Subtropical Maize Research Articles

Unraveling the genetic architecture of subtropical maize (Zea mays L.) lines to assess their utility in breeding programs

BackgroundMaize is an increasingly important food crop in southeast Asia. The elucidation of its genetic architecture, accomplished by exploring quantitative trait loci and useful alleles in various lines across numerous breeding programs, is therefore of great interest. The present study aimed to characterize subtropical maize lines using high-quality SNPs distributed throughout the genome.ResultsWe genotyped a panel of 240 subtropical elite maize inbred lines and carried out linkage disequilibrium, genetic diversity, population structure, and principal component analyses on the generated SNP data. The mean SNP distance across the genome was 70 Kb. The genome had both high and low linkage disequilibrium (LD) regions; the latter were dominant in areas near the gene-rich telomeric portions where recombination is frequent. A total of 252 haplotype blocks, ranging in size from 1 to 15.8 Mb, were identified. Slow LD decay (200–300 Kb) at r 2 ≤ 0.1 across all chromosomes explained the selection of favorable traits around low LD regions in different breeding programs. The association mapping panel was characterized by strong population substructure. Genotypes were grouped into three distinct clusters with a mean genetic dissimilarity coefficient of 0.36.ConclusionsThe genotyped panel of subtropical maize lines characterized in this study should be useful for association mapping of agronomically important genes. The dissimilarity uncovered among genotypes provides an opportunity to exploit the heterotic potential of subtropical elite maize breeding lines.

Response of recurrent selection on yield, kernel oil content and fatty acid composition of subtropical maize populations

High oil maize (HOM) has been successfully achieved by a long-term breeding program. The aim of this study was to evaluate eight cycles of recurrent selection for agronomic traits and HOM content in four subtropical white and yellow maize populations and the effect of selection on yield, kernel physical properties, oil content, fatty acid (FA) profile and lipophilic antioxidant (LAOX) capacity. Through the selection cycles the agronomic traits showed a low but significant increase of grain yield and reduction in diseases. These positive changes significantly affected kernel physical properties and distribution of ground particles related to grain hardness. Oil content in kernels increased through cycles of selection and had a positive correlation (r<0.001) with the germ size. The improved kernels belonging to the last cycles contained from 33 to 60% more oil compared to original counterparts. Oleic acid (OLA) and linoleic acid (LOA) accounted for approximately 79–85% of total FA and the relative proportions of these FA slightly decreased during selection. The percentage of Ω-3 linolenic acid increased in the oil belonging to the advanced HOM populations whereas LAOX significantly decreased with cycles of selection. There was not a relationship between the observed oil increase with grain yield; however, the monounsaturated oleic proportion tended to decrease in kernels belonging to the yellow populations. Fatty acids and LAOX capacity had a positive correlation with OLA and negative (r>0.65) with LOA. This research demonstrated that it was feasible to develop high-yielding subtropical populations with HOM content and acceptable agronomic performance and grain physical properties.

Dissecting Maize Productivity: Ideotypes Associated with Grain Yield under Drought Stress and Well‐watered Conditions

To increase maize (Zea mays L.) yields in drought-prone environments and offset predicted maize yield losses under future climates, the development of improved breeding pipelines using a multi-disciplinary approach is essential. Elucidating key growth processes will provide opportunities to improve drought breeding progress through the identification of key phenotypic traits, ideotypes, and donors. In this study, we tested a large set of tropical and subtropical maize inbreds and single cross hybrids under reproductive stage drought stress and well-watered conditions. Patterns of biomass production, senescence, and plant water status were measured throughout the crop cycle. Under drought stress, early biomass production prior to anthesis was important for inbred yield, while delayed senescence was important for hybrid yield. Under well-watered conditions, the ability to maintain a high biomass throughout the growing cycle was crucial for inbred yield, while a stay-green pattern was important for hybrid yield. While new quantitative phenotyping tools such as spectral reflectance (Normalized Difference Vegetation Index, NDVI) allowed for the characterization of growth and senescence patterns as well as yield, qualitative measurements of canopy senescence were also found to be associated with grain yield.

