Identification of QTLs for phosphorus utilization efficiency in maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) across P levels

It is necessary to develop maize plants that are productive when grown in phosphorus (P) deficient soils because of the high cost of P supplementation in soils. The shoot phosphorus utilization efficiency, the whole phosphorus utilization efficiency of plant and root/shoot ratio as well as the quantitative trait loci associated with these traits were determined for a F_2:3 population derived from the cross of two contrasting maize (Zea mays L.) genotypes, 082 and Ye107. A total of 241 F_2:3 families were evaluated in replicated trials under deficient and normal phosphorus conditions in 2007 at Southwest University. The genetic map constructed by 275 SSR and 146 AFLP markers spanned 1,681.3 cM in length with an average interval of 3.84 cM between adjacent markers. Phosphorus was determined in harvested plants separated into two portions, roots and shoots with leaves. The sum of the two portions was used as an expression for P in the whole plant. By using composite interval mapping, a total of 5–8 distinct QTLs were identified under deficient and normal phosphorus, respectively. SPUE and WPUE under deficient phosphorus were controlled by one QTL, which was in the interval bnlg1518-bnlg1526 (bins 10.04) on chromosome 10. The loci of QTLs were same for SPUE, WPUE and RSR under normal phosphorus, which were in the interval bnlg1518–bnlg1526 (bins 10.04) and P2M8/a-bnlg1839 (bins10.07) on chromosome 10. Unlike regions conferring phosphorus utilization efficiency and root shoot ratio under normal phosphorus, the region under deficient phosphorus showed that genes controlling phosphorus utilization efficiency or root shoot ratio might be different. These results may be useful to breeding programs in marker assisted selections to identify phosphorus tolerant genotypes. Junyi Chen and Li Xu contributed equally to this work.     

Genetic analysis of tropical quality protein maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) germplasm

Maize (Zea mays L.) is an important source of carbohydrates and protein in the diet in sub-Saharan Africa. The objectives of this study were to (i) estimate general (GCA) and specific combining abilities (SCA) of 13 new quality protein maize (QPM) lines in a diallel under stress and non-stress conditions, (ii) compare observed and predicted performance of QPM hybrids, (iii) characterize genetic diversity among the 13 QPM lines using single nucleotide polymorphism (SNP) markers and assess the relationship between genetic distance and hybrid performance, and (iv) assess diversity and population structure in 116 new QPM inbred lines as compared to eight older tropical QPM lines and 15 non-QPM lines. The GCA and SCA effects were significant for most traits under optimal conditions, indicating that both additive and non-additive genetic effects were important for inheritance of the traits. Additive genetic effects appeared to govern inheritance of most traits under optimal conditions and across environments. Non-additive genetic effects were more important for inheritance of grain yield but additive effects controlled most agronomic traits under drought stress conditions. Inbred lines CKL08056, CKL07292, and CKL07001 had desirable GCA effects for grain yield across drought stress and non-stress conditions. Prediction efficiency for grain yield was highest under optimal conditions. The classification of 139 inbred lines with 95 SNPs generated six clusters, four of which contained 10 or fewer lines, and 16 lines of mixed co-ancestry. There was good agreement between Neighbor Joining dendrogram and Structure classification. The QPM lines used in the diallel were nearly uniformly spread throughout the dendrogram. There was no relationship between genetic distance and grain yield in either the optimal or stressed environments in this study. The genetic diversity in mid-altitude maize germplasm is ample, and the addition of the QPM germplasm did not increase it measurably.     

Evaluation of waterlogging tolerance with the degree of foliar senescence at early vegetative stage of maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

Maize is highly susceptible to waterlogging, which is becoming one of worldwide abiotic threats in many agricultural areas. This study was evaluated to establish the screening method and to find tolerant maize genotypes. Six Korean maize inbred lines were subjected to waterlogging at V3 for 15 days using a big size pot with single maize plant (big pot method) and a box containing 31 maize plants at a time (box method). The degree of foliar senescence and the number of senescent leaves were better indicators for selecting waterlogging tolerant maize genotypes than SPAD value and plant height. The degree of foliar senescence revealed that KS124, KS140, and KS141 are tolerant, and KS85 is susceptible to waterlogging at the early growth stage. These responses of foliar senescence were in clear accordance with those of plant grain yield, which was supported by stress tolerance index for grain yield. The box method also showed the similar response of foliar senescence to the big pot method. Therefore, this box method based on foliar senescence may be simple and efficient for large-scale screening of maize germplasm against waterlogging stress. It was concluded that foliar senescence can be a good indicator for selecting tolerant maize genotypes against waterlogging at the early growth stage.     

