Genetic variation in maize (Zea mays L.) for germination and emergence at 10°C

Summary Full-sib, paternal half-sib and S1 families were derived from a population of maize, Pool 5, with a wide germplasm base. These families were used to study genetic variation for germination percentage, emergence percentage and time to emergence at 10°C.Genotypic variances were large for all characters, and the best families from the population were markedly superior to hybrids used as checks. Maternal effects appeared to be of overwhelming importance for germination percentage, but variances due to males were significant for emergence percentage and time to emergence. Inbreeding depression at the seedling level was also important for these two emergence characters. Germination and emergence characters were not correlated with seed weight.We concluded that Pool 5 should be a valuable source of genes for improving germination and emergence of maize at low temperatures.     

Genetic variation and selection criterion for digestibility traits of forage maize

Summary Variations in feeding value characteristics were studied on a factorial mating design, within a multilocal network. The in vitro digestibility of dry matter (IVDDM), neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), starch, soluble carbohydrate, protein and ash contents were evaluated on whole plant samples. The in vitro digestibility of the non-starch and non-soluble carbohydrate plant part (IVDNSC) was also computed from near infra-red reflectance spectroscopy estimations of IVDDM, starch and soluble carbohydrate contents. Variations due to general combining ability (GCA) were more important, for all traits, than variations due to specific combining ability (SCA). Variations due to the GCA × environment interaction were also sizeable and were partly explained by the earliness of the lines. IVDDM was primarily influenced by the relative proportions of cell walls and starch in the whole plant, which were particularly susceptible to the development stage of the plants, at harvest. When adjusted for earliness, there was generally no significant relation between the line per se value and GCA performance in the biochemical constituent contents of the plant. On the contrary, IVDNSC seemed to be a feeding value trait worth evaluating at the inbred line level. It was also almost independent of the ripening stage and could be easily computed from whole plant samples. IVDNSC, with a simultaneous control of starch content, might be a good selection criterion for evaluating and improving the nutritive value of forage maize.     

A selection procedure for ear drying-rates in maize

Summary A selection procedure to change the drying rates of maize (Zea mays L.) ears was developed and tested. Results indicated that drying rates were affected by hybrid genotype, ear maturity at harvest, number of kernels per row, ear diameter, and moisture content at harvest.Mass selection was initiated in an early synthetic, NDSG, in an attempt to produce both fast and slow drying strains. After two cycles resultant substrains were evaluated in the laboratory for direct and in the field for correlated selection responses. Laboratory results indicated that selection effectively changed moisture loss rates in NDSG, and confirmed earlier observations that an ear's moisture content at harvest affects its drying rate.Data from field experiments grown at five locations in 1980 revealed that each of two selection cycles for slow laboratory drying rate significantly reduced ear moisture content at harvest, in the field. This lower harvest moisture content appeared to result from a lower moisture content at physiological maturity rather than a faster drying rate. Other correlated selection responses included lowered yield, plant height, and ear weight due to selection for fast laboratory drying, and lowered plant height and ear weight from selection for slow drying.Data indicated that this selection procedure can be used to change moisture loss rate and ear moisture content at harvest. Results also contributed to understanding of factors involved in ear drying rates which may lead to more effective selection procedures.Journal Article No. 1385 of the North Dakota Agri. Exp. Stn., Fargo.     

Genetic characterization of 218 elite CIMMYT maize inbred lines using RFLP markers

Characterization of genetic diversity among maize inbred lines can facilitate organization of germplasm and improve efficiency of breeding programs. A set of 218 phenotypically diverse inbred maize lines developed at CIMMYT for hybrid production was characterized using 32 RFLP markers to: (1) analyze the genetic diversity present; (2) define potential heterotic groups based on clusters formed with marker data; and (3) identify the most representative testers for each potential heterotic group. Lines were clustered using five different genetic distance measurements to find consensus non-hierarchical clusters. Dendrograms were produced to study hierarchical classification within smaller groups of lines. A very high average allelic diversity was seen in this germplasm. Lines did not cluster based on phenotype, environmental adaptation, grain color or type, maturity, or heterotic response (as determined based on hybrid performance with testers), but lines related by pedigree usually did cluster together. Previously defined testers from opposite heterotic groups were not genetically differentiated, and did not represent well their heterotic group. Discrete clusters were difficult to find; thus, potential heterotic groups will be difficult to suggest using RFLP markers alone. However, suggestions on how to use molecular markers and cross performance information to refine heterotic groups and select representative testers are presented.     

