable for planting in acid soil should show high tolerance to both elements simultaneously. However, it is still not clear if the toxicity of Mn and Al on plant growth is antagonistic or synergistic, and the plants with Al tolerance are also tolerant to Mn toxicity. In this study, three barley genotypes (one Tibetan wild and two cultivated), differing in Al tolerance, were characterized for growth and physiological responses to Al or Mn toxicity as well as the combined treatment of the two toxic elements. Interestingly, it has been found that the combined treatment of both metals was less affected in comparison with Al or Mn treatment alone, in terms of plant growth, Al or Mn concentration in plant tissues, and photosynthetic parameters, indicating antagonistic interaction of Al and Mn for their effect on plant growth and physiological traits. The results also showed that there was a dramatic difference among barley genotypes in Mn toxicity tolerance and XZ16 showed much higher tolerance than other two genotypes. High Mn tolerance is mainly described to less Mn uptake and lower Mn concentration in plants, and Mn tolerance is independent of Al tolerance.     

Screening rough-seeded lupins (Lupinus pilosus Murr. and Lupinus atlanticus Glads.) for tolerance to calcareous soils

Soil- and solution-based screening methods were used to identify interspecific and intraspecific variation in lupins for tolerance to calcareous soils. Plants were grown for 21 days in a calcareous soil (pH 8.2; 50% CaCO₃; moisture content 90% of field capacity) for soil-based screening and in nutrient solution containing 15 mM KHCO₃ for solution-based screening. Chlorosis as an indicator of tolerance was recorded. Lupinus pilosus Murr. had the most tolerant genotypes and had the greatest range of intraspecific variation. Most genotypes of Lupinus atlanticus Glads. and Lupinus angustifolius L. were moderately intolerant, although two genotypes of L. atlanticus appeared to be tolerant. Lupinus albus L. had moderately tolerant to moderately intolerant genotypes, whilst the single genotypes of Lupinus cosentinii Guss. and Lupinus digitatus Forsk. appeared tolerant. In a field study six genotypes of L. pilosus identified in the soil-based screening as differing in their tolerance to the calcareous soil were grown on comparable calcareous (pH 8.3; topsoil 3% CaCO₃, subsoil 13% CaCO₃) and non-calcareous (pH 7.3) soils within a paddock. Chlorosis and nutrient concentrations in the youngest leaves were measured 53 days after sowing, whilst grain yield was estimated at harvest. Despite the soil containing a much lower CaCO₃ content than used in the screening method, the field study confirmed that moderately intolerant to intolerant genotypes had lower relative grain yields than more tolerant genotypes. Chlorosis rankings of the genotypes were correlated between field and the screening studies. It is suggested that the incorporation of genes conferring tolerance to calcareous soils into high yielding, agronomically suitable genotypes of L. pilosus should be an important objective in a lupin breeding program for calcareous soils.     

Polymorphism and physiology of arsenate tolerance in Holcus lanatus L. from an uncontaminated site

The polymorphism of arsenate tolerance in a Holcus lanatus L. population from an uncontaminated soil was investigated and a high percentage of tolerant individuals (65%) was found in the population studied. Influx of arsenate was highly correlated to arsenate tolerance within the population, with the most tolerant individuals having the lowest rates of arsenate influx. Isotherms for the high affinity arsenate uptake systems were determined in six tolerant and six non-tolerant genotypes. Tolerant plants had the lowest rates of arsenate influx. This was achieved by adaptation of the V_max of arsenate influx with the V_max of the high affinity uptake system saturating at lower substrate concentrations in the tolerant plants. The polymorphism is discussed with relation to adaptation to the extreme environments to which the plants are subjected on mine-spoil soils.     

Evaluating Agronomic Performance and Investigating Molecular Structure of Drought and Heat Tolerant Wild Alfalfa (<Emphasis Type="Italic">Medicago sativa</Emphasis> L.) Collection from the Southeastern Turkey

Drought is a major stress factor for agricultural production including alfalfa production. One way to counterbalance the yield losses is the introgression of drought tolerant germplasm into breeding programs. As an effort to exploit such germplasm, 16 individual plants were selected from the Southeastern Turkey from their natural habitat and clonally propagated in field trials with an ultimate goal to use the germplasm as parents for releasing a synthetic cultivar. Forage yield and forage quality traits were evaluated and molecular genetic diversity among genotypes were determined using inter simple sequence repeat markers. Genotypes showed a variation from growth habit to yield and quality traits indicating sufficient phenotypic variation for diverse breeding efforts (for grazing or harvesting) and long term selection schemes. A large amount of genetic variation was observed even with a limited number of marker and genotypes. However, no pattern of spatial genetic structure was observed for the scale of the study when genetic variation is linked to the geographic origin. We conclude that ex situ natural variation provides a wealth of germplasm that could be incorporated into breeding programs aiming to improve drought tolerance. We also suggest an extensive collection of seeds/plant tissue from unique plants with desirable traits rather than putting more efforts to create a spatial germplasm sampling efforts in narrow regions.     

