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.     

Grain yield and root characteristics of summer maize (Zea mays L.) under shade stress conditions

Low light is a major adversity affecting yield and quality of summer maize in the Huang‐Huai‐Hai region of China. We conducted a field experiment to explore the effects of shading on root development and yield formation in two summer maize hybrids (Zea mays L.), Denghai605 (DH605) and Zhengdan958 (ZD958). The experiment consisted of four treatments (CK: ambient sunlight, S1: shading from tasselling to physiological maturity stage, S2: shading from six‐leaf to tasselling stage, S3: shading from seeding to physiological maturity stage). Shading had a strong impact on the development of roots in the upper soil layer. Shading significantly decreased the root dry weight, root/shoot ratio, root length density, root absorption area and active absorption area. The results showed that treatments in a diminishing sequence of effects on root from S3, S1, S2 to CK. Overall, shading decreased the root morphologic and activity indices, and decreased yield in summer maize. During an average of 2 years, yields of ZD958 in S3, S2 and S1 decreased by 85%, 24% and 55%, yields of DH605 in S3, S2 and S1 decreased by 87%, 26% and 67%, in compare to CK. The results will be useful for hybrid selection and improving cultural practices for enhancing the maize shading resistance in Huang‐Huai‐Hai region.     

Effect of pollen storage by drying and deep-freezing on the expression of different agronomic traits in maize (Zea mays L.)

Summary Pollen from the inbred maize line HMv 1645 was used to study the effect of pollen treatments (drying and deep-freezing) on the phenotypic performance of the next generation. Fresh and artificially dried pollen samples with different water contents (56%, 18%, 13% and 10%) were used for sib pollinations immediately after collection or drying. Samples containing low amounts of water were then stored in liquid nitrogen for 7 days. Fertilization ability of the samples with 13% water was the highest after storage. Plant characteristics of the next generation originated from the seeds set by differently treated (fresh, dried to 13% water and deep-frozen) pollen were examined and statistically analysed. Pollen treatments due to the pollen storage procedure did not cause detectable changes in quantitative characters of the next generation.     

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.     

Varietal Differences in Development of Maize (Zea mays L.) Seedlings on Compacted Soils

Differences among open-pollinated tropical maize ( Zea mays L.) varieties in seedling development and establishment on compacted soils were studied. Soil dry density was used as an index of compaction. Three soil compaction levels and 12 traits associated with development and establishment of maize seedlings were investigated. A control (without compaction) was also included. Varietal differences were observed for most traits measured. Genetic differences for seedling development on compacted soil were detected. Varietal differences contributed about three times the contribution of compaction to total variability in the traits. Better seedling development and performance were obtained in moderately compacted soil than in the control. Shoot length, shoot dry weight and per cent dry matter in roots were good indicators of the tolerance of maize seedlings to compaction. A physiological strategy for maize seedling establishment on compacted soil was proposed. The implications of the results for seed testing were also highlighted. It was concluded that consideration should be given to the genotype of maize destined for use in ecologies prone to high soil densities. All varieties of maize grown in an agroecological zone could be screened to identify genotypes tolerant of higher soil densities. The seeds could then be multiplied and distributed to farmers.     

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.     

A QTL atlas for grain yield and its component traits in maize (Zea mays)

Grain yield and its component traits are essential targets in maize breeding. These traits are genetically complex and controlled by a large number of quantitative trait loci (QTL). The aim of this study was to compile reported QTL and major genes for grain yield and its component traits in a QTL atlas, as a valuable resource for the maize community. To this end, 1,177 QTL related to maize yield were collected from 56 studies published between 1992 and 2018. These QTL were projected to genetic map “IBM2 2008 Neighbors”, which led to the identification of 135 meta-QTL. Some genomic regions appear to be hotspots for yield-related meta-QTL, often affecting more than one of the investigated traits. Moreover, we catalogued 20 major maize loci associated with yield and identified 65 maize homologs of 21 rice yield-related genes. Interestingly, we found that a significant proportion of them are located in meta-QTL regions. Collectively, this study provides a reference for QTL fine-mapping and gene cloning, as well as for molecular marker-assisted breeding of yield-related traits in maize.     

