Mapping of post-flowering drought resistance traits in grain sorghum: association between QTLs influencing premature senescence and maturity

The identification of genetic factors underlying the complex responses of plants to drought stress provides a solid basis for improving drought resistance. The stay-green character in sorghum (Sorghum bicolor L. Moench) is a post-flowering drought resistance trait, which makes plants resistant to premature senescence under drought stress during the grainfilling stage. The objective of this study was to identify quantitative trait loci (QTLs) that control premature senescence and maturity traits, and to investigate their association under post-flowering drought stress in grain sorghum. A genetic linkage map was developed using a set of recombinant inbred lines (RILs) obtained from the cross B35 × Tx430, which were scored for 142 restriction fragment length polymorphism (RFLP) markers. The RILs and their parental lines were evaluated for post-flowering drought resistance and maturity in four environments. Simple interval mapping identified seven stay-green QTLs and two maturity QTLs. Three major stay-green QTLs (SGA, SGD and SGG) contributed to 42% of the phenotypic variability (LOD 9.0) and four minor QTLs (SGB, SGI.1, SGI.2, and SGJ) significantly contributed to an additional 25% of the phenotypic variability in stay-green ratings. One maturity QTL (DFB) alone contributed to 40% of the phenotypic variability (LOD 10.0), while the second QTL (DFG) significantly contributed to an additional 17% of the phenotypic variability (LOD 4.9). Composite interval mapping confirmed the above results with an additional analysis of the QTL × Environment interaction. With heritability estimates of 0.72 for stay-green and 0.90 for maturity, the identified QTLs explained about 90% and 63% of genetic variability for stay-green and maturity traits, respectively. Although stay-green ratings were significantly correlated (r=0.22, P ≤ 0.05) with maturity, six of the seven stay-green QTLs were independent of the QTLs influencing maturity. Similarly, one maturity QTL (DFB) was independent of the stay-green QTLs. One stay-green QTL (SGG), however, mapped in the vicinity of a maturity QTL (DFG), and all markers in the vicinity of the independent maturity QTL (DFB) were significantly (P ≤ 0.1) correlated with stay-green ratings, confounding the phenotyping of stay-green. The molecular genetic analysis of the QTLs influencing stay-green and maturity, together with the association between these two inversely related traits, provides a basis for further study of the underlying physiological mechanisms and demonstrates the possibility of improving drought resistance in plants by pyramiding the favorable QTLs. Received: 10 October 1998 / Accepted: 12 July 1999     

QTL analysis of drought-related traits and grain yield in relation to genetic variation for leaf abscisic acid concentration in field-grown maize

Abscisic acid (ABA) concentration is a quantitatively inherited trait which plays a pivotal role in the response of plants to drought stress. A recent study identified 17 quantitative trait loci (QTLs) controlling bulk-leaf ABA concentration (L-ABA) in a maize (Zea mays L.) population of 80 F4 random families tested for two years under droughted field conditions. Sixteen of the QTL regions influencing L-ABA also harboured QTLs for one or more of the following traits: stomatal conductance, a drought sensitivity index, leaf temperature, leaf relative water content, anthesis-silking interval, and grain yield. The analysis of the effects of each QTL region on the investigated traits indicated that L-ABA mainly represented an indicator of the level of drought stress experienced by the plant at the time of sampling because an increase in L-ABA was most commonly associated with a decrease in both stomatal conductance and grain yield as well as an increase in leaf temperature. Opposite results were observed at one QTL region on chromosome 7 near the RFLP locus asg8. A model is presented to interpret these contrasting results in terms of pleiotropic effects.Key words: Abscisic acid, ABA, drought stress, quantitative trait locus (QTL), molecular markers, Zea mays.      