Mapping genetic loci for tolerance to lime-induced iron deficiency chlorosis in grapevine rootstocks (Vitis sp.)

Iron is essential to plants for chlorophyll formation as well as for the functioning of various iron-containing enzymes. Iron deficiency chlorosis is a wide-spread disorder of plants, in particular, of those growing on calcareous soils. Among the different ways to control iron deficiency problems for crops, plant material and especially rootstock breeding is a suitable and reliable method, especially for fruit trees and grapes. The aim of the experiment was to characterize the genetic basis of grapevine chlorosis tolerance under lime stress conditions. A segregating population of 138 F1 genotypes issued from an inter-specific cross between Vitis vinifera Cabernet Sauvignon (tolerant) × V. riparia Gloire de Montpellier (sensitive) was developed and phenotyped both as cuttings and as rootstock grafted with Cabernet Sauvignon scions in pots containing non-chlorosing and chlorosing soils. Tolerance was evaluated by chlorosis score, leaf chlorophyll content and growth parameters of the shoots and roots. The experiments were performed in 2001, 2003 and 2006. The plants analysed in 2006 were reassessed in 2007. The most significant findings of the trial were: (a) the soil properties strongly affect plant development, (b) there are differences in tolerance among segregating genotypes when grown as cuttings or as rootstocks on calcareous soil, (c) calcareous conditions induced chlorosis and revealed quantitative trait loci (QTLs) implicated in polygenic control of tolerance, (d) rootstock strongly contributes to lime-induced chlorosis response, and (e) a QTL with strong effect (from 10 to 25 % of the chlorotic symptom variance) was identified on chromosome 13. This QTL colocalized with a QTL for chlorophyll content (R ² = 22 %) and a major QTL for plant development that explains about 50 % of both aerial and root system biomass variation. These findings were supported by stable results among the different years of experiment. These results open new insights into the genetic control of chlorosis tolerance and could aid the development of iron chlorosis-tolerant rootstocks.     

Genotypic variation in plant response to low boron and implications for plant breeding

Plant response to low B in the soil varies widely among species, and among genotypes within a species. Boron efficient genotypes are those that are able to grow well in soils in which other genotypes are adversely affected by B deficiency. This review considers the extent of variation in B efficiency in plant species and genotypes, the physiological nature of the efficiency mechanisms, what is known of the genetic basis for inheritance, screening techniques and the practical implications of the genotypic variations.Frequently, B efficiency is the sole reason for a difference between an average yield and complete crop failure. Severe yield losses can be effectively prevented by the inclusion of B efficiency as a selection criterion in crop breeding and improvement programmes for regions with low B soils. In addition, the expression of B deficiency primarily through male sterility, which is common in many species, creates opportunities for outcrossing in normally self-fertilised species. This, in turn, leads to two possibilities. Firstly, self fertilisation, and therefore maintenance of pure lines, cannot always be assumed in self pollinated species where B efficient and inefficient genotypes are grown side by side on low B soils. Secondly, B deficiency, in soil or artificial media, may be used as a fertility selective medium in which the male sterile B inefficient genotypes and the male fertile B efficient genotypes could hybridise naturally. This would be useful as a simple and economical method for creating heterozygous populations in breeding programmes as well as for producing hybrid seeds. Now that the roles of B in plant growth and development are beginning to be clarified, the efficiency mechanisms as well as the governing genetics can be explained. Practical benefits from the genetic diversity of B efficiency will be enhanced by a better understanding of B efficiency mechanisms and the molecular bases for their genetic control.     

