Heterosis for leaf photosynthesis,grain yield and yield components in maize

Summary One of the primary avenues of improving the biological efficiency of crop plants is through the improvement of the leaf and canopy photosynthetic rates. However, the question whether the superiority of hybrids in respect of productivity potential could be traced, in retrospective fashion, to the photosynthetic parameters should be answered first. Once established and standardized, such indices could be streamlined in innovative breeding to predict the heterotic combinations for final yield formation. To answer this question, the photosynthetic rate and other components of photosynthetic efficiency were monitored among eight inbred stocks of maize (selected for variable photosynthetic rate from a previous study) and their all possible one-way crosses. The results demonstrated extensive heterosis in respect of photosynthetic rate and other photosynthetic indices which, in turn, was also realized in terms of higher biomass productivity and yield. Nonetheless, high leaf photosynthesis alone did not result in higher grain yield. On the contrary, component interaction among the photosynthetic indices like photosynthetic rate, leaf area/plant, number of leaves and chlorophyll content on one hand and the complementary gene action on the other, could be held responsible for higher yields in hybrids. Since the components of photosynthetic efficiency can predict heterosis for biomass and grain yield upto a reasonable extent, judicious incorporation of such indices in selection parameters for applied genetic protocols can add yet another dimension to the strategies for future yield improvements.     

Evaluation of environments for preliminary testing of maize for yield and stover digestibility

Summary The identification of environments suitable for selection should enable plant breeders to test reliably a larger number of genotypes with given resources. This research was undertaken to evaluate discrimination ability (DA) and prediction ability (PA) of eight environments involving two locations. Eckartsweier (Eck) 1 and Voelkenrode (Voe); 2 yer. 1984 (84) and 1985 (85); and two harvests, silage harvest (I) and later (II). It was based on dry matter yield of plant (PDMY), ear (EDMY) and stover (SDMY) and on in vitro digestible organic matter (IVDOM), neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL) of stover of 12 inbred lines and their 66 diallel crosses in maize (Zea mays L.). Linear regression coefficient of the performance of genotypes in an environment on that averaged across all environments and its degree of determination were used as measures of the DA and PA of environments, respectively. In hybrids, the DA of environments differed significantly for all traits except SDMY and ADL. Environment Eck851 showed better PA (0.63) than other environments for PDMY, EDMY, IVDOM, NDF, and ADF. Among the 12 two-environment combinations. EckI was a better predictor for PDMY, EDMY, IVDOM and NDF (PA0.80). The more productive environments showed better DA and PA for PDMY and FDMY than less productive ones. For IVDOM and NDF the first harvest provided better discrimination and prediction than the second harvest. In inbred lines the DA of environments differed significantly for EDMY only. In the present study, PA and DA seemed to be functions of the environments, PA was improved by combining two environments, and some environments seemed to be suitable for preliminary selection of genotypes for such diverse traits as yield and digestibility.     

Selection of sites for preliminary maize yield trials in the rainforest zone of South-Western Nigeria

Summary Grain-yield data obtained from 49 varieties of maize (Zea mays L.), grown at each of five sites for two years, were analyzed with the objective of identifying two or three sites where mean performance adequately predicted the mean performance of all five sites combined. Regression of overall varietal mean on site or site-combination means was done and the predictive potential of a site or combination of sites was judged by four criteria: (i) r²0.9, (ii) b1.0, (iii) high yield (i.e., yield greater than or equal to overall mean yield) and (iv) consistent yield (i.e., yield did not fall below the mean yield by more than 10%). In addition, a trial differentiation index, D, was used to compare the site combinations selected on the basis of the above criteria with the differentiation potential of all five sites combined. None of the five individual sites and the ten two-site combinations satisfactorily predicted overall mean yield. However, five three-sites combinations had values of D similar to that of all the five sites. Rank-correlation coefficients of the three-site combinations with all five sites ranged from 0.90 to 0.79. It was concluded that certain three-site combinations could be satisfactorily used for preliminary maize yield trials in the rainforest ecological zone of S.W. Nigeria.     