Combining ability and GGE biplot analyses for resistance to northern leaf blight in tropical and subtropical elite maize inbred lines

The incidence and severity of northern leaf blight (NLB) disease has increased in Southern Africa in the past years with previously resistant cultivars being affected; implying more resistant sources have to be identified and inheritance of NLB resistance investigated. Therefore, 45 F1 hybrids generated in a half diallel mating of ten elite maize inbred lines were evaluated in six environments for combining ability, genotype × environment interaction and effect of NLB disease on grain yield. General and specific combining ability (GCA and SCA) were highly significant (P < 0.001) for both NLB disease and grain yield. The GCA/SCA ratio was close to unity for both NLB (0.96) and grain yield (0.89), indicating predominance of additive over non-additive gene action for the traits in these inbred lines. Parent P2 and P7 had good GCA for both NLB disease resistance and high grain yield. The NLB disease ratings on the maize hybrids and inbreds ranged from 1.0 to 8.5 (approximately 0–75 % severity). Negative slope coefficients of the linear regression indicated maize yield decrease of 280–610 kg ha−1 for NLB final disease severity of about 25–75 %, respectively, stressing the need for resistant cultivars to manage the disease. Genotype and (genotype × environment) (GGE) biplots indicated absence of crossover interaction and revealed positive associations among environments, signifying the suitability of all the environments for disease screening. Therefore, the significant additive effects for NLB disease and grain yield entail that breeding progress would be made through selection and a few disease ‘hot-spot’ sites, such as Cedara (South Africa) and Mpongwe (Zambia) can be used for disease screening.

Infrared Thermal Imaging as a Rapid Tool for Identifying Water‐Stress Tolerant Maize Genotypes of Different Phenology

AbstractThe main task of this research was to evaluate canopy temperature and Crop Water Stress Index (CWSI) by assessing genotype variability of maize performance for different water regimes. To that end, three hundred tropical and subtropical maize hybrids with different phenology in terms of date of anthesis were evaluated. The influence of phenology on the change in canopy temperatures and CWSI was not equal during the three dates of measurement. At the end of vegetative growth (82 days after sowing, DAS) and at the blister stage (DAS 97), a high significant difference in temperatures and CWSI (P &lt; 0.001) were obtained between the early‐ and late‐maturity genotypes. During anthesis (DAS 89), phenology had a significant effect (P &lt; 0.01) only for the well‐watered genotypes, while under water‐stress conditions, no differences were found between early and late genotypes in terms of canopy temperature and CWSI. High significant differences (P &lt; 0.001) in stomatal conductance (gs) between early and late genotypes for different treatments were observed. A relationship (R2 = 0.62) between gs and canopy temperature was obtained. Under a water‐stress canopy, temperature was measured at anthesis, which was negatively correlated with grain yield of the early (r = −0.55)‐ and late (r = −0.46)‐maturity genotypes in the water‐stressed condition.

Genetic analysis of maturity and flowering characteristics in maize (Zea mays L.)

ObjectiveTo elucidate the pattern of inheritance and determine the relative magnitude of various genetic effects for maturity and flowering attributes in subtropical maize. MethodsFour white grain maize inbred lines from flint group of corn, two with late maturity and two with early maturity, were used. These contrasting inbred lines were crossed to form four crosses. Six generations (P1, P2, F1, F2, BC1, and BC2) were developed for each individual cross. These were evaluated in triplicate trial for two consecutive years. ResultsBoth dominance gene action and epistatic interaction played major role in governing inheritance of days to pollen shedding, 50% silking, anthesis silking interval and maturity. ConclusionsPreponderance of dominance gene action for these traits indicated their usefulness in hybrid programs of subtropical maize.

Genetic characterization of a core set of a tropical maize race Tuxpeño for further use in maize improvement.