Aluminium tolerance of root hairs underlies genotypic differences in rhizosheath size of wheat (Triticum aestivum) grown on acid soil

We found significant genetic variation in the ability of wheat (Triticum aestivum) to form rhizosheaths on acid soil and assessed whether differences in aluminium (Al(3+) ) tolerance of root hairs between genotypes was the physiological basis for this genetic variation. A method was developed to rapidly screen rhizosheath size in a range of wheat genotypes. Backcrossed populations were generated from cv Fronteira (large rhizosheath) using cv EGA-Burke (small rhizosheath) as the recurrent parent. A positive correlation existed between rhizosheath size on acid soil and root hair length. In hydroponic experiments, root hairs of the backcrossed lines with large rhizosheaths were more tolerant of Al(3+) toxicity than the backcrossed lines with small rhizosheaths. We conclude that greater Al(3+) tolerance of root hairs underlies the larger rhizosheath of wheat grown on acid soil. Tolerance of the root hairs to Al(3+) was largely independent of the TaALMT1 gene which suggests that different genes encode the Al(3+) tolerance of root hairs. The maintenance of longer root hairs in acid soils is important for the efficient uptake of water and nutrients.     

Improving salinity tolerance in crop plants: a biotechnological view

Salinity limits the production capabilities of agricultural soils in large areas of the world. Both breeding and screening germplasm for salt tolerance encounter the following limitations: (a) different phenotypic responses of plants at different growth stages, (b) different physiological mechanisms, (c) complicated genotype × environment interactions, and (d) variability of the salt-affected field in its chemical and physical soil composition. Plant molecular and physiological traits provide the bases for efficient germplasm screening procedures through traditional breeding, molecular breeding, and transgenic approaches. However, the quantitative nature of salinity stress tolerance and the problems associated with developing appropriate and replicable testing environments make it difficult to distinguish salt-tolerant lines from sensitive lines. In order to develop more efficient screening procedures for germplasm evaluation and improvement of salt tolerance, implementation of a rapid and reliable screening procedure is essential. Field selection for salinity tolerance is a laborious task; therefore, plant breeders are seeking reliable ways to assess the salt tolerance of plant germplasm. Salt tolerance in several plant species may operate at the cellular level, and glycophytes are believed to have special cellular mechanisms for salt tolerance. Ion exclusion, ion sequestration, osmotic adjustment, macromolecule protection, and membrane transport system adaptation to saline environments are important strategies that may confer salt tolerance to plants. Cell and tissue culture techniques have been used to obtain salt tolerant plants employing two in vitro culture approaches. The first approach is selection of mutant cell lines from cultured cells and plant regeneration from such cells (somaclones). In vitro screening of plant germplasm for salt tolerance is the second approach, and a successful employment of this method in durum wheat is presented here. Doubled haploid lines derived from pollen culture of F₁ hybrids of salt-tolerant parents are promising tools to further improve salt tolerance of plant cultivars. Enhancement of resistance against both hyper-osmotic stress and ion toxicity may also be achieved via molecular breeding of salt-tolerant plants using either molecular markers or genetic engineering.     

Linking waterlogging tolerance with Mn2+ toxicity: a case study for barley

Vast agricultural areas are affected by flooding, causing up to 80% yield reduction and resulting in multibillion dollar losses. Up to now, the focus of plant breeders was predominantly on detrimental effects of anoxia, while other (potentially equally important) traits were essentially neglected; one of these is soil elemental toxicity. Excess water triggers a progressive decrease in soil redox potential, thus increasing the concentration of Mn²⁺ that can be toxic to plants if above a specific threshold. This work aimed to quantify the relative contribution of Mn²⁺ toxicity to waterlogging stress tolerance, using barley as a case study. Twenty barley (Hordeum vulgare) genotypes contrasting in waterlogging stress tolerance were studied for their ability to cope with toxic (1 mm ) amounts of Mn²⁺ in the root rhizosphere. Under Mn²⁺ toxicity, chlorophyll content of most waterlogging‐tolerant genotypes (TX9425, Yerong, CPI‐71284‐48 and CM72) remained above 60% of the control value, whereas sensitive genotypes (Franklin and Naso Nijo) had 35% less chlorophyll than 35% of controls. Manganese concentration in leaves was not related to visual Mn²⁺ toxicity symptoms, suggesting that various Mn²⁺ tolerance mechanisms might operate in different tolerant genotypes, i.e. avoidance versus tissue tolerance. The overall significant (r = 0.60) correlation between tolerance to Mn²⁺ toxicity and waterlogging in barley suggests that plant breeding for tolerance to waterlogging traits may be advanced by targeting mechanisms conferring tolerance to Mn²⁺ toxicity, at least in this species.     