Inheritance of tillering and flowering-time in early maturing maize

Summary A diallel cross and F₂ populations derived from eight early maturing maize inbreds were used to investigate the inheritance of tillering and flowering-time (anthesis), and the possible relationship between tillering and flowering-time. Incomplete dominance for increased tillering was observed; potence ratios, representing the overall degree of dominance, ranged from 0.26 to 0.52. Dominance for early flowering ranged from incomplete with a potence ratio of 0.55 to overdominance with a potence ratio of 1.40. Broad-sense heritabilities were low for both characters. The genetic component of variation for tillering was due to general combining ability effects; specific combining ability effects were not significant. A significant negative linear relationship between tillering and flowering-time was found. Lack of independent assortment of tillering and flowering-time in F₂ populations indicated that the two characters are genetically related.     

Hybrid reconstruction in maize

Summary The production of seed of the maize single cross hybrid F68^*NE2 is uneconomic because of the low grain yield of the maternal line. Therefore the aim was to produce it from newly developed inbred lines obtained by reshuffling the genes in the hybrid, accompanied by selection. Thus in open pollinating generations derived from this hybrid, i.e. in C0, C1, C2, C3 and C4, honeycomb selection for grain yield improvement was applied. Selfing of one ear and open pollination of another ear of selected prolific C4 plants yielded 20 pairs of S1/half sib progenies. Plants grown from remnant S1 seed corresponding to superior progeny pairs were selfed. In each S2-line a single plant was selected and selfed. The S3-lines were evaluated for yield. Two S3-lines, i.e. 6D and 2B, attracted attention because they yielded two and a half times as much as the best commercial inbred line B73.The S1-and S2-lines were tested for combining ability with the related inbred lines NE2 and F68 by means of honeycomb design experiments and for combining ability with unrelated, freely available inbred lines by means of randomized complete block designs. Two S2-lines, i.e. 5C and 6E, were selected for their good combining ability. The six single cross hybrids produced by crossing the four S3-lines 6D, 2B, 5C, and 6E were compared with the original hybrid F68^*NE2 in a honeycomb design at two sites. The grain yields of the single cross hybrids 6D^*6E and 5C^*6E were similar to that of F68^*NE2. However, these two reconstructed hybrids can be produced in a cheaper way because the new maternal inbred lines yield as good as B73 (line 5C) or much better (line 6D).     

Populations of maize with more rapid and reliable seedling emergence than cornbelt dents at low temperatures

Summary Selected races, populations and genotypes of maize (Zea mays L.) from regions where maize is sown under cool conditions were evaluated in controlled-environment rooms for time to seedling emergence and percentage emergence at low temperatures. The objective of the study was to identify populations with the ability to emerge more rapidly and more reliably than Cornbelt Dent, the race most widely used to produce cultivars for temperate regions.Several populations emerged markedly faster and more reliably than Cornbelt Dent populations. All of these populations contained germplasm of highland Mexican origin, except for Gaspé Flint, and the fastest population in each experiment always contained a high proportion of germplasm from the Cónico race. The advantage of populations containing Cónico germplasm was greatest in environments where emergence was slowest.     

Genetic control of kernel modification found in South African quality protein maize inbred lines

Summary The genetic control of endosperm modification in 12 opaque-2 maize (Zea mays L.) inbred lines was investigated by means of a diallel cross experiment conducted across two environments. Kernel vitreousness and kernel hardness were determined by partially dominant genes. Additive gene action was largely responsible for kernel modification. A favourable general combining ability for kernel vitreousness and kernel hardness was positively correlated with an accumulation of dominant kernel modifying genes. South African sources of endosperm modifiers have been found to be similar to those used in other quality protein maize breeding programmes. Certain inbred lines displayed sufficient genetic potential for use in a quality protein maize hybrid breeding programme.     