The tropical maize race Tuxpeño is a well-known race of Mexican dent germplasm which has greatly contributed to the development of tropical and subtropical maize gene pools. In order to investigate how it could be exploited in future maize improvement, a panel of maize germplasm accessions was assembled and characterized using genome-wide Single Nucleotide Polymorphism (SNP) markers. This panel included 321 core accessions of Tuxpeño race from the International Maize and Wheat Improvement Center (CIMMYT) germplasm bank collection, 94 CIMMYT maize lines (CMLs) and 54 U.S. Germplasm Enhancement of Maize (GEM) lines. The panel also included other diverse sources of reference germplasm: 14 U.S. maize landrace accessions, 4 temperate inbred lines from the U.S. and China, and 11 CIMMYT populations (a total of 498 entries with 795 plants). Clustering analyses (CA) based on Modified Rogers Distance (MRD) clearly partitioned all 498 entries into their corresponding groups. No sub clusters were observed within the Tuxpeño core set. Various breeding strategies for using the Tuxpeño core set, based on grouping of the studied germplasm and genetic distance among them, were discussed. In order to facilitate sampling diversity within the Tuxpeño core, a minicore subset of 64 Tuxpeño accessions (20% of its usual size) representing the diversity of the core set was developed, using an approach combining phenotypic and molecular data. Untapped diversity represents further use of the Tuxpeño landrace for maize improvement through the core and/or minicore subset available to the maize community.

Registration of Maize Germplasm Lines Tx736, Tx739, and Tx740 for Reducing Preharvest Aflatoxin Accumulation

Maize (Zea mays L.) production in Texas, the southern United States, and much of the developing world is constrained by preharvest contamination from aflatoxins—potent mycotoxins produced by the fungus Aspergillus flavus. When consumed, aflatoxin can lead to impaired growth, liver cancer, or death of both humans and livestock. Because of these effects, the presence of this mycotoxin is tightly regulated in the U.S. food supply. No complete resistance to A. flavus or aflatoxin is known to exist in maize; however, multiple quantitative traits and some sources of resistance have been identified. We propose the release of three maize lines that demonstrate reduced preharvest aflatoxin accumulation. Tx736 (Reg. No. GP‐578, PI 662937) is a germplasm line derived from modified pedigree selection of a cross between Tx772, a line with reduced aflatoxin accumulation, and T246, a temperate line with good yield and Texas adaptation, backcrossed to Tx772 once. Lines Tx739 (Reg. No. GP‐579, PI 662938) and Tx740 (Reg. No. GP‐580, PI 662939) were selected by pedigree methods from S3 plants selfed out of heterotic groups A, C, and E from Agricomseeds (Santa Cruz, Bolivia). These three lines were field tested as lines per se and as testcross hybrids with introduced inoculation in multienvironmental field trials. In trials, these lines and hybrids had between 30 and 73% lower aflatoxin content than commercial checks. These germplasm lines will serve as unique sources for novel traits and alleles that reduce aflatoxin in elite temperate and subtropical maize.

Utilization Potential of the Temperate Maize Inbreds Integrated with Tropical Germplasm

A diallel mating experiment was conducted using ten maize lines, which consisted of five tropical, subtropical maize inbreds and five temperate maize inbreds developed by consecutive backcross and introducing tropical germplasm. Totally, ninety crosses were made based on Griffing method III. The objectives of this study were to evaluate: (1) combining ability of the ten lines; (2) heterosis and heterotic performance of these crosses between the temperate lines integrated with tropical germplasm and the same tropical lines. Ninety crosses were evaluated in Kunming, Yunnan province in 2008 and 2009. One local elite hybrid Yunrui 6 was used in the trials as a check. The results showed that the inbreds integrated with tropical germplasm YML598 and YML58 had highly significant GCA effects in most traits, which should have high use potential in maize hybrid breeding programs. And it was found that combining ability for grain yield was closely related to combining ability for most of yield component factors, yield per plant had positive correlation with most of traits except anthesis silking interval and round tip. And it was showed that most of hybrids with relative heterosis H ≥10% and highly significant SCA effects for yield per plant were the crosses between tropical subtropical lines and temperate lines, which suggested that the five tropical sub-tropical inbreds and five temperate inbreds still belonged to their previous heterotic group respectively, and their heterosis did not be changed. Thus, using tropical subtropical lines as donor, backcrossing consecutively with temperate lines may be an effective way to improve the latter.