Dealing with the genotype x environment interaction via a modelling approach: a comparison of QTLs of maize leaf length or width with QTLs of model parameters

Quantitative genetics of adaptive traits is made difficult by the genotypexenvironment interaction. A classical assumption is that QTLs identified in both stressed and control conditions correspond to constitutive traits whereas those identified only in stressed treatments are stress-specific and correspond to adaptive traits. This hypothesis was tested by comparing, in the same set of experiments, two ways of analysing the genetic variability of the responses of maize leaf growth to water deficit. One QTL detection was based on raw phenotypic traits (length and width of leaf 6) of 100 recombinant inbred lines (RILs) in four experiments with either well-watered or stressing conditions in the field or in the greenhouse. Another detection followed a method proposed recently which consists of analysing intrinsic responses of the same RILs to environmental conditions, determined jointly over several experiments. QTLs of three responses were considered: (i) leaf elongation rate per unit thermal time in the absence of stress, (ii) its response to evaporative demand in well-watered plants, and (iii) its response to soil water status in the absence of evaporative demand. The QTL of leaf length differed between experiments, but colocalized in seven cases out of 13 with QTLs of the intrinsic leaf elongation rate, even in experiments with stressing conditions. No colocalization was found between QTLs of leaf length under water deficit and QTLs of responses to air or soil water status. By contrast, QTLs of leaf width colocalized in all experiments, regardless of environmental conditions. The classical method of identifying the QTL of constitutive versus adaptive traits therefore did not apply to the experiments presented here. It is suggested that identification of the QTL of parameters of response curves provides a promising alternative for dealing with the genetic variability of adaptive traits.     

Two consensus quantitative trait loci clusters controlling anthesis–silking interval, ear setting and grain yield might be related with drought tolerance in maize

Unravelling the molecular basis of drought tolerance will provide novel opportunities for improving crop yield under water-limited conditions. The present study was conducted to identify quantitative trait loci (QTLs) controlling anthesis–silking interval (ASI), ear setting percentage (ESP) and grain yield (GY). The mapping population included 234 F₂ plants derived from the cross X178 (drought tolerant) × B73 (drought susceptible). The corresponding F_2:3 progenies, along with their parents, were evaluated for the above-mentioned traits under both well-watered and water-stressed field conditions in three different trials carried out in central and southern China. Interval mapping and composite interval mapping identified 45 and 65 QTLs for the investigated traits, respectively. Two QTL clusters influencing ASI and ESP on chromosomes 1 (bin 1.03) and 9 (bins 9.03–9.05) were identified in more than two environments, showing sizeable additive effects and contribution to phenotypic variance; these two QTL clusters influenced GY only in one environment. No significant interaction was detected between the two genomic regions. A comparative analysis of these two QTL clusters with the QTLs controlling maize drought tolerance previously described in three mapping populations confirmed and extended their relevance for marker-assisted breeding to improve maize production under water-limited conditions.     

Quantitative trait loci for yield and correlated traits under high and low soil nitrogen conditions in tropical maize

The first objective of this study was to map and characterize quantitative trait loci (QTL) for grain yield (GY) and for secondary traits under varying nitrogen (N) supply. To achieve this objective, a segregating F_2:3 population previously developed for QTL mapping under water-limited conditions was used. The population was evaluated in Mexico under low N conditions in the dry winter season and under low and high N conditions in the wet summer season. From eight QTLs identified for GY under low N conditions, two were also detected under high N conditions. Five QTLs were stable across the two low N environments and five co-localized with QTLs identified for the anthesis-silking interval (ASI) or for the number of ears per plant (ENO) under low N conditions. The percentage of the phenotypic variance expressed by all QTLs for ASI and ENO was quite different when evaluated under low N conditions during the dry winter (40% for ASI and 22% for ENO) and the wet summer seasons (22% for ASI and 46% for ENO). The results suggest optimizing different breeding strategies based on selection index depending on the growing season. Good QTL colocalization was observed for ASI (four QTLs) and ENO (three QTLs) when looking at QTL identified under low N and water-limited conditions in the same population. The results suggest that that both secondary traits can be used in breeding programs for simultaneous improvement of maize against low N and drought stresses.     