Stability and genetic improvement of maize yield in Argentina

Summary Yield stability and the genetic improvement of maize (Zea mays L.) grain yield in the humid pampas of Argentina were evaluated.Stability parameters were computed for 15 varieties grown in 35 environments. To analyze genetic improvement of grain yield, data were obtained from trials grown in four locations and twelve years. Two locations represented the typical maize area, while the remaining two were considered marginal. Simple correlation and linear regression coefficients were computed to study the relationships between yield, stability parameters, number of days to 50% tassel emergence, and year of release. Genetic improvement of grain yield was analyzed from linear regression of the average yield of the three highest-yielding varieties (as percentage of the average yield of five common checks) on year of trial.Yield differed significantly among varieties. Significant variety x environment (linear) interaction was also detected. Significant linear relationships were found between regression coefficient for yield (stability parameter 1) and days to tassel emergence, stability parameter 1 and yield, year of release and days to tassel emergence, and year of release and stability parameter 1. Thus, newer maize varieties tended to flower later and had greater responsiveness to favorable environments than did older varieties. Yields have increased for both the typical and marginal areas, with average yearly increases of 114 and 182 kg/ha, respectively.Breeding programs with nurseries located in the typical maize area have raised yield potential in both areas studied. However, if present trends continue, future releases could prove inadequate in low-yielding or short season environments.     

An evaluation for genetic variation within maize inbred lines maintained by sib-mating and self-pollination

Summary Maintaining maize (Zea mays L.) inbred lines without genetic change is an important concern of maize breeders and seed producers. Long-time inbred lines, however, have been shown to be genetically unstable, and with selection pressure acting on the resulting genetic variation, these lines have evolved into different strains. The objective of this study was to compare maintenance of line integrity under reproduction by sib-mating and self-pollination in ear-to-row progenies. Ten lines, varying from 5 to 35 generations of previous ear-to-row selfing, were maintained for 11 successive generations under each method of reproduction. Alternate generations were compared by using a randomized complete-block design with each family of lines as a separate experiment. Data were collected for 10 plant and yield traits: pollen shed and silk emergence dates, plant and ear heights, tassel branch number, kernel row number, ear length and diameter, grain yield, and 300-kernel weight. Summarizing for F-tests over all traits and experiments, 30% were significant among sib-mated generations and 56% among selfed generations. Also, overall generation means of the two methods were different for 51% of the comparisons, and the selfed lines were less vigorous for 79% of the significant comparisons. It was concluded that sib-mating lessens the effect of genetic instability and that a reproduction system of sib-mating with intermittent generations of selfing may be more beneficial than continuous ear-to-row selfing for certain lines.Joint contribution: USDA-ARS, and Journal Paper No. J-11880 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA 50011. Project No. 2194.     

Genetic diversity of Tainan-white maize inbred lines and prediction of single cross hybrid performance using RAPD markers

To evaluate the genetic diversity of 13 maize inbred lines, and to determine the correlation between genetic distance and single cross hybrid performance, we employed the randomly amplified polymorphic DNA(RAPD)- a PCR-based technique. Six of these lines came from the Taichung population, and others derived from seven different sites. Forty different primers were used to give a total of 646 reproducible amplification products, 547 (84.7%) of them being polymorphic. Genetic divergence was determined using Jaccard's similarity coefficient, and a final dendrogram was constructed by UPGMA (unweighted pair-group method with arithmetical averages) cluster analysis in the CLUSTER procedure of the SAS system. The RAPD analysis was a useful tool in determining the extent of genetic diversity among Tainan-white maize inbred lines in the present case. Cluster analysis showed that the 13 inbred lines could be classified into distinct heterotic groups. There was no significant linear regression of grain dry weight heterosis value and mean performance of hybrids on genetic distance. And their coefficients of determination(R²) are small, so that predictive value is limited. The present results showed that the Jaccard's similarity coefficients based on RAPD data cannot be used to precisely predict the F₁ hybrids yield performance and heterosis value. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Mass selection for kernel depth in early maize

