Evaluation of maize plant introductions for cold tolerance

Summary We evaluated cold-tolerance responses of 144 plant introductions (PI) of maize (Zea mays L.), attempting to include in our sample at least one PI adapted to each of the countries or ecological zones respresented in the maize germplasm collection at the North Central Regional Plant Introduction Station at Ames, Iowa, USA. Plant introductions were grown for 42 days in plastic boxes (26 cm long × 19 cm wide × 10 cm deep) in a growth chamber maintained at 10±1 °C. Cold-tolerance responses of each PI were evaluated by three traits: 1) percentage emergence (recorded 30 days after planting), 2) emergence index, an estimate of rate of emergence, and 3) seedling dry weight (sampled 42 days after planting). Estimates of variances and heritabilities for these three traits were large, suggesting that genotypic variation for cold tolerance in the maize germplasm collection would be sufficient to permit selection advance. Furthermore, genotypic correlations among the three traits were high: therefore, improvement by index selection should be possible. Correlations between the cold-tolerance traits and days from planting to 50% silk emergence (an estimate of maturity obtained at Ames. Iowa, USA) were low. Days from planting to 50% silking emergence for the 25 most cold-tolerant PI's ranged from 46 for PI 214279 from Canada to 106 for PI 331440 from Ethiopia. It should be possible, therefore, to develop cold-tolerant genotypes adapted to all latitudes.Contribution from the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa 50011, and the Agricultural Research Service, U.S. Department of Agriculture, cooperating. Journal Paper No. J-8780 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Projects No. 1018 and 2152     

Mass selection for improved cold and density tolerance of two maize populations

Summary Data of planting and plant density are two cultural practices influencing grain yield of maize (Zea mays L.). Our study was designed to evaluate the usefulness of a mass selection scheme to improve cold and density tolerance of the BS2 and BS3 maize populations. Populations were planted at an early planting date and a high plant density, and three cycles of mass selection for well-filled ears on erect plants were conducted at earch of three Corn Belt locations (i.e., Waseca, MN; Ames, IA; Portageville, MO).Results showed that selection improved cold tolerance traits of BS3, but not of BS2. Mass selection did not increase density tolerance of either population at any location. Selection did not improve response to planting dates, although the early planting date did improve agronomic performance and grain yield of all entries. We concluded that mass selection at high plant densities and early planting dates at diverse geographical locations did not produce cycles adapted to specific environmental conditions.     

Variation and inheritance of response to acetochlor among maize inbred lines and hybrids

Summary The variation of response to acetochlor was studied in a two-year experiment carried out by subjecting 18 maize (Zea mays L.) inbred lines to three herbicide rates (0, 2.5 and 5 l a.i./ha). In both years some inbred lines consistently exhibited an evident susceptibility, with symptoms consisting of the seedling curling up below the soil surface and causing impaired field emergence. The results were poor plant density and lower grain yield in comparison to control. In contrast, other lines showed a satisfactory level of tolerance.Then, to gather data on the inheritance of response to acetochlor, four tolerant inbreds (T) and four susceptible inbreds (S) were crossed to obtain four T×T, four S×S, four S×T and the corresponding four T×S two-way hybrids. These hybrids were studied together with parental lines by applying the same herbicide rates used in the previous trial. The S×S hybrids showed susceptibility to the herbicide and the T×T were tolerant, whereas the S×T and the T× S hybrids showed a tolerance very close to that of the T×T hybrids. No difference was found between S×T and the corresponding T×S hybrids as to herbicide response. On average, the 16 hybrids exhibited greater tolerance than the eight parental lines, with each hybrid group being more tolerant than its parental line group. These results indicate that tolerance to acetochlor is prevailingly dominant, that action of extranuclear genes should be ruled out, and that the level of plant vigour can affect herbicide reactiveness.     

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.     

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.     

Variation in cold tolerance and spring growth among Italian white clover populations

Seven populations collected at different altitudes in northern Italy, two Ladino breeding populations and two control cultivars (AberHerald and Grasslands Huia) of white clover (Trifolium repens L.) were included in a series of experiments analysing: (i) levels of cold tolerance using artificial and field-based methods; (ii) relationships between these measures of cold tolerance; (iii) components of spring yield, various physiological traits, and their relationships with cold tolerance. Rates of seedling and growing point mortality in the populations over winter, assessed in separate field experiments, were closely related (r = 0.85). Grasslands Huia showed the highest death rates, and material originating from high altitudes the lowest. The LT₅₀ value, i.e., the temperature at which 50% of the growing points would die, estimated by an artificial freezing test, was significantly correlated with field-based measures of seedling (r = 0.64) and growing point (r = 0.84) mortality. The existence of these correlations is of potential interest for the development of indirect selection criteria for complex and expensive-to-evaluate traits such as winter survival in field plots. Besides being reliable, in this study the artificial assessment was also sensitive, providing a greater degree of separation of the populations means than field-based measures. Of the several physiological traits (water content,concentrations of water soluble and total non-structural carbohydrate, and water soluble protein content of stolons) measured at a mid-winter sampling date under field conditions, the only character showing significant variation between populations was soluble protein content. There was a slight trend for material with a higher protein content to exhibit greater field-based values of cold tolerance. High altitude populations tended to have low spring yields. The highest spring yield was found in one of the Ladino populations. The study identified two populations which combined, to differing extents, cold tolerance and spring yield characteristics that would be of potential use in breeding for specified agronomic/climatic zones. This revised version was published online in August 2006 with corrections to the Cover Date.     