Relationships among American and Spanish populations of maize

Summary Two experiments were carried out with two objectives. First, to establish the phenetic relationships among the maize (Zea mays L.) landraces from Galicia (Northwestern Spain) maintained at the Misión Biológica de Galicia. Second, to assess the resemblance between a collection of Spanish populations (including the landraces from Galicia) and a set of US Corn Belt varieties. For the first objective 73 varieties from Galicia, along with 9 hybrid checks, were grown in 9×9 simple lattices at two locations for two years. For the second objective 131 populations from the US Corn Belt and Spain, along with 9 hybrid checks, were grown for three years in unreplicated experiments. Cluster analyses were carried out with the first principal components that accounted for a significant amount of the total variation. Four groups were found among the landraces from Galicia. The populations from Spain and America were classified as belonging to nine main groups. The replicated experiment was more accurate than the unreplicated one. However, it is concluded that an unreplicated test grown in several environments is accurate enough to detect the main groups, although some inaccuracies should be expected.     

Breeding for yield and nutritive value in forage maize: An easy criterion for stover quality,and genetic analysis of Lancaster variety

Summary The maize populations BS13 (S2) C4 and Lancaster are compared with respect to their potential as forage maize. In light of the preliminary results, the Lancaster population was chosen for a more thorough study.The determination of NDF gave a sufficiently precise estimate of the stover digestibility within our experimental conditions. The heritability of stover and grain production was 0.59 and 0.30, respectively, whilst that of the stover NDF was 0.32.The additive genetic correlation between NDF and stover production, possibly attributable to the architecture of the plant, favours the selection of genotypes that produce more and better forage.     

Heterotic relationships among European maize inbreds

European flint maize (Zea mays L.)cannot be considered an uniform group of germplasm based on its origin and area of adaptation. However, maize breeders have not taken full advantage of the variability within the European flint germplasm. The objective of this work was to study the heterotic relationships among European maize inbreds from different origins. Nine European flint inbreds were crossed in a diallel that was evaluated in three environments in northwestern Spain. The variability within the European flint germplasm and the agronomic value of some inbreds could be utilized for maize breeding programs as an alternative to the systematic introduction of U.S. dent germplasm that is narrowing the germplasm base of breeding programs even in places where it is poorly adapted. Some European flint inbreds may also be valuable sources of earliness (F7 and EP42), resistance to root lodging (EA1070), and yield (EP42).These results suggest that, within the European flint germplasm, there could be some heterotic patterns, such as ‘north-central Europe × southern Europe’, which could provide an alternative to the heterotic pattern ‘European flint × U.S. dent’. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cytoplasmic-nucleargenic effects in the C121 (Athens) Zea mays L. inbred with three cytoplasms

Summary Seeds of the long-time maize (Zea mays L.) inbred CI 21 (Athens) were prepared with the following cytoplasms: GA 199 and GT 112. Each source of cytoplasm as the female parent was corssed to the CI 21 (A) as the pollen parent. The F₁, as the female, was backcrossed successively to the CI 21 (A) genotype until the CI 21 (A) nucleus was substituted into each of the cytoplasms. Thus, each cytoplasmic source of seed assumed to have the same nuclear genotype.The cytoplasms were compared in all possible combinations for one year with 10 replications. Among the characters studied, significant differences were obtained for the following: effect of herbicide, reaction to Fusarium sp., silking on a given day, tasseling, plant height, ear height, lodging, number of ears, and yield.These results revealed a favorable interaction of GA 199 cytoplasm with the CI 21 (A) genotype for desirable characters. Apparently, plasmon-sensitive effects caused the unfavorable results. Results from these experiments also provide further support for cytoplasmic effects on agronomic characters in maize.Part of the dissertation submitted by the senior author to the Dep. of Agronomy, University of Georgia, Athens, Georgia 30602, USA.     