Summary Effectiveness of mass selection for kernel depth in an early maize (Zea mays L.) synthetic, i.e. NDSAB, was evaluated in eight North Dakato environments. Four cycles of selection for deeper kernels were evaluated for grain yield and associated traits by using corresponding randomly selected subpopulations of NDSAB to adjust out inbreeding effects on selected subpopulations. General combining ability (GCA) effects of selected subpopulations also were evaluated for these traits.Subpopulations selected from one to four cycles for deep kernels were reciprocally crossed to corresponding randomly selected subpopulations in a factorial mating design. Subpopulations, selfed subpopulations, and subpopulation crosses were evaluated for yield and associated traits. Subpopulations selected for deep kernels generally had deeper kernels, higher yields, heavier kernels, more kernel rows per ear, and lower test weights than corresponding randomly selected subpopulations. Differences in GCA effects among selected and unselected subpopulations generally corresponded to differences between selected and unselected parental subpopulations. Inbreeding effects from selfing were observed for all traits, but no average heterosis was detected when subpopulation crosses were compared to parental subpopulations. Reciprocal differences were observed for all traits except kernel depth, but these differences were inconsistent relative to whether deep kerneled females made superior hybrids to unselected females.Results supported the use of mass selection for kernel depth as a means of grain yield improvement in the short term for NDSAB. However, negative associations of kernel depth with test weight may restrict long term use of this procedure.Journal Article No. 1505 of the North Dakota Agric. Exp. Stn., Fargo     

Phenotypic and genetic relationships between yield components in maize

Summary Physiological components of kernel development — LAG period, effective filling period duration (EFPD) and grain filling rate (GFR) — ear moisture release (U), ear size (row number and kernels per row), days from emergence to silking and number of leaves, were examinated on 45 F₁ hybrids (10×10 diallel cross) in order to study their genetic relationships with yield. Combining ability analysis revealed that all trait variability derived mainly from g.c.a. effects. LAG period and EFPD were the traits most affected by genotype-environment interaction.Covariation analysis (path method) based on mean phenotypic values and on g.c.a. effects yielded similar information. It is shown that GFR and EFPD are both related to plant yield, but GFR made the most important contribution. On the contrary, a significant relationship between yield and LAG was not detected. Ear size components were also positively related to yield and had negative effects on GFR. These results indicate that, for our material, the dry matter accumulation rate is the main limiting factor of yield.Considering s.c.a. effects, kernel number per row made the most important contribution.     

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.     

Genetic and environmental effects on abscisic acid accumulation in leaves of field-grown maize

Summary This study analyzes the components of phenotypic variation for abscisic acid (ABA) content in maize (Zea mays L.) leaves and the correlations with drought sensitivity index (DSI) and silk delay (SD), involved in the reaction to water deficit. Eight early- and seven medium-maturity inbreds were examined in field trials: in 1990 with low irrigation volume and in 1991 with low and high irrigation volumes. ABA concentration and DSI were investigated at growth stages (S) corresponding to stem elongation (S3), appearance of the first husks (S4), and mid-end of silking (S5). The ABA concentration was significantly higher in conditions of water deficit and in the later growth stage. The genetic component for ABA concentration attained higher relative values than those shown by DSI in the same growth stages and by SD; moreover, it increased from growth stage 3 to stage 5. The genotype × year and genotype × irrigation volume interactions were smaller for ABA concentration than for DSI and SD. The broad sense heritability on a plant basis, estimated in drought conditions, for ABA concentration ranged from 21.4 to 55.1% according to maturity group and growth stage. A wide variation was observed among lines for ABA concentration: the medium-maturity group showed a three-fold range (from 219 to 605 ng ABA g^–1 dry weight). No clear relationships between ABA concentration, DSI and SD were found. These results indicate the feasibility of a selection for ABA concentration within segregating populations derived from crosses between the inbred lines herein tested.Abbreviations ABA - abscisic acid - DSI - drought sensitivity index - DW - dry weight - SD - silk delay     

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.     

The impact of genotype x soil texture interaction on the efficiency of selection for yield in maize (Zea mays L.)