Meta-analysis of quantitative trait loci for grain yield and component traits under reproductive-stage drought stress in an upland rice population

A recombinant inbred population developed from a cross between high-yielding lowland rice (Oryza sativa L.) subspecies indica cv. IR64 and upland tropical rice subspecies japonica cv. Cabacu was used to identify quantitative trait loci (QTLs) for grain yield (GY) and component traits under reproductive-stage drought stress. One hundred fifty-four lines were grown in field trials in Indonesia under aerobic conditions by giving surface irrigation to field capacity every 4 days. Water stress was imposed for a period of 15 days during pre-flowering by withholding irrigation at 65 days after seeding. Leaf rolling was scored at the end of the stress period and eight agronomic traits were evaluated after recovery. The population was also evaluated for root pulling force, and a total of 201 single nucleotide polymorphism markers were used to construct the molecular genetic linkage map and QTL mapping. A QTL for GY under drought stress was identified in a region close to the sd1 locus on chromosome 1. QTL meta-analysis across diverse populations showed that this QTL was conserved across genetic backgrounds and co-localized with QTLs for leaf rolling and osmotic adjustment (OA). A QTL for percent seed set and grains per panicle under drought stress was identified on chromosome 8 in the same region as a QTL for OA previously identified in three different populations.     

QTLs mapping of morphological traits related to salt tolerance in <Emphasis Type="Italic">Medicago truncatula</Emphasis>

To understand the complex inheritance of tolerance to salt stress in Medicago truncatula, quantitative trait loci (QTLs) analysis was performed using a set of recombinant inbred lines (RILs) derived from a cross between the tolerant line Jemalong A17 and susceptible line F83005.5. The RILs and parental lines were grown in individual pots filled with sterilized sand in a greenhouse under 0 and 50 mM NaCl. Plants were harvested after a period of 60 days. Fourteen quantitative traits related to aerial and root growths were measured. Broad-sense heritability of measured traits ranged from 0.21 to 0.83 and from 0.05 to 0.62 in control and in salt-stressed conditions, respectively. Established correlations between measured traits are dependent on treatment effect. We identified and mapped 10 QTLs in control conditions and 19 in salt stress. No major QTL was identified indicating that tolerance to salt stress is governed by several genes with low effects. The QTLs detected under control and under salt-stressed conditions almost did not share the same map locations suggesting that the loci that are not stable across treatments reflect adaptation to this constraint. The maximum of QTLs was observed on the chromosome 8. The usefulness of these QTLs, identified in greenhouse conditions, for marker-assisted selection should therefore be evaluated under field conditions, and validated in other genetic backgrounds.     

Genomic dissection of drought resistance in durum wheat × wild emmer wheat recombinant inbreed line population

Drought is the major factor limiting wheat productivity worldwide. The gene pool of wild emmer wheat, Triticum turgidum ssp. dicoccoides , harbours a rich allelic repertoire for morpho-physiological traits conferring drought resistance. The genetic and physiological bases of drought responses were studied here in a tetraploid wheat population of 152 recombinant inbreed lines (RILs), derived from a cross between durum wheat (cv. Langdon) and wild emmer (acc# G18-16), under contrasting water availabilities. Wide genetic variation was found among RILs for all studied traits. A total of 110 quantitative trait loci (QTLs) were mapped for 11 traits, with LOD score range of 3.0–35.4. Several QTLs showed environmental specificity, accounting for productivity and related traits under water-limited (20 QTLs) or well-watered conditions (15 QTLs), and in terms of drought susceptibility index (22 QTLs). Major genomic regions controlling productivity and related traits were identified on chromosomes 2B, 4A, 5A and 7B. QTLs for productivity were associated with QTLs for drought-adaptive traits, suggesting the involvement of several strategies in wheat adaptation to drought stress. Fifteen pairs of QTLs for the same trait were mapped to seemingly homoeologous positions, reflecting synteny between the A and B genomes. The identified QTLs may facilitate the use of wild alleles for improvement of drought resistance in elite wheat cultivars.     