Genetic mapping for resistance to gray leaf spot in maize

The molecular marker technology has been used on mapping of quantitative trait loci (QTLs) associated with plant resistance. The objectives of this research were to estimate genetic parameters and to map genomic regions involved in the resistance to gray leaf spot in maize. Ninety F₃ families from the BS03 (susceptible) and BS04 (resistant) cross were used. Field trials were performed using a 10 × 10 square lattice design with three replications. Data from 62 SSR markers were used for linkage analysis. The locations of the QTLs on the linkage groups were determined by composite interval mapping method and the phenotypic variance explained by each marker was determined by regression analysis. Several QTLs associated to disease resistance were identified in the population BS03 × BS04. Some QTLs showed significant effects over the different environments studied. The existence of significant QTLs in common among different environments indicates these genomic regions as possible new tools for marker-assisted selection in maize breeding programs.     

Maturity interaction and black layer occurrence in opaque-2 and normal hybrids in maize (Zea mays L.)

Summary Six opaque-2 lines and their normal counterparts were crossed in diallel crosses. The crosses were grown in 1970 in a split plot randomized complete block design. Harvests were made at 7-day intervals starting at 28 days after pollination and countinuing through 63 days.The average kernel weight of opaque-2 hybrids was inferior to that of the normal. Nevertheless, the opaque-2 gene performed differently in different hybrids. In B14 x B37 single cross the mutant had similar kernel weight as its normal counterpart in the first and second harvests. In contrast a wide difference was found between the opaque-2 and the normal, both at early and late stages of development in W64A x A545 background.The normal hybrids had greater cob weight, ranging from 9'7 to 11.8% more than the opaque-2. The difference in cob weight of the opaque-2 and the normal remained constant over the different harvest dates.At physiological maturity, the opaque-2 hybrids averaged 3.5% higher moisture content than the normal. In general, a slower accumulation of dry matter in the kernels was accompanied by a retention of more moisture.Shelling percentage was higher for the normal hybrids. Black layer, an indicator of physiological maturity, was formed at about the same time in the opaque-2 and normal.Journal paper No. 6094.     

Differences in reciprocal crosses of maize inbred lines diverse for protein content

Summary Forty-two crosses and their reciprocals in maize (Zea mays L.) involving inbred lines highly diverse for protein content were evaluated in four environments. Data were recorded on crude protein content of grain, protein yield, grain yield, 1000 kernel weight, dry matter content of ear at harves,, days to 50% slking, plant height, ear height, and early vigor. No significant variation due to reciprocal differences was observed for protein content and early vigor. For all other traits the variance component due to reciprocal x environment interactions was significant while the variance component due to reciprocal differences was significant only for kernel weight, dry matter content of ear, plant height, and ear height. The variance components due to nuclear differences and their interactions with environments were always highly significant and larger than the components due to reciprocal differences and reciprocal x environment interactions. The instability and low magnitude of reciprocal differences indicated that it might be difficult to exploit them commercially. However, seeing the presence of reciprocal differences for most of the traits studied, the evaluation of breeding materials for these differences seems to be important.The research work was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 142.     

Participatory plant breeding with maize in Mexico and Honduras

Maize is a staple food crop in many developing countries. However, if seven major maize producing countries are excluded from this group, data indicate that only 34% of the maize area is planted with improved seed despite considerable effort invested in maize breeding. This has led researchers to investigate other options, such as farmer-participatory plant breeding, for delivering the benefits of plant breeding knowledge and technology to farmers in developing countries. This paper describes short-term results from participatory maize breeding studies in Mexico and Honduras. Results from three selection cycles in Mexico suggest that stratified mass selection without pollination control, with selections carried out by researchers in farmers' fields, may be effective at improving yield in farmers' local varieties. In Honduras, mass selection with pollination control, where selections were done by collaborating farmers in their own fields on their own varieties, showed trends (non-significant) towards yield improvement. Farmer selection seemed to offer the greatest yield benefit over experiment station selection on the farm with the lowest yield potential, suggesting that farmer-participatory approaches may be most advantageous in marginal environments where experiment station conditions differ most dramatically from farmers' conditions. These studies highlighted the importance of seed systems knowledge in designing participatory plant breeding programs. For cross-pollinated crops, they also highlighted the need to balance progress from selection and demands on farmers' time and labor in choosing breeding methods. Further work is needed to investigate farmer-participatory breeding approaches that can address post-harvest traits. This revised version was published online in August 2006 with corrections to the Cover Date.     