Breeding for yield, in mixtures of common beans (Phaseolus vulgaris L.) and maize (Zea mays L.)

Summary Despite the growing industrialization, technification and transformation that is happening in the agriculture around the world, and despite that agricultural research has always concentrated its effort on sole crops, multiple cropping systems have historically been important for common bean production in tropical countries. The reasons for this fact, are economical and social, as well as biological. Bean breeders have always been questioned on their work, because the development of new varieties is usually done in sole crop, but the varieties are grown in either systems. This paper addresses a set of questions that are usually presented to the breeders, in light of the evidence obtained from many trials conducted in Brazil and in the U.S.A.: Will the genotypes bred for sole crop conditions, perform well when grown in intercrop; How different should a genotype be, for cultivation in intercropping compared to genotypes developed for sole crop conditions; Is there a need for special breeding programs for intercropping and How could a breeding program focus the question of multiple (associated) cropping?     

Genetic dissection of haploid male fertility in maize (Zea mays L.)

Haploid genome doubling is a key limiting step of haploid breeding in maize. Spontaneous restoration of haploid male fertility (HMF) provides a more promising method than the artificial doubling process. To reveal the genetic basis of HMF, haploids were obtained from the offspring of 285 F_2:3 families, derived from the cross Zheng58 × K22. The F_2:3 families were used as the female donor and Yu high inducer No. 1 (YHI‐1) as the male inducer line. The rates of HMF from each family line were evaluated at two field sites over two planting seasons. HMF displayed incomplete dominance. Transgressive segregation of haploids from F_2:3 families was observed relative to haploids derived from the two parents of the mapping population. A total of nine quantitative trait loci (QTL) were detected, which were distributed on chromosomes 1, 3, 4, 7 and 8. Three major QTL, qHMF3b, qHMF7a and qHMF7b were detected in both locations, respectively. These QTL could be useful to predict the ability of spontaneous haploid genome doubling, and to accelerate the haploid breeding process by introgression or aggregation of those QTL.     

Quantitative trait loci mapping of maize (Zea mays) ear traits under low-phosphorus stress

We constructed an F_2:3 population of 180 individuals from a cross between the maize genotypes 082 (high phosphorus use efficiency) and Ye107 (low phosphorus use efficiency). We used the population to perform quantitative trait loci (QTL) mapping for eight ear traits (ear diameter [ED], ear length [EL], ear weight [EW], ear kernel weight [EKW], row number per ear [ERN], bald tip length [BTL], kernel number per row [RKN] and number of seeds per plant [NSP]) in three low-phosphorus environments. A total of 36 QTL for ear traits were detected in at least one of three single environments and were located on all maize chromosomes except for chromosome 10; the explained phenotypic variation ranged from 5.91% to 12.80%. The QTL JAqEKW5-1, JAqEL3-1, JAqEW1-1, JAqBTL1-1 and JAqNSP5-2 were identified and stably expressed in single-environment and joint-environment analyses. Some novel QTL that have not been reported previously were also detected. Most of the identified QTL had stable effects in all low-phosphorus environments, suggesting that they may be useful for molecular breeding to develop low-phosphorus tolerant maize varieties.     

Development and mapping of gene-tagged SNP markers in laccases of maize (Zea mays L.)

Laccases, EC 1.10.3.2 or p -diphenol : dioxygen oxidoreductases, have been proposed to be involved in the oxidative polymerization of monolignols into lignins in plants. While 17 laccases have been identified in Arabidopsis , only five ( ZmLac1–5 ) have so far been identified in maize. By a bioinformatic approach, 14 putative laccases were identified in maize. One putative laccase was identical to ZmLac1 , while five were highly homologous to either ZmLac4 or ZmLac5 . Sequence alignment of allelic sequences enabled the development of TaqMan single nucleotide polymorphism (SNP) markers for nine putative laccases. Four of these gene-tagged SNP markers were validated in a doubled-haploid mapping population of 140 individuals, mapping these loci to chromosomes 1, 2, 3 and 7, respectively.     