Glyphosate tolerance in maize (Zea mays L.). 1. Differential response among inbred lines

Summary Variation in susceptibility to the safe broad-spectrum herbicide glyphosate was investigated in maize. Eleven inbred lines, grown in a growth chamber, were evaluated for their tolerance to the herbicide at 2.4 mM (0.2 kg a.i. in 400 I ha^-1 of water). Following treatment with glyphosate at the three-leaf stage, significant variation in damage, expressed as visual injury ratings scored 7, 14 and 21 days after the application of the herbicide, was found. Effects on dry weight and shoot height were consistent with visual scores and the carbon-exchange rate was found to be a sensitive index of differential injury.Biochemical characterization of 5-enol-pyruvyl-shikimate-3-phosphate (EPSP) synthase, the main target of the herbicide, ruled out the possibility that this differential susceptibility was due to variations in the sensitivity of the enzyme. On the contrary, a positive correlation was found between in vivo tolerance and EPSP synthase levels, measured at different stages during seedling growth. This result suggests that a naturally occurring difference in EPSP synthase levels in the tissues may contribute to the differential response observed in vivo in maize inbreds.Abbreviations CER carbon-exchange rate - EPSP 5-enol-pyruvyl-shikimate-3-phosphate - ID₅₀ concentration causing 50% inhibition     

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.     

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.     

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.     

Six cycles of selection for adaptation in two exotic populations of maize

Summary The maturity rating of maize (Zea mays L.) grown in Galicia (Northwestern Spain) varies from FAO 200 to 600. Later germplasm has usually too much moisture content in the kernel at normal harvesting time so serious problems with storing could appear. Besides, farmers have tended to use earlier varieties in the last few years. All this imposes limitations on the amount of germplasm available to develop varieties adapted to this area.To study the possibility of adapting late, exotic material to the environment of the Atlantic coast of Galicia we carried out six cycles of individual selection on two non-adapted populations of maize (Purdue A and Purdue B). The criterion of selection was early silking and the criterion of response was moisture content of kernel at harvesting.The original populations and the populations obtained after each cycle of selection were crossed to the hybrids CM105 × CM109 (tester Reid) and H99 × H95 (tester Lancaster) and were evaluated in four environments to study the efficiency of the selection scheme. There were significant reductions in days to silking (6.1 and 6.7 days from cycle 0 to cycle 6 for Purdue A and Purdue B, respectively) and grain moisture at harvesting (3.0 and 3.9% from cycle 0 to cycle 6 for Purdue A and Purdue B, respectively). There were also reductions in plant height and yield in both populations.In general, the crosses population × Lancaster were higher for yield than the crosses population × Reid. Yield of the population crosses by both testers decreased after the six cycles of selection probably because of the earlier maturity of the selected populations. Some inbreeding depression may also have occurred.     

Glyphosate tolerance in maize (Zea mays L.). 2. Selection and characterization of a tolerant somaclone

Summary The progeny of 104 regenerated maize plants were screened for tolerance to the safe broad-spectrum herbicide glyphosate during seed germination and early growth. Seven somaclones showed varying degrees of resistance to the application of the herbicide at 1.2 mM (0.1 kg a.i. in 400 1 ha^-1 of water). Plants capable of a normal growth following treatment with 2.4 mM (0.2 kg ha^-1) glyphosate at the three leaf stage were selfed, and their progeny analyzed. A family able to tolerate the exposure to glyphosate at 2.4 mM was isolated and shown to maintain a photosynthetic rate comparable with control after the application of the herbicide.The selfed progeny of the tolerant somaclone was characterized as to the properties of two targets of glyphosate, the shikimate pathway enzymes 5-enol-pyruvyl-shikimate-3-phosphate (EPSP) synthase and 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase. In vitro tests ruled out the possibility that the tolerance was due to altered forms of these enzymes. Families showed significant variability with regard to EPSP and DAHP synthase levels, measured at different stages during seedling growth; however, not even these traits were correlated with in vivo response to glyphosate. The possible role of other physiological processes in determining the increased tolerance to the herbicide is discussed.Abbreviations DAHP 3-deoxy-D-arabino-heptulosonate-7-phosphate - EPSP 5-enol-pyruvyl-shikimate-3-phosphate - CER carbon-exchange rate - ID₅₀ concentration causing 50% inhibition     

Contribution of autochthonous maize populations for adaptation to European conditions