Comparative LD mapping using single SNPs and haplotypes identifies QTL for plant height and biomass as secondary traits of drought tolerance in maize

Drought often delays developmental events so that plant height and above-ground biomass are reduced, resulting in yield loss due to inadequate photosynthate. In this study, plant height and biomass measured by the Normalized Difference Vegetation Index (NDVI) were used as criteria for drought tolerance. A total of 305 lines representing temperate, tropical and subtropical maize germplasm were genotyped using two single nucleotide polymorphism (SNP) chips each containing 1536 markers, from which 2052 informative SNPs and 386 haplotypes each constructed with two or more SNPs were used for linkage disequilibrium (LD) or association mapping. Single SNP- and haplotype-based LD mapping identified two significant SNPs and three haplotype loci [a total of four quantitative trait loci (QTL)] for plant height under well-watered and water-stressed conditions. For biomass, 32 SNPs and 12 haplotype loci (30 QTL) were identified using NDVIs measured at seven stages under the two water regimes. Some significant SNP and haplotype loci for NDVI were shared by different stages. Comparing significant loci identified by single SNP- and haplotype-based LD mapping, we found that six out of the 14 chromosomal regions defined by haplotype loci each included at least one significant SNP for the same trait. Significant SNP haplotype loci explained much higher phenotypic variation than individual SNPs. Moreover, we found that two significant SNPs (two QTL) and one haplotype locus were shared by plant height and NDVI. The results indicate the power of comparative LD mapping using single SNPs and SNP haplotypes with QTL shared by plant height and biomass as secondary traits for drought tolerance in maize.

Use of thermography for high throughput phenotyping of tropical maize adaptation in water stress

In this study the suitability of thermal imaging for phenotyping was investigated as part of a breeding experiment carried out by the International Maize and Wheat Improvement Centre (CIMMYT) at Tlaltizapán experimental station in Mexico. Different subtropical maize genotypes with two replications were screened with respect to their tolerance to water stress. Thermal images of the canopy of 92 different maize genotypes were acquired on two different days in the time interval between anthesis and blister stages (grain filling 1), whereby each picture contained five plots of different genotypes and canopy temperatures calculated for each plot. Significantly, lower canopy temperatures were found in well-watered genotypes compared with water-stressed genotypes. Furthermore significant differences ( p < 0.001) between genotypes under water stress were detected using thermal images. A close correlation ( p < 0.01–0.001) between canopy temperature or modified Crop water stress index with NDVI and SPAD values was obtained. It may be concluded that genotypes better adapted to drought conditions exhibited lower temperatures. Thermography is a potentially promising method to accelerate the screening process and thereby enhance phenotyping for drought adaptation in maize.

Genotypic and phenotypic relationship between physiological and grain yield related traits in four maize (Zea mays L.) crosses of subtropical climate

A study to estimate genotypic and phenotypic correlations among physiological and grain yield related traits in four maize crosses, derived from mating of four inbred lines of subtropical climate, was conducted during 2006 and 2007 at Cereal Crops Research Institute (CCRI) Pirsabak, Nowshera, Pakistan. Physiological plant traits included days to 50% silk emergence, leaf area, plant height and days to maturity while ear length, number of kernel rows ear-1, 100 grain weight and percent shelling were included as grain yield related traits. The association of these physiological traits with grain and stover yields was also investigated in this study. Days to silk and maturity were significantly and negatively associated with ear length both genotypicaly and phenotypicaly in two crosses. However, in the other two crosses, this association was non significant with negative magnitudes regarding ear length, grain yield with other related traits and stover yield. Leaf area and plant height exhibited positive and significant relationship with grai and stover yields and all other yield associated traits. Based on the results of the current study, it was concluded that association between physiological traits and grain yield related traits was mostly positive for leaf area and plant height. However, days to 50% silking and days to maturity exhibited negative association with most of the grain yield related traits. It can be inferred that positive relationship indicated favorable genes controlling these traits which were present in the parents and might be utilized for the improvement of these lines in future breeding programs. Key words: Genotypic and phenotypic relationship, physiological and grain yield related traits, generations, subtropical maize.