Identification of genetic factors affecting plant density response through QTL mapping of yield component traits in maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

It is generally believed that grain yield per unit area of modern maize hybrids is related to their adaptability to high plant population density. In this study, the effects of two different plant densities (52,500 and 90,000 plants/hm²) on 12 traits associated with yield were evaluated using a set of 231 F_2:3 families derived from two elite inbred lines, Zheng58 and Chang7-2. Evaluation of the phenotypes expressed under the two plant density conditions showed that high plant density condition could decrease the value of 10 measured yield component traits, while the final grain yield per hectare and the rate of kernel production were increased. Twenty-seven quantitative trait loci (QTLs) for 10 traits were detected in both high and low plant density conditions; among them, some QTLs were shown to locate in five clusters. Thirty QTLs were only detected under high plant density. These results suggest that some of the yield component traits perhaps were controlled by a common set of genes, and that kernel number per row, ear length, row number per ear, cob diameter, cob weight, and ear diameter may be influenced by additional genetic mechanisms when grown under high plant density. The QTLs identified in this study provide useful information for marker-assisted selection of varieties targeting increased plant density.     

Creation of new maize germplasm using alien introgression from <Emphasis Type="Italic">Zea mays</Emphasis> ssp. <Emphasis Type="Italic">mexicana</Emphasis>

Zea mays ssp. mexicana, an annual wild relative of maize, has many desirable characteristics for maize improvement. To transfer alien genetic germplasm into maize background, F₁ hybrids were generated by using Z. mays ssp. mexicana as the female parent and cultivated maize inbred line Ye515 as the male parent. Alien introgression lines, with a large range of genetic diversity, were produced by backcross and successive self-pollinations. A number of alien introgression lines with the predominant traits of cultivated maize were selected. Genomic in situ hybridization (GISH) proved that small chromosome segments of Z. mays ssp. mexicana had been integrated into the maize genome. Some outstanding alien introgression lines were evaluated in performance trials which showed 54.6% hybrids had grain yield greater than that of hybrid check Yedan12 which possessed 50% Ye515 parentage, and 17.1, 9.9% hybrids had grain yield competitive or greater than those of Nongda108 and Zheng958, which were elite commercial hybrids in China, respectively. The results indicated that some of the introgression lines had excellent agronomic traits and combining ability for maize cultivar, and demonstrated that Z. mays ssp. mexicana was a valuable source for maize breeding, and could be used to broaden and enrich the maize germplasm.     

Breeding potential of inbred lines derived from five maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) populations

This review on the resistance to wheat blast disease focus on the latest knowledge useful for the breeders, but also takes into account the lacks in these knowledge. To tackle this disease, it is relevant to apply a breeding strategy which has previously proven its efficacy for obtaining rice varieties with a high level of partial and durable resistance to blast. But, incomplete information is available on wheat blast resistance. Therefore, firstly, it is necessary to adjust this breeding strategy considering the worst hypothesis corresponding to every lack of knowledge. Next, the possible invalidation of every hypothesis can allow simplifying the breeding schema and its implementation. For every lack of knowledge, the practical consequences of the corresponding worst hypothesis, the study of its validity and the consequences of its possible invalidation are explained. Scientific arguments, materials and methods details are provided with the latest available references.     

Combining ability,heterosis and genetic diversity in tropical maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) under stress and non-stress conditions

Drought and low soil fertility are considered the most important abiotic stresses limiting maize production in sub-Saharan Africa. Knowledge of the combining ability and diversity of inbred lines with tolerance to the two stresses and for those used as testers would be beneficial in setting breeding strategies for stress and nonstress environments. We used 15 tropical maize inbred lines to (i) evaluate the combining ability for grain yield (GY), (ii) assess the genetic diversity of this set of inbred lines using RFLP, SSR, and AFLP markers, (iii) estimate heterosis and assess the relationship between F₁ hybrid performance, genetic diversity and heterosis, and (iv) assess genotype × environment interaction of inbred lines and their hybrids. The F₁ diallel hybrids and parental inbreds were evaluated under drought stress, low N stress, and well-watered conditions at six locations in three countries. General combining ability (GCA) effects were highly significant (P < 0.01) for GY across stresses and well-watered environments. Inbred lines CML258, CML339, CML341, and CML343 had the best GCA effects for GY across environments. Additive genetic effects were more important for GY under drought stress and well-watered conditions but not under low N stress, suggesting different gene action in control of GY. Clustering based on genetic distance (GD) calculated using combined marker data grouped lines according to pedigree. Positive correlation was found between midparent heterosis (MPH) and specific combining ability (SCA), GD and GY. Hybrid breeding program targeting stress environments would benefit from the accumulation of favorable alleles for drought tolerance in both parental lines.     