Early vigor, earliness and cold tolerance are the main potential contributions of European maize (Zea mays L.) for breeding programs for adaptation to areas with short growing seasons and cold springs. The objective of this research was to determine the potential contributions of populations from different European regions to breeding for adaptation. Six Spanish and six French maize populations differing on variability for earliness, vigor and cold tolerance were crossed in a complete diallel without reciprocals. The populations and their crosses were evaluated in the field and in a cold chamber. Minimum temperatures were the main environmental trait affecting genotype × environment interaction, probably due to the cold sensitivity of the genotypes with the best performance in the field. The best population cross, based on specific heterosis for adaptation-related traits in the field, was Viana × Rastrojero, but this cross was cold sensitive. Tuy × Lazcano should be the best choice for a breeding program for adaptation, based on performance in the field and cold tolerance. As conclusions, there was variability for earliness, vigor and cold tolerance among the populations and crosses involved in this study, being tolerant to cold conditions the populations with medium growing cycle originated in areas with short growing seasons. The highest yielding crosses were cold sensitive.     

Developmental expression of glutathione-S-transferase in maize and its possible connection with herbicide tolerance

Summary Crop improvement for tolerance to specific herbicides is an important breeding target, since molecules performing well with regard to environmental safety are frequently not completely selective for crops. The glutathione (GSH)/glutathione-S-transferase (GST) system is a general mechanism of detoxification that in higher plants may confer tolerance to some herbicides. GSH level and GST activity were measured in different maize inbred lines, in the absence or in the presence of EPTC (a thiocarbamate) and of Alachlor (a chloroacetanilide); a wide genetic variability was observed for these parameters, which appear to be involved in plant tolerance to herbicides. Isozyme analysis was performed on roots, leaves, scutellum, pollen, coleoptile, mesocotyl of the same inbreds: it revealed the presence of many GST forms in maize, showing high polymorphism; they are controlled by at least five genes, the expression of which is developmentally regulated in the different tissues analyzed.     

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     

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.     

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.     

Variation in the expression of specificity in two maize diseases

Summary Ten inbred lines from the open-pollinated maize variety Jarvis were selected from 51 randomly collected lines to represent a wide range of susceptibility to one isolate each of Bipolaris maydis or Colletotrichum graminicola. Ten isolates of each pathogen were selected for a range of virulence on a maize line with average resistance. Resistance and virulence ratings were based on lengths of lesions that developed on leaves of greenhouse-grown seedlings inoculated with 5 l droplets of suspensions of known spore concentrations. For each disease the ten maize lines were inoculated in all possible combinations with the ten pathogen isolates. The experiment was run six times with each pathogen. Analysis of variance for individual trials indicated a significant interaction between maize lines and B. maydis isolates in all six trials and between maize lines and C. graminicola isolates in four of six trials. For both diseases. the combined analysis over all six trials revealed no significant interaction. Apparently the expression of specificity in these host-pathogen interactions is variable.     

Variation and covariation of silage maize digestibility estimated from digestion trials with sheep

Summary Variation and covariation for agronomic and digestibility traits of silage maize are reported from a compilation of 22 years of experiments with standard sheep. Genotype effects of DOM and DCF were highly significant, even when genotypes were nested in earliness groups or brown-midrib hybrids discarded (Table 2). The genetic variance of crude fiber content was low, but the variance of the DCF was high. The genetic variance of DOM was about 4 times lower than genetic variance of DCF, but broad sense heritability of DOM was higher because of lower residual variance (Table 3). Genetic correlations between grain or crude fiber content and DOM had similar absolute values, 0.65, so each of these two traits was an important but not the unique determinant of silage maize quality. There was no correlation between DCF and grain or crude fiber content. Yield was not related to DOM or DCF within each group of earliness, allowing some quality improvement without agronomic drift (Table 4). Except for late hybrids, most of DOM differences between groups of earliness came from lowering of minimum value, while maximum values were similar. It was the contrary for DCF, with similar minimum values for all groups (Table 5). There was no obvious correlation between year of registration of hybrids and DOM or DCF, but extra new variation seemed obtained only for low values (Figs 1, 2; Table 6). IVDOM according to the APC process was a poor predictor of DOM, especially when brown-midrib hybrids and earliness effects were discarded; but because heritability of this trait was similar to DOM heritability, such enzymatic processes could probably be used to avoid drift towards poor DOM with hybrids bred for higher stalk strength.Abbreviations APC Amylase Pepsine Cellulase solubility - DDM sheep digestibility of dry matter - DOM sheep digestibility of organic matter - DCF sheep digestibility of crude fiber - DM dry matter - IVDCW in vitro digestibility of cell wall - IVDDM in vitro digestibility of dry matter - IVDOM in vitro digestibility of organic matter - NIRS near infra-red reflectance spectroscopy - NDF neutral detergent fiber - bm₁-brown-midrib-1 allele, bm₃-brown midrib-3 allele     

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.