Inducer line generated double haploid seeds for combined waxy and opaque 2 grain quality in subtropical maize (Zea mays. L.)

The implementation of modern inducer lines in maize breeding can substantially decrease the time required to create elite inbred lines. In industrialized countries, this method has already largely replaced conventional backcross methods. However, the application of in vivo gynogenesis for inducing doubled haploids is still limited to European and US maize germplasms and has still to be adapted for exotic plant material. The reliability of three modern European inducer lines from the University of Hohenheim (Germany) was investigated for the production of haploid progenies from subtropical specialty maize. Three Chinese hybrids heterozygous for waxy maize and opaque 2 were used as maternal donor material, as maize double recessive for waxy and opaque 2 may improve the nutrition of ethnic minorities in Southeast Asia. However, many false positives were detected by flow cytometry among putative haploid seeds based on anthocyanin pigmentation because the color expression was inhibited in almost 50% of the induced seeds from this maternal plant material. Based on flow cytometry, the haploid induction rates were high with 10.2–12.3%, and the chromosome doubling rates were around 50%; therefore the principal potential of producing DH was confirmed for subtropical maize. However inducer lines for the precise and fast recognition of truly induced haploid seeds still need to be developed.

Genetic association mapping identifies single nucleotide polymorphisms in genes that affect abscisic acid levels in maize floral tissues during drought

In maize, water stress at flowering causes loss of kernel set and productivity. While changes in the levels of sugars and abscisic acid (ABA) are thought to play a role in this stress response, the mechanistic basis and genes involved are not known. A candidate gene approach was used with association mapping to identify loci involved in accumulation of carbohydrates and ABA metabolites during stress. A panel of single nucleotide polymorphisms (SNPs) in genes from these metabolic pathways and in genes for reproductive development and stress response was used to genotype 350 tropical and subtropical maize inbred lines that were well watered or water stressed at flowering. Pre-pollination ears, silks, and leaves were analysed for sugars, starch, proline, ABA, ABA-glucose ester, and phaseic acid. ABA and sugar levels in silks and ears were negatively correlated with their growth. Association mapping with 1229 SNPs in 540 candidate genes identified an SNP in the maize homologue of the Arabidopsis MADS-box gene, PISTILLATA, which was significantly associated with phaseic acid in ears of well-watered plants, and an SNP in pyruvate dehydrogenase kinase, a key regulator of carbon flux into respiration, that was associated with silk sugar concentration. An SNP in an aldehyde oxidase gene was significantly associated with ABA levels in silks of water-stressed plants. Given the short range over which decay of linkage disequilibrium occurs in maize, the results indicate that allelic variation in these genes affects ABA and carbohydrate metabolism in floral tissues during drought.

Heterosis, molecular diversity, combining ability and their interrelationships in short duration maize (Zea mays L.) across the environments