Mapping of QTLs associated with cadmium tolerance and accumulation during seedling stage in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Cadmium (Cd) is a non-essential element and toxic to plants. To investigate the genetics of Cd tolerance and accumulation in rice, quantitative trait loci (QTL) associated with Cd tolerance and accumulation at the seedling stage were mapped using a doubled haploid (DH) population derived from a cross between a japonica JX17 and an indica ZYQ8. A total of 22 QTLs were found to be associated with shoot height (SH), root length (RL), shoot dry weight (SDW), root dry weight (RDW), total dry weight (TDW) and chlorophyll content (CC), and 10 and 12 QTLs were identified under the control and Cd stress conditions, respectively. For Cd tolerant coefficient (CTC), 6 QTLs were detected on chromosomes 1, 3, 5, 8 and 10. Under Cd stress, 3 QTLs controlling root and shoot Cd concentrations were mapped on chromosome 6 and 7. One QTL for shoot/root rate of Cd concentration was identified on chromosome 3. The results indicated that Cd tolerance and accumulation were quantitatively inherited, and the detected QTLs may be useful for marker-assistant selection (MAS) and identification of the genes controlling Cd tolerance and accumulation in rice.     

Density effects on environmental variance and expected response to selection in maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

The study aimed to address the optimal plant population density in maize that maximizes phenotypic expression and differentiation, and lessens environmental effects on genotypic expression in terms of the response to selection. A set of seven short-season hybrids (Rom set) was tested under rainfed conditions (2006, 2007) in Romania, and a set of seven long-season hybrids (Gr set) was tested with irrigation (2007) in Greece. Experimentation was conducted under ultra-low (ULD), low (LD), middle (MD), and high (HD) densities (0.74, 2.51, 4.20, 8.40 plants/m² for the Rom set, and 0.74, 3.13, 6.25, 8.33 plants/m² for the Gr set). Phenotypic expression and differentiation for grain yield were highest at the ULD. Coefficient of variation (CV) for grain yield, ear length and kernel row number decreased as density decreased. Environmental conditions and hybrid plant-yield potential (i.e., maximum yield per plant) were crucial for the optimal density that achieved the lowest environmental variance. For the Rom set the lowest CV for grain yield was obtained at the LD in the unfavourable season and at the ULD in the favourable season. The less acquired variance was achieved at the ULD for the highest yielding hybrids and at the LD for the lowest yielding hybrids, revealing a negative association between plant-yield potential and optimal density. Concluding, a density proximal to the ULD approximates absence of competition in maize, and optimizes three determinant parameters for successful selection: selection intensity, heritability and phenotypic differentiation.     

Fine mapping of a <Emphasis Type="Italic">Xantha</Emphasis> mutation in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

The recessive mutation of the XANTHA gene (XNT) transforms seedlings and plants into a yellow color, visually distinguishable from normal (green) rice. Thus, it has been introduced into male sterile lines as a distinct marker for rapidly testing and efficiently increasing varietal purity in seed and paddy production of hybrid rice. To identify closely linked markers and eventually isolate the XNT gene, two mapping populations were developed by crossing the xantha mutant line Huangyu B (indica) with two wild type japonica varieties; a total of 1,720 mutant type F₂ individuals were analyzed for fine mapping using polymorphic InDel markers and high dense microsatellite markers. The XNT gene was mapped on chromosome 11, within in a fragment of ~100 kb, where 13 genes are annotated. The NP_001067671.1 gene within the delimited region is likely to be a candidate XNT gene, since it encodes ATP-dependent chloroplast protease ATP-binding subunit clp A. However, no sequence differences were observed between the mutant and its parent. Bioinformatics analysis demonstrated that four chlorophyll deficient mutations that were previously mapped on the same chromosome are located outside the XNT region, indicating XNT is a new gene. The results provide useful DNA markers not only for marker assisted selection of the xantha trait but also its eventual cloning.     