Summary This study was undertaken to investigate the implications of genotype x soil texture interaction on response to selection in maize. Mass honeycomb selection for yield was applied for 11 cycles from the F₂ of the single cross maize hybrid F68×NE2 in a field B with silty-clay-loam soil texture. Response to selection compared to the original single cross hybrid was estimated both in absence of competition and under solid stand in the selection field B and in a nearby field A differing in soil texture (clay-loam).A strong crossover type of interaction occurred both under solid stand and in the absence of competition in the two tests the improved population outyielded the hybrid in field B in the two densities, but lagged behing the hybrid in field A. The results suggest that interaction between genotype and soil texture might affect efficiency of selection detrimentally unless provision is taken for parallel selection early in the crop improvement program in fields differing in soil texture.     

Assessing the genetic diversity of Portuguese maize germplasm using microsatellite markers

A collection of Portuguese maize accessions representing a valuable source of genes for introduction into modern cultivars is stored at the Portuguese Plant Germplasm Bank (Banco Português de Germoplasma Vegetal—BPGV). To assess genetic diversity among inbreds, microsatellite analysis was carried out for 54 inbred lines representing the diversity of Portuguese dent and flint maize germplasm. Fifty American and other European elite inbreds were also analysed for comparison. Fifteen microsatellite loci distributed throughout the maize genome were chosen based on their repeat unit and base composition. A total of 80 alleles were detected with an average allele number of 5.33 per locus. Polymorphism information content (PIC) values and observed genetic distances showed the existence of large variability among inbreds. Cluster analysis indicated that almost all of the inbreds could be distinguished from each other and Portuguese inbreds were present in all clusters formed. These associations were consistent with the known pedigree records of the inbreds, confirming a mixed origin of Portuguese materials. Comparative analysis of microsatellite diversity among groups was established according to important traits for both breeding and line identification. This revealed that, although most of the genetic diversity (>95%) was attributable to differences among inbreds of different groups, the existence of phenotypic differentiation in endosperm colour, kernel type and cob colour could be suggested for grouping. These findings support the joint use of molecular and morphological traits in management of the germplasm collection. In this study, SSR markers proved to be effective to characterise and identify maize inbred lines, and demonstrate associations among them. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic variation in digestibility of forage maize (Zea mays L.) and its estimation by near infrared reflectance spectroscopy (NIRS). An analysis

Summary Genetic variation is a major source of differences in digestibility of forage maize. Other sources are environmental conditions, location effects, and cultural practice. These factors initiate differences in cell-wall content and cell-wall digestibility. In vitro techniques with rumen microflora account efficiently for such differences. Cell-wall content can be determined easily and can also be predicted properly by Near Infrared Reflectance Spectroscopy (NIRS). However, no currently available chemical and physical laboratory techniques are able to discriminate between digestible and indigestible cell-wall fractions. Present NIRS functions do not predict cell-wall digestibility properly in maize. If further research on NIRS functioning overcomes this barrier, NIRS can become a useful breeding tool for digestibility in forage maize.     

Six years of maize selection in ‘Kitale synthetic II’, ‘Ecuador 573’, and ‘kitale composite a’ using methods of the comprehensive breeding system

Summary Evaluation of the results of six years of selection in the Kitale maize breeding methods study proved reciprocal recurrent selection to be an effective interpopulation improvement method. Ear-to-row selection was effective in improving Kitale Composite A (KCA). Data from a diallel of the cycle-6 ear-to-row substrains showed that where less than 10 lines were selected, inbreeding appeared to depress gain, but differences among the various ear-to-row experiments were not significant. Intererossing the substrains was predicted to reduce the effect of inbreeding. Yield gains in reciprocal recurrent selection and ear-to-row selection were associated with increases in ears per 100 plants. The reciprocal recurrent selection variety cross yield gain was estimated at 3.5% per year (7%/cycle) and that of the best ear-to-row procedure at 2.6% per year. The variety cross syn-2 from reciprocal recurrent selection was predicted to improve at the same rate as the best ear-to-row procedure (E7) with one long growing season per year. Under a cropping system with two similar seasons per year, however, ear-to-row selection should be more effective than reciprocal recurrent selection or its derived syn-2. Mass selection at 10% selection intensity produced significant improvement, but not at 2% selection intensity.S₁ and three half-sib selection methods were ineffective in KCA. Inbreeding and the lack of linkage equilibrium in KCA were discussed as contributing. Theoretical substrain compositing required 30 to 40 lines selected to equal the effectiveness of ear-to-row selection with 10 lines. Full-sib selection was effective in improving yield at 1.2% per year. All the recurrent selection methods showed a significant increase in ears per 100 plants. Population size was shown to be important in reducing losses due to inbreeding.If a hybrid or variety cross is potentially useable, reciprocal recurrent selection provides the best population approach. Initially, the variety cross syn-2 could be released with no additional breeding effort. Transition to variety crosses or traditional hybrids from extracted lines could be done as farmer husbandry improves and seed production facilities develop. Otherwise, open-pollinated varieties superior to existing maizes can be effectively developed using ear-to-row selection. The other methods may be effective in breeding populations when larger effective population sizes are used, but they were not effective in KCA when only 10 lines were recombined each cycle.Contribution from the East African Agriculture and Forestry Research Organization of the East African Community, the U.S. Agency for International Development (Project 618-11-110-657, East African Food Crop Research), the Kenya Ministry of Agriculture, and the Agricultural Research Service of the U.S. Department of Agriculture.     