Yield and Agronomic Characters of Tropical Maize (Zea mays L.) Cultivars under Different Irrigation Regimes

Drought spells are unpredictable under tropical conditions and can occur at every growth stage of the maize plants. Little is known about the reactions of tropical maize cultivars to water shortage. A set of Thai cultivars was examined in the field during the dry season. Three stress situations were imposed: prolonged stress throughout the growing season, pre- and post-anthesis stress. Pre-anthesis water stress delayed flowering and especially the data of silking. Thus, the anthesis-silking interval was wider. Prolonged water stress decreased grain yield mainly due to a low number of kernels and/or thousand kernel weight (TKW). Post-anthesis stress mainly reduced TKW grain set per ear. Relief from pre-anthesis stress increased grain yield by a grater number of kernels and higher TKW as compared to prolonged stress. Genotypic differences were high for the anthesis-silking interval following pre-anthesis stress. Low yielding, early but generally drought-stable cultivars existed as well as cultivars which were generally high yielding even under water stress because of a good residual yield. Some cultivars were resistant to pre-anthesis stress but not to post-anthesis stress and vice versa. It can be concluded that ample genotypic variability exists for adaptation to varying situations of pre-and post-anthesis stress within tropical maize cultivars.     

Assessment of salinity tolerance based upon seedling root growth response functions in maize (Zea mays L.)

Root growth response of 10-days-oldseedlings of 100 maize accessions at, 0 mM, 60 mM, 80 mM and 150 mM NaCl concentration was assessed in solution culture. The non-linear least square method was used to quantify the salt tolerance of maize accessions. The estimated salinity threshold, Ct, the NaCl concentration at which root growth starts to decrease, C0, and C50, the concentrations at which roots stop growing and 50% of its control value revealed considerable differences between the accessions. No general consistency for tolerance was, however, found between the estimates of Ct and C50. Different genetic systems appeared to be involved in controlling the inheritance of Ct and C50.Both Ct and C50 appeared to quantify accession tolerance, and the expression of root growth as a function of NaCl concentrations provides a useful guideline for salt tolerance. Estimates of broad sense heritability for relative root length were moderate in size (0.62 to 0.82), suggesting the scope for enhancing salt tolerance in maize through selection and breeding. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of water limitation on yield advantage and water use in wheat (Triticum aestivum L.)/maize (Zea mays L.) strip intercropping

Wheat (Triticum aestivum L.)/maize (Zea mays L.) strip intercropping is widely practiced in arid regions of northwestern China because of its high land use efficiency. However, its sustainability has been questioned because it consumes much more water than sole cropped wheat or maize. The present study was conducted to investigate the effects of water limitation on the yield advantage and water use of this system. Three field experiments were conducted in the Hetao Irrigation District in Inner Mongolia during the growing seasons of 2012–2014. Each experiment comprised two water applications, in which one was full irrigation and the other was a period of water limitation during the co-growth period of intercropping.The interspecific competition in wheat/maize intercropping was intensified by water stress. For water limitation applied during the wheat booting/maize V5 stage (Exp. I, second irrigation was not applied), the yield advantage of intercropped wheat (IW) over sole wheat was enhanced, whereas that of intercropped maize (IM) over sole maize was reduced compared with full irrigated treatments; for water limitation applied during the wheat jointing/maize V2 stage (Exp. II, first irrigation was not applied), the yield advantages of both IW and IM were greatly reduced; for water limitation applied during the wheat grain filling/maize V9 stage (Exp. III, third irrigation was not applied), the yield advantage of IW was slightly improved, whereas that of IM was reduced. The yield advantage of intercropping under limited irrigation was 25%, 3%, and 18% in Exps. I–III, respectively, whereas that under full irrigation ranged between 22 and 24%.Under well-watered conditions, wheat/maize intercropping used 24–29% more water than the weighted means of sole crops with the water use efficiency equivalent to sole crops. After the application of water limitation, 60 mm irrigation water was saved by intercropping every year, whereas the reduction of water use ranged from 25.1 to 70.8 mm; the changes in water use of intercropping relative to sole crops was reduced to 18–24%; the changes in water use efficiency stayed at nearly zero in Exps. I and III but decreased to a value of −13% in Exp. II. These results indicated that water limitation could be applied during wheat booting or filling stage in wheat/maize intercropping to save irrigation water in our study area.     