Identification of QTLs controlling low-temperature germinability and cold tolerance at the seedling stage in rice (<Emphasis Type="Italic">Oryza Sativa</Emphasis> L.)

Both low-temperature germinability (LTG) and cold tolerance at the seedling stage (CTS) are important traits for rice. In this study, a rice population of recombinant inbred lines (RILs), derived from the backcross population of a cross between Dongnong422 and Kongyu131, was developed to detect quantitative trait loci (QTL) affecting LTG and CTS by using seed of different storage times. Correlation analysis indicated that there was no significant relationship between LTG and CTS, suggesting that cold tolerance might be genetic differences for LTG and CTS. In total, Twelve and twenty-three major QTLs were detected for LTG and CTS, respectively, which could explain greater than 10% of the phenotypical variation. Eight (qCG12-1, qGI12-1, qGV9-1, qMLIT12-1, qPV6-1, qMDG12-1, qLDWcold10-1, qLFWcold10-1) significant QTLs were mapped for different storage time, it concluded that such QTLs were not affected by environment (storage time) and were closely related QTLs to cold tolerance. One or more QTLs were identified for each trait with some of these QTLs co-locating, qMLIT7-1, qCG7-1, and qGI7-1 for LTG, qLFWcold10-1, and qLDWcold10-1 for CTS with contributions over 15% were mapped common marker interval, respectively, co-location of QTLs for different traits can be an indication that a locus has pleiotropic effects on multiple traits due to a common mechanistic basis. Two lines, RIL128 and RIL73, might be valuable to improve the LTG and CTS through a combination of crosses. The identified QTLs might be applicable to improve the rice cold tolerance by the marker-assisted selection approach.     

The ND EarlyQPM program: developing the next generation of healthier maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) products

There is a need to develop the next generation of healthier crop products for an improved human and animal nutrition. Maize (Zea mays L.) cultivars with improved amino acid profiles are essential to diets focused on this crop. Breeders have added crop value with the development of quality protein maize (QPM). However, QPM cultivars have never been an option to farmer and ranchers in short-season environments. The objectives of this research were: (1) to adapt QPM genotypes to the northern U.S. through the North Dakota (ND) EarlyQPM Program, and (2) to develop new early generation SS and non-SS short-season QPM lines and populations for breeding purposes. Fifty-three inbred lines, including 47 QPM donor lines, five experimental North Dakota State University (NDSU) lines, and one ex-PVP line from industry, were selected to produce 94 early-QPM backcross populations. Considering earliness, protein content, and amino acid levels, 218 early generation lines were selected for producing testcrosses with industry testers. Experiments evaluating testcrosses were arranged in 10 × 10 and 12 × 12 partially balanced lattice designs across three ND locations in 2013 and 2014. A total of 48 lines were selected for further development, 17 representing the Stiff Stalk (SS) heterotic group and 31 representing the non-SS group. Selected lines showed, in hybrid combinations, not only above average grain yield, rate of dry down, and total protein content but also high levels of lysine, tryptophan, and methionine essential amino acids for feedstock nutrition. The results of this research show, for the first time, the successful adaptation of QPM genotypes to short-season environments. As a result, new ND EarlyQPM germplasms and lines have been developed for potential release.     

The impact of GM seed admixture on the non-GM harvest product in maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

Recently, the introduction of GM maize in agricultural production in the EU and elsewhere has raised the issue of adventitious presence of GM seeds in conventional seed lots. Adventitious presence may occur in all arable farming, and at any step in the production of seeds or grain, or in processing of harvested product in the food/feed chain. As of today, there are no official thresholds governing the adventitious presence of GM seeds in conventional seed lots in Europe. However, it is assumed that GM admixture in seed lots could have a considerable influence on the level of adventitious presence in the non-GM harvested product. The experiments highlighted in this paper aim at the consequences of adventitious presence of GM maize seeds in conventional seed lots. It is shown for varieties belonging to the same maturity group that the final GM rate (% seeds) in the harvest product is nearly same as the initial seed admixture (% seeds). This corresponds to Hardy–Weinberg expectations. The variation depends mainly on the flowering coincidence, the site and climatic conditions. In cases where the admixed seeds are of different maturity group, the level of cross-pollination in the harvest product is reduced. Furthermore a comparison between the visual GM seed detection and real-time PCR detection was done. It is evident that the result of the real-time PCR detection method has a more variable uncertainty associated with its results than the visual seed testing method. The accuracy of prediction from % GM seed to % GM DNA depends on the reference material used for calibration curves.     