干旱胁迫下水稻叶片光合速率与叶绿素含量的相关性及其基因定位

以水稻重组自交系珍汕97B×IRAT109 F9代群体195个株系为材料,用213个简单重复系列（SSR）标记构建了基于该群体的连锁图谱,对水稻叶片叶绿素含量和光合速率在干旱和正常条件下的数量性状位点（QTL）和双基因互作进行了分析,同时分析了叶绿素含量与光合速率的相关关系. 结果表明：叶绿素含量与光合速率在正常供水下呈极显著正相关（r=0.185　7,表示在1%水平上显著）,但在干旱下则表现无关（r=0.076　6）.控制叶绿素含量的基因很复杂,主效QTL有13个,位于1、2、3、4、5、6、10号染色体上;其中,在干旱处理下检测到的主效QTL有6个,位于1、2、3、4、5号染色体上;在正常供水下检测到的主效QTL有7个,位于2、3、4、6、10号染色体上.在干旱和正常条件下它们分别解释了47.39%和56.19%的表型变异;在2种处理下均检出的主效QTL是2、3、4号染色体上的qCC2a、qCC2b、qCC3a、qCC3c、 qCC4a、 qCC4b; 它们位于同一染色体的相同区段.在干旱和正常条件下检测到4个QTL与光合速率有关;其中干旱下有3个(qPR2、 qPR10、 qPR11),正常条件下1个(qPR10).它们分别被定位于2、10、11号染色体,共解释13.94%的表型变异. 叶绿素含量互作效应位点有16对,涉及除10号染色体外的所有染色体;干旱下,有4对互作基因,共解释1857%的表型变异,分别位于1-7、2-4、5-8、6-12号染色体上;正常供水下,有12对互作基因,共解释38.49%的表型变异,分别位于1-3、1-4、1-8、2-4、2-5、3-5、4-11、4-12、5-9、7-12、8-11 号染色体上,其中3-5号染色体不同区段上有两对互作效应位点.     

Identification of QTLs for root characteristics in maize grown in hydroponics and analysis of their overlap with QTLs for grain yield in the field at two water regimes

We investigated the overlap among quantitative trait loci (QTLs) in maize for seminal root traits measured in hydroponics with QTLs for grain yield under well-watered (GY-WW) and water-stressed (GY-WS) field conditions as well as for a drought tolerance index (DTI) computed as GY-WS/GY-WW. In hydroponics, 11, 7, 9, and 10 QTLs were identified for primary root length (R1L), primary root diameter (R1D), primary root weight (R1W), and for the weight of the adventitious seminal roots (R2W), respectively. In the field, 7, 8, and 9 QTLs were identified for GY-WW, GY-WS, and DTI, respectively. Despite the weak correlation of root traits in hydroponics with GY-WW, GY-WS, and DTI, a noticeable overlap between the corresponding QTLs was observed. QTLs for R2W most frequently and consistently overlapped with QTLs for GY-WW, GY-WS, and/or DTI. At four QTL regions, an increase in R2W was positively associated with GY-WW, GY-WS, and/or DTI. A 10 cM interval on chromosome 1 between PGAMCTA205 and php20644 showed the strongest effect on R1L, R1D, R2W, GY-WW, GY-WS, and DTI. These results indicate the feasibility of using hydroponics in maize to identify QTL regions controlling root traits at an early growth stage and also influencing GY in the field. A comparative analysis of the QTL regions herein identified with those described in previous studies investigating root traits in different maize populations revealed a number of QTLs in common.     

正常与水分胁迫下水稻叶片叶绿素含量的QTL分析

随着分子标记技术的发展,利用不同的遗传群体对叶绿素的分子遗传机理进行了一些探索,定位了一些控制叶绿素含量的数量性状基因座(Quantitative trait loci, QTL)。该研究着眼于当前干旱严重影响农业生产的形势,以水稻重组自交系‘珍汕97B’בIRAT109’ F₉代群体195个株系为材料,在正常与水分胁迫环境下研究叶片叶绿素含量与光合速率的变化及相关性,定位不同水分条件下影响叶绿素含量的QTL,为阐明干旱环境下水稻叶绿素含量的分子遗传机理、分子标记辅助育种和节水抗旱稻培育提供理论基础和依据。研究表明叶绿素含量与光合速率在正常供水下呈极显著正相关(r=0.185 7^**),但在干旱下则表现无关(r=0.076 6)。QTL定位共检测到13个影响叶绿素含量的主效QTL,分别位于第1、2、3、4、5、6、10染色体:其中在干旱处理下检测到6个,其联合贡献率为47.39%;在正常供水下检测到7个,联合贡献率达56.19%。检测到显著互作效应位点16对:其中干旱处理下有4对显著互作,联合贡献率为18.57%;正常供水下有12对显著互作,联合贡献率达38.49%。     