Microsatellite marker based genetic diversity analysis among cotton (<Emphasis Type="Italic">Gossypium hirsutum</Emphasis>) accessions differing for their response to drought stress

Genetic diversity is an essential input for any plant breeding programme. To assess the genetic divergence among the newly identified drought tolerant lines and elite cotton genotypes including popular varieties, a total of 51 distinctly polymorphic markers were identified after screening 142 genome-wide SSR markers. The identified polymorphic markers detected a total of 140 alleles with a mean of 2.75 alleles per loci and average polymorphism information content of 0.45. Jaccard coefficient based dissimilarity index between the genotypes ranged from 0.18 to 0.82 indicating existence of wide variation between and within the drought tolerant and susceptible genotypes at the DNA level. Cluster and factorial analyses have provided the structure of genetic diversity present and clearly distinguished the drought tolerant and susceptible cotton genotypes. Clustering pattern was in congruence with the source or pedigree of genotypes. The information generated in the present study on genetic divergence among genotypes having differential response to drought will help in selection of suitable lines as parents for developing drought tolerant cultivars in cotton. The polymorphic markers and diverse lines identified in the study will be of immense utility in molecular mapping and marker assisted breeding to achieve drought tolerance in cotton.     

The development of potential screens based on shoot calcium and iron concentrations for the evaluation of tolerance in Egyptian genotypes of white lupin (Lupinus albus L.) to limed soils

European cultivars of white lupin (Lupinus albus L.) grow poorly in limed or calcareous soils. However, Egyptian genotypes are grown successfully in highly calcareous soil and show no stress symptoms. To examine their physiological responses to alkaline soil and develop potential screens for tolerance, three experiments were conducted in limed and non-limed (neutral pH) soil. Measurements included net CO2 uptake, and the partitioning of Fe2+ and Fe3+ and soluble and insoluble Ca in stem and leaf tissue. Intolerant plants showed clear symptoms of stress, whereas stress in the Egyptian genotypes and in L pilosus Murr. (a tolerant species) was less marked. Only the intolerant plants became chlorotic and this contributed to their reduced net CO2 uptake in the limed soil. In contrast, Egyptian genotypes and L pilosus showed no change in net CO2 uptake between the soils. The partitioning of Ca and Fe either resulted from the stress responses, or was itself a stress response. L pilosus and some Egyptian genotypes differed in soluble Ca concentrations compared with the intolerant cultivars, although no significant difference was apparent in the Ca partitioning of the Egyptian genotype Giza 1. In a limed soil, Giza 1 maintained its stem Fe3+ concentration at a level comparable with that of plants grown in non-limed soil, whereas stem [Fe3+] of an intolerant genotype increased. Gizal increased the percentage of plant Fe that was Fe2+ in its leaf tissue under these conditions; that of the intolerant genotype was reduced. The potential tolerance of the Egyptian genotypes through these mechanisms and the possibility of nutritional-based screens are discussed.     

Mixed planting with a leguminous plant outperforms bacteria in promoting growth of a metal remediating plant through histidine synthesis

The effectiveness of plant growth promoting bacteria (PGPB) in improving metal phytoremediation is still limited by stunted plant growth under high soil metal concentrations. Meanwhile, mixed planting with leguminous plants is known to improve yield in nutrient deficient soils but the use of a metal tolerant legume to enhance metal tolerance of a phytoremediator has not been explored. We compared the use of Pseudomonas brassicacearum, Rhizobium leguminosarum, and the metal tolerant leguminous plant Vicia sativa to promote the growth of Brassica juncea in soil contaminated with 400 mg Zn kg^–1, and used synchrotron based microfocus X-ray absorption spectroscopy to probe Zn speciation in plant roots. B. juncea grew better when planted with V. sativa than when inoculated with PGPB. By combining PGPB with mixed planting, B. juncea recovered full growth while also achieving soil remediation efficiency of >75%, the maximum ever demonstrated for B. juncea. μXANES analysis of V. sativa suggested possible root exudation of the Zn chelates histidine and cysteine were responsible for reducing Zn toxicity. We propose the exploration of a legume-assisted-phytoremediation system as a more effective alternative to PGPB for Zn bioremediation.     

Selection of Brassica rapa genotypes for tolerance to boron toxicity

Although boron (B) is a micronutrient essential for the growth of vascular plants, it reduces growth and seed yield when present in excessive amounts. A hydroponic assay of nineteen Brassica rapa genotypes resulted in the identification of two tolerant genotypes, WWY Sarson and Local at a range of boron concentrations (15–165 μM). The most tolerant and sensitive genotypes were assessed for shoot boron concentrations in a soil assay with 4, 29 and 54 mg B kg⁻¹ soil. The soil assay confirmed the results of the hydroponic screening. Shoot boron uptake was at least three times lower and shoot boron concentrations about 10 times lower in the tolerant than sensitive genotypes, indicating that boron tolerance involved boron exclusion from the shoot.     