Five inbred lines, 10 single cross maize (Zea mays L.) hybrids and two standard cultivars as check were used to study the combining abilities and heterosis under three environmental conditions. Random amplified polymorphic DNAs (RAPDs) markers were used to study the genetic diversity (GD) and further to analyze relationship of RAPDs based GD with combining ability and heterosis in short duration maize. Spearman’s rank correlation coefficients and linear regressions were analyzed to identify the most important factor determining heterosis and per se performance of the hybrids. Variances due to general combining ability (GCA), specific combining ability (SCA) and their interactions with environment were found to be significant. Twenty random primers generated 179 RAPD fragments. Of these, 102 RAPD fragments were polymorphic. GD was determined using Jaccard’s similarity coefficient, and a dendogram was constructed by UPGMA cluster analysis. The RAPDs based GD exhibited non-significant negative or positive association, non-significant linear regression along with very low coefficient of determination (R2) with SCA, high and mid parent heterosis (HP and MP) and per se performance of the hybrids. Significant positive correlations and regressions along with high coefficients of determination were recorded for SCA with HP, MP and per se performance of the hybrids. The HP and MP also established significant positive association and linear regression along with high coefficient of determination with per se performance of hybrids whereas the parental mean did not establish any significant correlations with the GD, HPH, MPH and grain yield of F1s. The present investigation, therefore, did not find any role of RAPDs based GD in determining hybrid heterosis and hybrid performance in short duration sub-tropical maize. The SCA, however, has emerged as the most important factor in determination of heterosis as well as per se performance of the hybrids in short duration maize.

Combining Ability and Reciprocal Cross Effects of Elite Quality Protein Maize Inbred Lines in Subtropical Environments

ABSTRACTGeneral combining ability (GCA), specific combining ability (SCA), and reciprocal cross effects information facilitate efficient utilization of inbred lines in a breeding program. A diallel analysis of nine quality protein maize (Zea mays L.) inbred lines was evaluated over seven environments in Zimbabwe. Hybrid × environment interaction was significant for all the traits except for tryptophan content and Quality Index. Thus, phenotyping for tryptophan content and Quality Index can be done using a few environments. There was preponderance of GCA effects for tryptophan content, protein content, kernel endosperm modification, and anthesis dates, while SCA effects were both significant and dominant for grain yield, and were also significant for Quality Index and anthesis dates. Reciprocal effects were significant for Quality Index, tryptophan, and anthesis dates, but on the average they accounted for &lt;13% of the variation among hybrids; hence, they were less important. The cross CML181f × CML176 between heterotic group B inbreds was the highest yielding. Inbred CZL03016 exhibited the most desirable GCA effects for kernel modification, while CML264Q showed the best GCA effects for Quality Index, tryptophan, and protein content. CML264Q would be crucial for use as a donor in breeding for these traits in subtropical maize programs.

QTL mapping for reaction to Phaeosphaeria leaf spot in a tropical maize population

Phaeosphaeria leaf spot (PLS) is an important disease in tropical and subtropical maize (Zea mays, L.) growing areas, but there is limited information on its inheritance. Thus, this research was conducted to study the inheritance of the PLS disease in tropical maize by using QTL mapping and to assess the feasibility of using marker-assisted selection aimed to develop genotypes resistance to this disease. Highly susceptible L14-04B and highly resistant L08-05F inbred lines were crossed to develop an F(2) population. Two-hundred and fifty six F(2) plants were genotyped with 143 microsatellite markers and their F(2:3) progenies were evaluated at seven environments. Ten plants per plot were evaluated 30 days after silk emergence following a rating scale, and the plot means were used for analyses. The heritability coefficient on a progeny mean basis was high (91.37%), and six QTL were mapped, with one QTL on chromosomes 1, 3, 4, and 6, and two QTL on chromosome 8. The gene action of the QTL ranged from additive to partial dominance, and the average level of dominance was partial dominance; also a dominance x dominance epistatic effect was detected between the QTL mapped on chromosome 8. The phenotypic variance explained by each QTL ranged from 2.91 to 11.86%, and the joint QTL effects explained 41.62% of the phenotypic variance. The alleles conditioning resistance to PLS disease of all mapped QTL were in the resistant parental inbred L08-05F. Thus, these alleles could be transferred to other elite maize inbreds by marker-assisted backcross selection to develop hybrids resistant to PLS disease.