QTL analysis of shading sensitive related traits in maize under two shading treatments

During maize development and reproduction, shading stress is an important abiotic factor influencing grain yield. To elucidate the genetic basis of shading stress in maize, an F(2:3) population derived from two inbred lines, Zhong72 and 502, was used to evaluate the performance of six traits under shading treatment and full-light treatment at two locations. The results showed that shading treatment significantly decreased plant height and ear height, reduced stem diameter, delayed day-to-tassel (DTT) and day-to-silk (DTS), and increased anthesis-silking interval (ASI). Forty-three different QTLs were identified for the six measured traits under shading and full light treatment at two locations, including seven QTL for plant height, nine QTL for ear height, six QTL for stem diameter, seven QTL for day-to-tassel, six QTL for day-to-silk, and eight QTL for ASI. Interestingly, three QTLs, qPH4, qEH4a, and qDTT1b were detected under full sunlight and shading treatment at two locations simultaneously, these QTL could be used for selecting elite hybrids with high tolerance to shading and high plant density. And the two QTL, qPH10 and qDTS1a, were only detected under shading treatment at two locations, should be quit for selecting insensitive inbred line in maize breeding procedure by using MAS method.     

Combining quantitative trait Loci analysis and an ecophysiological model to analyze the genetic variability of the responses of maize leaf growth to temperature and water deficit

Ecophysiological models predict quantitative traits of one genotype in any environment, whereas quantitative trait locus (QTL) models predict the contribution of alleles to quantitative traits under a limited number of environments. We have combined both approaches by dissecting into effects of QTLs the parameters of a model of maize (Zea mays) leaf elongation rate (LER; H. Ben Haj Salah, F. Tardieu [1997] Plant Physiol 114: 893-900). Response curves of LER to meristem temperature, water vapor pressure difference, and soil water status were established in 100 recombinant inbred lines (RILs) of maize in six experiments carried out in the field or in the greenhouse. All responses were linear and common to different experiments, consistent with the model. A QTL analysis was carried out on the slopes of these responses by composite interval mapping confirmed by bootstrap analysis. Most QTLs were specific of one response only. QTLs of abscisic acid concentration in the xylem sap colocalized with QTLs of response to soil water deficit and conferred a low response. Each parameter of the ecophysiological model was computed as the sum of QTL effects, allowing calculation of parameters for 11 new RILs and two parental lines. LERs were simulated and compared with measurements in a growth chamber experiment. The combined model accounted for 74% of the variability of LER, suggesting that it has a general value for any RIL under any environment.     

QTL Mapping of Agronomic Waterlogging Tolerance Using Recombinant Inbred Lines Derived from Tropical Maize (Zea mays L) Germplasm

Waterlogging is an important abiotic stress constraint that causes significant yield losses in maize grown throughout south and south-east Asia due to erratic rainfall patterns. The most economic option to offset the damage caused by waterlogging is to genetically incorporate tolerance in cultivars that are grown widely in the target agro-ecologies. We assessed the genetic variation in a population of recombinant inbred lines (RILs) derived from crossing a waterlogging tolerant line (CAWL-46-3-1) to an elite but sensitive line (CML311-2-1-3) and observed significant range of variation for grain yield (GY) under waterlogging stress along with a number of other secondary traits such as brace roots (BR), chlorophyll content (SPAD), % stem and root lodging (S&RL) among the RILs. Significant positive correlation of GY with BR and SPAD and negative correlation with S&RL indicated the potential use of these secondary traits in selection indices under waterlogged conditions. RILs were genotyped with 331 polymorphic single nucleotide polymorphism (SNP) markers using KASP (Kompetitive Allele Specific PCR) Platform. QTL mapping revealed five QTL on chromosomes 1, 3, 5, 7 and 10, which together explained approximately 30% of phenotypic variance for GY based on evaluation of RIL families under waterlogged conditions, with effects ranging from 520 to 640 kg/ha for individual genomic regions. 13 QTL were identified for various secondary traits associated with waterlogging tolerance, each individually explaining from 3 to 14% of phenotypic variance. Of the 22 candidate genes with known functional domains identified within the physical intervals delimited by the flanking markers of the QTL influencing GY and other secondary traits, six have previously been demonstrated to be associated with anaerobic responses in either maize or other model species. A pair of flanking SNP markers has been identified for each of the QTL and high throughput marker assays were developed to facilitate rapid introgression of waterlogging tolerance in tropical maize breeding programs.     