Response of maize inbred lines to a defoliation treatment inducing tolerance to cold at germination

Defoliation during maize (Zea mays L.) kernel development has been observed to induce tolerance to cold of germinating seeds in responsive genotypes. The objectives of this study were to evaluate the response to defoliation of immature embryo and mature seed germinability at cold and to verify if the response was influenced by the developmental stage at which the treatment was applied. In three environments, six inbred lines (B73, IABO78, Lo1016, Lo964, Mo17, Os420) were defoliated (D) approximately 20 days after pollination (DAP) or not defoliated (ND). Immature embryos were excised three days after defoliation and germinated in vitro at 9 or 25 ^∘C. At maturation, kernel germination was tested at the same temperatures. Defoliation improved cold tolerance and mean time to germination (MTG) at 9 ^∘C of both embryos and kernels of Lo1016. To study the effect of kernel developmental stage on response to defoliation, plants of B73, Lo1016 and Lo964 were defoliated at 15, 18, 21, 24, 27, 30, 33, 36, and 39 DAP, or not defoliated. At the same DAP, immature grains were analyzed for dry weight, water and abscisic acid (ABA) content. In Lo1016, low amounts of kernel ABA were detected at all stages, while in Lo964 and B73 ABA increased during development. Lo1016 mature kernels showed an improvement of cold tolerance due to defoliation at all times, while the other genotypes did not. In conclusion inbred lines showed variability for mature seed and immature embryo tolerance to cold at germination and for the ability to acquire tolerance after defoliation.     

Properties of maternal haploid maize plants and potential application to maize breeding

Summary Presented are the results of a two-year study of haploid maize plants in the field. The haploids were produced with the aid of inducer line ZMS. In total, 604 and 1030 haploids were obtained and studied in the first and second years, respectively. Tassels of haploid plants were found to be almost completley sterile. Fertility of ears was studied by pollinating them with the pollen from diploid inbred lines, the cross resulting in almost all of the haploid ears carrying kernels. On average 27.4 kernels per ear of haploid plant were obtained in the first year of study and 26.3 in the second. These gave rise to normal diploid plants. This property allows genotypes selected at the level of haploid plants to be involved in breeding process. Unusual plants were found among haploids, phenotypically resembling homozygous lines. It was assumed that the plants had resulted from spontaneous chromosome doubling in haploids. The results of comparative studies of progenies of unusual plants and inbred lines derived from the same synthetic population are presented.     

Mapping QTLs in breeding for drought tolerance in maize (Zea mays L.)

Summary Grain yield in the maize (Zea mays L) plant is sensitive to drought in the period three weeks either side of flowering. Maize is well-adapted to the use of restriction fragment length polymorphisms (RFLPs) to identify a tight linkage between gene(s) controlling the quantitative trait and a molecular marker. We have determined the chromosomal locations of quantitative trait loci (QTLs) affecting grain yield under drought, anthesis-silking interval, and number of ears per plant. The F₃ families derived from the cross SD34(tolerant) × SD35 (intolerant) were evaluated for these traits in a two replicated experiment. RFLP analysis of the maize genome included non-radioactive DNA-DNA hybridization detection using chemiluminescence. To identify QTLs underlying tolerance to drought, the mean phenotypic performances of F₃ families were compared based on genotypic classification at each of 70 RFLP marker loci. The genetic linkage map assembled from these markers was in good agreement with previously published maps. The phenotypic correlations between yield and other traits were highly significant. In the combined analyses, genomic regions significantly affecting tolerance to drought were found on chromosomes 1,3,5,6, and 8. For yield, a total of 50% of the phenotypic variance could be explained by five putative QTLs. Different types of gene action were found for the putative QTLs for the three traits.     

Genetic variation of the rate of leaf appearance in maize: Possible yield prediction at the early stage

Summary Evidence of genetic variation for early vigour is presented using maize line × tester crosses. The leaf appearance rate and associated variance components are affected by a physiological stress attributable to the transition to autotrophic nutrition by the plant. At this stage, specific combining ability plays an important role in the total genetic variation. It is concluded that differing genetic controls exist in early and late material, and that this difference is also manifested in leaf initiation and elongation rates. The ground coverage rate, as a component of plant development, is genetically correlated to total dry matter yield. The genetic correlations vary according to the growth stage.