Tropical and subtropical maize in Asia : production systems, constraints, and research priorities

This book examines future technological and policy prospects for the sustainable intensification of rainfed upland maize production in Asia, and derives R&D priorities for specific maize production environments and markets. Village-level and farmer-group surveys were conducted to characterize upland maize production environments and systems in China, India, Indonesia, Nepal, the Philippines, Thailand, and Vietnam. Survey findings, particularly farmer-identified constraints to maize production, complemented with other relevant data, were used in country-level, R&D priority-setting workshops. High on the list of farmer constraints was drought, estimated to affect three production environments that are home to about 48 million rural poor and produce an estimated 16 million tons of maize, and others such as downy mildew, stem borers, soil erosion/landslides, waterlogging, poor agricultural extension/ technology transfer services, and poor access to low-interest credit and markets. Farmers felt that socioeconomic and policy-related constraints impact maize productivity more than technical constraints do. It is important to recognize that technology is not the only key to increasing productivity and bettering the conditions of marginal maize farmers in Asia. There is a growing trend towards commercializing and intensifying maize production that is different from the staple food self-sufficiency paradigm that has been the cornerstone of agricultural policy in most developing countries. Appropriate government policies could help alleviate the adverse consequences of commercialization and promote sustainable intensification of maize production, especially in marginal environments inhabited by resource-poor subsistence farmers

Soil sustainability indicators following conservation tillage practices under subtropical maize and bean crops

Conservation management systems such as no tillage may enhance sequestration of soil C. The soil properties that contribute to soil C storage under such systems are still largely unknown, especially in subtropical agroecosystems. We investigated the influence of tillage [mouldboard plough (MP) and no tillage (NT)] on soil organic C, microbial biomass and activity, structural stability and mycorrhizal status of a field cultivated with maize ( Zea mays L.) or bean ( Phaseolus vulgaris L.) on a Vertisol in Northern Tamaulipas, Mexico. Crop type, tillage system and soil depth had a significant effect on soil organic C, aggregate stability and bulk density. Soil organic C, microbial biomass C and N and dehydrogenase and phosphatase activities were greater with NT than with MP, particularly under bean cultivation. In the 0–5 cm layer, microbial biomass C and N were, on average, about 87 and 51% greater in the soils cultivated with bean and maize, respectively, under NT than under MP. Higher levels of mycorrhizal propagules, glomalin related soil protein (GRSP) and stable aggregates were produced under NT than under MP in both crops. The no-tillage system can be considered an effective management practice for carrying out sustainable agriculture under subtropical conditions, due to its improvement of soil physical and biochemical quality and soil C sequestration.

Genetic Diversity among CIMMYT Maize Inbred Lines Investigated with SSR Markers: II. Subtropical, Tropical Midaltitude, and Highland Maize Inbred Lines and their Relationships with Elite U.S. and European Maize

Characterization of genetic diversity of maize (Zea mays L.) germplasm is of great importance in hybrid maize breeding. The objectives of this study were to (i) investigate genetic diversity in CIMMYT subtropical, tropical midaltitude and highland inbreds with simple sequence repeat (SSR) markers, (ii) identify appropriate testers for the development of new inbred lines, (iii) compare this sample to U.S. and European elite maize lines and CIMMYT tropical lowland inbreds, and (iv) use the marker and pedigree information as a guide to understanding the heterotic relationship among the CIMMYT maize lines (CMLs) and their potential practical use in maize breeding programs worldwide. Inbreds included in the study were assayed with 79 SSR markers. The CIMMYT inbred lines originated from 35 mostly broad‐based populations and pools with mixed origins. A total of 566 alleles were scored, (averaging 7.2 and ranging from 2 to 16 alleles per locus). The modified Roger's distance (MRD) between pairs of inbreds averaged 0.78, with a range of 0.45 to 0.93. Unweighted paired group method using arithmetic averages (UPGMA) cluster analysis revealed no clear clustering. This reflects a mixed composition of CIMMYT subtropical, tropical midaltitude and highland maize populations and pools, and indicates that large amounts of variation have been incorporated into CIMMYT germplasm. Temperate heterotic groups were separated based on the markers, and nontemperate CIMMYT maize was genetically distinct from temperate lines. Discrete clusters were difficult to identify within and often between megaenvironments. Specific recommendations for nontemperate hybrid maize breeding are made.