Mapping QTLs associated with drought resistance in sorghum (Sorghum bicolor L. Moench)

Drought is a major abiotic stress factor limiting crop production. Identification of genetic factors involved in plant responses to drought stress will provide a solid foundation to improve drought resistance. Sorghum is well adapted to hot dry environments and regarded as a model for studying drought resistance among the grasses. Significant progress in genome mapping of this crop has also been made. In sorghum, rapid premature leaf death generally occurs when water is limited during the grain filling period. Premature leaf senescence, in turn, leads to charcoal rot, stalk lodging, and significant yield loss. More than 80% of commercial sorghum hybrids in the United States are grown under non-irrigated conditions and although most of them have pre-flowering drought resistance, many do not have any significant post-flowering drought resistance. Stay-green is one form of drought resistance mechanism, which gives sorghum resistance to premature senescence under soil moisture stress during the post-flowering period. Quantitative trait locus (QTL) studies with recombinant inbred lines (RILs) and near-isogenic lines (NILs) identified several genomic regions associated with resistance to pre-flowering and post-flowering drought stress. We have identified four genomic regions associated with the stay-green trait using a RIL population developed from B35 × Tx7000. These four major stay-green QTLs were consistently identified in all field trials and accounted for 53.5% of the phenotypic variance. We review the progress in mapping stay-green QTLs as a component of drought resistance in sorghum. The molecular genetic dissection of the QTLs affecting stay-green will provide further opportunities to elucidate the underlying physiological mechanisms involved in drought resistance in sorghum and other grasses.     

Targeted discovery of quantitative trait loci for resistance to northern leaf blight and other diseases of maize

To capture diverse alleles at a set of loci associated with disease resistance in maize, heterogeneous inbred family (HIF) analysis was applied for targeted QTL mapping and near-isogenic line (NIL) development. Tropical maize lines CML52 and DK888 were chosen as donors of alleles based on their known resistance to multiple diseases. Chromosomal regions (“bins”; n = 39) associated with multiple disease resistance (MDR) were targeted based on a consensus map of disease QTLs in maize. We generated HIFs segregating for the targeted loci but isogenic at ~97% of the genome. To test the hypothesis that CML52 and DK888 alleles at MDR hotspots condition broad-spectrum resistance, HIFs and derived NILs were tested for resistance to northern leaf blight (NLB), southern leaf blight (SLB), gray leaf spot (GLS), anthracnose leaf blight (ALB), anthracnose stalk rot (ASR), common rust, common smut, and Stewart’s wilt. Four NLB QTLs, two ASR QTLs, and one Stewart’s wilt QTL were identified. In parallel, a population of 196 recombinant inbred lines (RILs) derived from B73 × CML52 was evaluated for resistance to NLB, GLS, SLB, and ASR. The QTLs mapped (four for NLB, five for SLB, two for GLS, and two for ASR) mostly corresponded to those found using the NILs. Combining HIF- and RIL-based analyses, we discovered two disease QTLs at which CML52 alleles were favorable for more than one disease. A QTL in bin 1.06–1.07 conferred resistance to NLB and Stewart’s wilt, and a QTL in 6.05 conferred resistance to NLB and ASR.     

Sensitivities of rice grain yield and other panicle characters to late-stage drought stress revealed by phenotypic correlation and QTL analysis

A lowland rice variety (Zhenshan97B) was crossed with an upland variety (IRAT109) to construct a set of recombinant inbred lines (RILs). The population was evaluated under both well-watered (control) and drought-stress (drought) conditions for 2 years. Panicle water potential (PWP), panicle length (PL), grain number per plant (GNP), primary branch number (PBN), second branch number (SBN), spikelet density (SPD), and dry grain weight per plant (DGW) were measured. Phenotypic correlation and path analysis were used to interpret the ranking of importance of other panicle traits to grain yield. Comparison of such rankings under control and drought conditions showed varied responses of panicle traits to drought stress. It was indicated that GNP was an important contributor to DGW under control, and even more important under drought. Thirty-two quantitative trait loci (QTLs) for panicle traits and dry grain weight were identified, with contribution rates ranging from 3.33% to 22.66%. Eleven epistatic QTLs were detected. Cases of collocated QTLs under control and drought were found for PL, SPD, GNP, PBN, and SBN, but not for DGW and PWP. The effectiveness of selection under normal and stressed conditions is discussed.     