Cluster analysis of RFLP data from related maize inbred lines of the BSSS and LSC heterotic groups and comparison with pedigree data

Summary In this study, 31 elite inbred lines of maize (Zea mays L.) were analyzed with 149 clone-enzyme combinations for their respective RFLP profiles. Objectives were (1) to determine the utility of RFLPs for estimation of genetic similarties among 16 inbred lines from the Iowa Stiff Stalk Synthetic (BSSS) and among 15 inbred lines from the Lancaster Sure Crop (LSC) heterotic groups and (2) to compare genetic similarities based on molecular markers with those based on pedigree information. Coefficients of genetic similarity (GS) and coancestry (f) between pairs of lines from the same heterotic group were calculated from RFLP and pedigree data, respectively. For lines from the BSSS heterotic group, cluster analyses based on RFLP and pedigree data revealed similar associations. GS and f values were closely correlated (r=0.70) for related BSSS lines. For lines from the LSC heterotic group, considerable discrepancies existed between the GS and f values, especially for those pairs involving inbreds Va22 and Lo924. Effect of selection and/or erroneous pedigree records are discussed as possible explanations for the low correlation of GS and f values (r=0.07) for related LSC lines. RFLPs seem useful for investigation of relationships among maize inbreds, verification of pedigree records, and quantification of the degree of relatedness.     

Variation among Andean races of maize for cold tolerance during heterotrophic and early autotrophic growth

Cold in the initial growth stages is an important stressfactor for maize grown in regions with a temperate climate,particularly in case of early sowing. Sources of tolerancehave been identified in adapted genotypes, but promisinggenes for cold tolerance should also be found in materialdeveloped under the lower-temperature margins of the cropdistribution. This research was conducted in order to testAndean maize accessions for cold tolerance expressed duringboth the heterotrophic and early autotrophic growth stages.Experiments were conducted in controlled environments tostudy cold tolerance traits (germination %, germinationindex and plant growth rate) at continuous 10^°C (heterotrophic growth) and at varying 10–16^°C (autotrophic growth). An experiment was also performed inthe field with early sowing (both heterotrophic and autotrophic growth). In each experiment, a control trialwas conducted in more favourable conditions (i.e. continuous25^°C in a controlled environment or late planting inthe field) so that cold tolerance traits could also beexamined as the ratio between the stress and the controltrial. None of the accessions was superior for all coldtolerance traits. However, several Andean maize accessionsoutperformed the US Corn-belt hybrid checks for one or moretraits, both in heterotrophic and autotrophic growth. Overall, BOZM 855, PMS 636, Poblacion D, Poblacion E andBOZM 696 were the best accessions, suggesting that they canbe a promising source of genes for improving cold toleranceof adapted maize genotypes.     

Storage of maize (Zea mays L.) pollen at −196°C in liquid nitrogen

Summary The water content of pollen has a decisive influence on its storability in liquid nitrogen. Pollen with an initial high water content cannot be stored successfully at extremely low temperatures, so a certain degree of drying must be carried out before storage. Provided the viability of the pollen is not significantly reduced during drying, the pollen remains viable and fertile when kept at –196°C.     

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.