The early stress response of maize (Zea mays L.) to chloride salinity

Chloride is a micronutrient required for photosynthesis but when applied in the concentration of a macronutrient, it may also promote growth by regulating turgor. However, if chloride accumulates excessively, it can induce toxicity. The aim of this study was to identify physiological dysfunctions in maize (Zea mays L.) that arise in response to excessive chloride ion accumulation. For this, a novel water sensor was employed for the first time allowing the in vivo measurement of water content in the plant by using two near IR‐wavelengths with different absorption of water. This enabled to analyse whether water imbalances occurred. Chloride was given together with calcium as companying counter cation. Results show that most of the tested maize genotypes were able to maintain growth, photosynthesis and normal water content when stressed with concentrations as high as 757.1 mg chloride/kg soil dry matter. Leaf blades accumulated only 8.5 mg chloride/g dry matter, with the most genotypes not even showing salt stress necrosis at the leaves. A comparison between more tolerant and more sensitive genotypes revealed that restriction of chloride root‐to‐shoot translocation is a trait of chloride tolerance.     

Dissection of the genetic architecture for grain quality‐related traits in three RIL populations of maize (Zea mays L.)

To manipulate the composition of the maize kernel to meet future needs, an understanding of the molecular regulation of kernel quality‐related traits is required. In this study, the quantitative trait loci (QTL) for the concentrations of grain protein, starch and oil were identified using three sets of RIL populations in three environments. The genetic maps and the initial QTL were integrated using meta‐analyses. A total of 38 QTL were identified, including 15 in population 1, 12 in population 2 and 11 in population 3. The individual effects ranged from 2.87% to 13.11% of the phenotypic variation, with seven QTL each contributing over 10%. One common QTL was found for the concentrations of grain protein and starch in bin 3.09 in the three environments and the three RIL populations. Of the 38 initial QTL, 22 were integrated into eight mQTL by meta‐analysis. mQTL3 and mQTL8 of the key mQTL in which the initial QTL displayed R² > 10% included six and three initial QTL for grain protein and starch concentrations from two or three populations, respectively. These results will provide useful information for marker‐assisted selection to improve the quality of the maize kernel.     

Effect of Mulch from Selected Multipurpose Trees (MPTs) on Growth, Nitrogen Nutrition and Yield of Maize (Zea mays L.)

Leaf extracts and mulch from 14 multipurpose trees were used to test their effects on maize germination, growth and yield. Maize germination was significantly reduced by leaf extracts of all species with increasing extract concentration. The most drastic reductions were caused by Gliricidia sepium , Tetrapleura tetraptera , Lonchocarpus sireceus , Senna siamea and Leucaena leucocephala . Terminalia superba , Tetrapleura tetraptera , Pithecelobium dulce , Gliricidia sepium and Senna siamea significantly reduced maize root growth at the lowest extract concentration, while shoot length was most significantly reduced by Gliricidia sepium , Leucaena leucocephala , Alchornea coordifolia , Pithecelobium dulce , Terminalia superba , and Tetrapleura tetraptera at all concentrations. Growth of maize in pots and yield in the field were controlled by nitrogen supply and uptake. Fast-decomposing leaf material rich in nitrogen promoted growth and yield of maize. Thus Leucaena leucocephala and Gliricidia sepium , both nitrogen-fixing legumes with high nitrogen contents and fast-decomposing leaf materials, significantly increased maize growth and yield. This suggests that the negative effects of leaf extracts observed in the laboratory are ecologically irrelevant in the field.