Diallel analysis of grain filling rate and grain filling period in tropical maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

Grain yields of maize in the lowland tropics are generally limited by short days and high temperatures that minimize durations of incident light. Little has been published on the effects of this limiting environment on the genetics of grain filling rate (GFR), and grain filling period (GFP) in tropical maize germplasm. This study sought to address these limitations. A set of 8 elite maize inbreds of tropical origin and their 28 diallel hybrids were grown in three seasons at Waimanalo, Hawaii, USA. Seasonal differences included > 100% differences in values of photosynthetic active radiation (PAR) during grain formation. Information was sought on the performance variations and the genotype by season interactions for GFR, GFP, days to mid-silk (DTS), kernel weight, with estimates of general (GCA) and specific combining ability (SCA) and their interactions with seasons. Significant differences occurred for inbreds, hybrids, and genotype by season interactions, GCA and SCA effects and their interactions with seasons, which could be attributed primarily to the differences in PAR values among seasons in Hawaii during grain filling. Additive genetic effects predominated for GFR and GFP. Breeding approaches that take advantage of additive gene effects including hybrid breeding with evaluations in multiple Hawaii seasons may be used to alter GFR and GFP.     

Sequence characterized amplified region markers tightly linked to the dwarf mosaic resistance gene <Emphasis Type="Italic">mdm1 (t)</Emphasis> in maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

Maize dwarf mosaic is one of the devastating and wide spread viral diseases in the world. The present investigation was carried out to develop DNA markers closely linked to the resistance gene mdm1 (t). Linkage between the markers and phenotypes was confirmed by analyzing an F₂ population obtained from a cross between a resistant parent ‘Huangzaosi’ and a susceptible parent ‘Mo17(478)’. Four AFLP markers were found in the maize dwarf mosaic resistant plants. By using (BSA) bulked segregant analysis, two of the four AFLP markers were transformed into Sequence-characterized amplified regions markers (SCARs), nominated Rsun-1 and Rsun-2. The two amplified fragment length polymorphism (AFLP) markers, RHC-1and RHC-2, from the amplification products of primer combination E-AGC/M-CAA and E-AGC/M-GAA, showed linkage with the mdm1 (t) gene in a genetic distance 1.6 and 2.0 cM, respectively. The results indicate that the new SCAR markers will be valuable to distinguish resistant plants from susceptible plants in plantlets growing in seedbeds. The markers developed in this study are suitable for marker-assisted selection for maize dwarf mosaic resistance.     

Residual macronutrient concentration and follower maize (<Emphasis Type="Italic">Zea mays</Emphasis>) crop performance in soilless growth medium previously cropped with six <Emphasis Type="Italic">Musa</Emphasis> genotypes

The nutrient mining abilities of six Musa genotypes: ‘Agbagba’, ‘PITA 22’, ‘Nsukka Local’, ‘FHIA 17’, ‘Fougamou’, and ‘BITA 7’, grown in organic medium formulated by decomposing rice husks with poultry manure in volume proportions of 3:2, were determined at the Teaching and Research Farm of the Department of Crop Science, University of Nigeria, Nsukka, Nigeria. Results indicated significant variation in nutrient mining with respect to Musa genotype and duration of growth. The medium previously cropped with ‘Agbagba’ recorded highest residual mean values for P and Ca, while the medium cropped with ‘Fougamou’ had the highest residual concentration of Mg and S. Residual amount of K was highest in the medium where ‘PITA 22’ and ‘Nsukka Local’ were previously grown. Peak residual concentration of P and K occurred in the medium in which Musa plants were grown only for 4 months, decreasing thereafter, in contrast to Ca, Mg, and S. Residual N tended to increase with longer growth duration of the Musa crops. Follower crop of maize grown in the medium previously cropped with Musa genotypes showed variable performance. While medium previously cropped with ‘PITA 22’ and ‘BITA 7’ favored growth attributes and leaf greenness of the follower maize plant, ‘Fougamou’ favored maize fresh weight attributes. Ectomycorrhiza association was observed in medium previously cropped to ‘BITA 7’ and ‘PITA 22’. Generally, ‘Agbagba’ and ‘Fougamou’ seemed to be the most reticent nutrient miners while uptake of P and K appeared to be low 4 months after planting (MAP) in contrast to Ca, Mg, and S uptake. However, substrate previously cropped with ‘PITA 22’, ‘BITA 7’ and ‘Fougamou’ gave rise to superior growth and fresh weight attributes, respectively, of the maize, the follower crop. A prospect of utilizing the ectomycorrhiza association observed with some genotypes to upgrade Musa yields and those of associated or follower crops exist.     