Identification of quantitative trait loci under drought conditions in tropical maize. 1. Flowering parameters and the anthesis-silking interval

Drought is an important climatic phenomenon which, after soil infertility, ranks as the second most severe limitation to maize production in developing countries. When drought stress occurs just before or during the flowering period, a delay in silking is observed, resulting in an increase in the length of the anthesis-silking interval (ASI) and in a decrease in grain yield. Selection for reduced ASI in tropical open-pollinated varieties has been shown to be correlated with improved yields under drought stress. Since efficient selection for drought tolerance requires carefully managed experimental conditions, molecular markers were used to identify the genomic segments responsible for the expression of ASI, with the final aim of developing marker-assisted selection (MAS) strategies. An F₂population of 234 individuals was genotyped at 142 loci and F₃ families were evaluated in the field under several water regimes for male flowering (MFLW), male sterility (STER), female flowering (FFLW) and ASI. The genetic variance of ASI increased as a function of the stress intensity, and the broad-sense heritabilites of MFLW, FFLW and ASI were high under stress conditions, being 86%, 82% and 78%, respectively. Putative quantitative trait loci (QTLs) involved in the expression of MFLW and/or FFLW under drought were detected on chromosomes 1, 2, 4, 5, 8, 9 and 10, accounting for around 48% of the phenotypic variance for both traits. For ASI, six putative QTLs were identified under drought on chromosomes 1, 2, 5, 6, 8 and 10, and together accounted for approximately 47% of the phenotypic variance. Under water stress conditions, four QTLs were common for the expression of MFLW and FFLW, one for the expression of ASI and MFLW, and four for the expression of ASI and FFLW. The number of common QTLs for two traits was related to the level of linear correlation between these two traits. Segregation for ASI was found to be transgressive with the drought-susceptible parent contributing alleles for reduced ASI (4 days) at two QTL positions. Alleles contributed by the resistant line at the other four QTLs were responsible for a 7-day reduction of ASI. These four QTLs represented around 9% of the linkage map, and were stable over years and stress levels. It is argued that MAS based on ASI QTLs should be a powerful tool for improving drought tolerance of tropical maize inbred lines.     

Maize leaf temperature responses to drought: Thermal imaging and quantitative trait loci (QTL) mapping

Leaf temperature has been shown to vary when plants are subjected to water stress conditions. Recent advances in infrared thermography have increased the probability of recording drought tolerant responses more accurately. The aims of this study were to identify the effects of drought on leaf temperature using infrared thermography. Furthermore, the genomic regions responsible for the expression of leaf temperature variation in maize seedlings (Zea mays L.) were explored. The maize inbred lines Zong3 and 87-1 were evaluated using infrared thermography and exhibited notable differences in leaf temperature response to water stress. Correlation analysis indicated that leaf temperature response to water stress played an integral role in maize biomass accumulation. Additionally, a mapping population of 187 recombinant inbred lines (RILs) derived from a cross between Zong3 and 87-1 was constructed to identify quantitative trait loci (QTL) responsible for physiological traits associated with seedling water stress. Leaf temperature differences (LTD) and the drought tolerance index (DTI) of shoot fresh weight (SFW) and shoot dry weight (SDW) were the traits evaluated for QTL analysis in maize seedlings. A total of nine QTL were detected by composite interval mapping (CIM) for the three traits (LTD, RSFW and RSDW). Two co-locations responsible for both RSFW and RSDW were detected on chromosomes 1 and 2, respectively, which showed common signs with their trait correlations. Another co-location was detected on chromosome 9 between LTD and shoot biomass, which provided genetic evidence that leaf temperature affects biomass accumulation. Additionally, the utility of a thermography system for drought tolerance breeding in maize was discussed.