Tolerance to ion toxicities enhances wheat (Triticum aestivum L.) grain yield in waterlogged acidic soils

Aims

The high concentrations of Mn, Fe and Al in acid soils during waterlogging impair root and shoot growth more severely in intolerant than tolerant wheat genotypes. This study aims to establish whether this difference in vegetative growth and survival during waterlogging (1) is verifiable across a range of tolerant/intolerant genotypes and acid soils, and (2) results in improved recovery after cessation of waterlogging and enhanced grain yield.

Methods

Wheat genotypes contrasting in their tolerance to ion toxicities were grown in four acid soils until 63DAS and maturity, with a 42-day waterlogging treatment imposed at 21 DAS.

Results

The shoot Al, Mn and Fe concentrations increased by up to 5-, 3- and 9-fold respectively due to waterlogging in various soils. Compared to the intolerant lines, Al-, Mn- and Fe-tolerant genotypes maintained a relatively lower increase in shoot concentrations of Al (79 vs. 117%), Mn (90 vs. 101%) and Fe (171 vs. 252%) and demonstrated better waterlogging tolerance at the vegetative stage expressed in relative root (38% vs. 25%) and shoot (62% vs. 52%) growth. After cessation of waterlogging and the continued growth to maturity, tolerant genotypes maintained a relatively lower plant concentration of Al, Mn and Fe, but produced a higher above-ground biomass (74% vs. 56%) and most importantly demonstrated improved waterlogging tolerance (a relative grain yield of 78% vs. 54%) compared to intolerant genotypes. Maturity following waterlogging stress was delayed less in tolerant than intolerant genotypes (114 vs. 124%, respectively), which would reduce the potential yield loss where post-anthesis coincides with drought.

Conclusions

The results confirm the validity of a novel approach of enhancing waterlogging tolerance of wheat genotypes grown in acid soil via increased tolerance to ion toxicities.     

Soil CO2 venting as one of the mechanisms for tolerance of Zn deficiency by rice in flooded soils

We sought to explain rice (Oryza sativa) genotype differences in tolerance of zinc (Zn) deficiency in flooded paddy soils and the counter‐intuitive observation, made in earlier field experiments, that Zn uptake per plant increases with increasing planting density. We grew tolerant and intolerant genotypes in a Zn‐deficient flooded soil at high and low planting densities and found (a) plant Zn concentrations and growth increased with planting density and more so in the tolerant genotype, whereas the concentrations of other nutrients decreased, indicating a specific effect on Zn uptake; (b) the effects of planting density and genotype on Zn uptake could only be explained if the plants induced changes in the soil to make Zn more soluble; and (c) the genotype and planting density effects were both associated with decreases in dissolved CO₂ in the rhizosphere soil solution and resulting increases in pH. We suggest that the increases in pH caused solubilization of soil Zn by dissolution of alkali‐soluble, Zn‐complexing organic ligands from soil organic matter. We conclude that differences in venting of soil CO₂ through root aerenchyma were responsible for the genotype and planting density effects.     

Soil aluminium effects on uptake,influx, and transport of nutrients in sorghum genotypes

Sorghum [Sorghum bicolor (L.) Moench] is the fifth most important cereal crop of the world. In South America, it is grown mainly on acid soils, and its production on these soils is limited by deficient levels of available P, Ca, Mg, and micronutrients, and toxic levels of Al and Mn. A greenhouse experiment was undertaken to evaluate the genotypic differences in sorghum for uptake (U), inhibition (IH), influx (IN) into roots, and transport (TR) to shoot for nutrients at three levels of soil Al saturation (2, 41, 64%). Overall shoot nutrient U, IN, and TR showed a significant inverse correlation with soil Al saturation and shoot Al concentration, and a significant positive correlation with shoot and root dry weight. The nutrient uptake parameters differentiated genotypes into most and least efficient categories at various levels of soil Al saturation. The nutrient uptake parameters showed significant differences with respect to soil Al saturation, genotypes, and their interactions. In the current study, Al tolerant genotypes recorded higher IN and TR for P, K, Ca, Mg, Zn, and Fe than Al-sensitive genotypes. Therefore, these U, IN, and TR traits could be used in selection of sorghum plants adaptable to acid soils. Sorghum genotypes used in this study showed intraspecific genetic diversity in U, IN, and TR for essential nutrients. It was concluded that selection of acid soil tolerant genotypes and further breeding of acid (Al) tolerant sorghum cultivars are feasible.IICA/EMBRAPA/World BankIICA/EMBRAPA/World BankIICA/EMBRAPA/World Bank