Fine mapping of grain weight QTLs using near isogenic lines from a Cross between <Emphasis Type="Italic">Oryza sativa</Emphasis> and <Emphasis Type="Italic">O. grandiglumis</Emphasis>

In a previous study, we reported the grain weight QTL, tgw2 in the 150 F2:3 lines derived from a cross between Oryza sativa subssp. Japonica cv. Hwaseongbyeo and HG101. This QTL was confirmed in F4 lines (CR1242) segregating for the target region. For fine mapping of tgw2, one F₅ plant homozygous for the O. grandiglumis DNA in the target region was selected from CR1242 and crossed with Hwaseongbyeo to produce the F₂ and F₃ populations. QTL analysis using 490 F₂ plants confirmed the existence of tgw2 with an R ² value of 28.0%. This QTL explained 61.3% of the phenotypic variance for 1,000-grain weight in 64 F₃ lines. Substitution mapping with 47 F₃ lines and 74 F₄ plants with informative recombination breakpoints in the target region was carried out to narrow down the position of the tgw2. The result indicated that tgw2 was located in the 384-kb interval between two SSR markers, RM12813 and RM12836. Annotation data of BACs in this 384-kb region revealed that forty-five putative genes exist in this interval including the GW2 gene responsible for grain weight and width. Considering the position of the QTL tgw2, it appears that tgw2 is functionally related to the gene GW2. However, the possibility that another unknown mechanism might be responsible for regulation of grain weight at tgw2 cannot be ruled out. Four QTLs for grain length, grain width, and grain thickness were also located in the same interval suggesting that a single gene is involved in controlling these four traits. Substitution mapping also indicated that two QTLs for grain weight and culm length, tgw2 and cl2, were tightly linked.     

Diallel analysis of grain yield and resistance to seven diseases of 12 African maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) inbred lines

Maize (Zea mays L.) is grown on 15 million ha in eastern and southern Africa. Several diseases are of common occurrence in the region and regularly result in significant yield losses. A collaborative regional disease nursery (REGNUR) project was initiated in 1998 to identify and increase access to disease resistant germplasm, generate and disseminate information on disease and insect resistance sources, and facilitate the development of resistant cultivars by project partners. A diallel among 12 elite inbred lines was formed with the specific objective of evaluating the combining ability of these inbred lines for grain yield and resistance to seven diseases. The trial was grown at six sites in 2001. Results showed that both general (GCA) and specific combining ability effects were significant for most diseases. On the average, GCA accounted for 69% of resistance to diseases and only 37% of variation for grain yield. Correlations between GCA effects for disease scores were generally non-significant, implying that it is possible to pyramid genes for disease resistance in inbred lines. This underscores the need for screening for resistance to prevailing diseases using artificial inoculation or reliable hot-spots. Based on GCA effects for grain yield and across diseases, P12 and P6 were the best inbred lines. The crosses P4 × P9 (6.7 t ha⁻¹) and P4 × P12 (6.9 t ha⁻¹) were the best hybrids in the earlier maturity group, while P3 × P9 (8.3 t ha⁻¹) and P2 × P8 (7.4 t ha⁻¹) were the best hybrids in the late maturity group.     

Association mapping of salt tolerance traits at germination stage of rapeseed (<Emphasis Type="Italic">Brassica napus</Emphasis> L.)

Soil salinization has a major effect on crop yield. Seed germination is a crucial prerequisite for the successful cultivation of rapeseed in saline soils. Limited information is available about rapeseed salt tolerance at germination stage. Here, we carried out an association mapping of salt tolerance QTLs at seed germination stage. We evaluated salt tolerance of 214 rapeseed inbred lines at germination stage, and the 60 K Brassica Infinium^® SNP array was used to genotype the association panel. Finally, a total of 110 single nucleotide polymorphisms (SNPs) were associated with three salt tolerance indices and these SNPs were integrated into 44 QTLs. In addition, 56 possible candidate genes were located in or near the genomic region where the above QTLs were mapped. Thus, our results provide valuable information for understanding the genetic control of rapeseed salt tolerance at germination stage and may facilitate marker-based breeding for rapeseed salt tolerance.