Tolerant Inbred Lines Research Articles

Genetic characterization and population structure of maize populations using SSR markers

Maize is a major staple food in sub-Saharan Africa. However, due to the threats posed by biotic and abiotic constraints to its production, it is a prerequisite to conserve germplasm with diverse genetic background and to seek for favourable genotypes with such background for improvement of superior genotypes. This study assessed the diversity of 70 white maize populations developed at CSIR-Savanna Agricultural Research Institute with resistance/tolerance to drought and low soil nitrogen (Low-N) using 31 SSR markers. A total of 288 alleles were detected among the germplasm used with a range of 4 to 17 alleles per locus and an average allele number of 9.60 alleles per locus. Polymorphic Information Content values for the SSR markers ranged from 0.32 to 0.85 with a mean value of 0.68. The average heterozygosity obtained from the markers was 0.54 and gene diversity ranged from 0.35 to 0.87 with a mean gene diversity value of 0.71 indicating a high level of polymorphism among the populations. Clustering based on Jaccard's similarity coefficient and an Unweighted Pair Group Method with Arithmetic Mean (UPGMA) revealed 5 clusters for the 70 populations. Evanno method for population structure clustering further revealed two sub-populations. Sub-population 1 was more genetically diverse with an Fst value of 0.18 than sub-population 2 which recorded an Fst value of 0.013. These results indicated that the SSR used were very polymorphic for diversity studies. Also, the studied maize populations are diverse and heterozygous, making them ideal source populations for extraction of drought and low-N tolerant inbred lines.

Dissecting molecular mechanisms underlying salt tolerance in rice: a comparative transcriptional profiling of the contrasting genotypes

BackgroundSalinity expansion in arable land is a threat to crop plants. Rice is the staple food crop across several countries worldwide; however, its salt sensitive nature severely affects its growth under excessive salinity. FL478 is a salt tolerant indica recombinant inbred line, which can be a good source of salt tolerance at the seedling stage in rice. To learn about the genetic basis of its tolerance to salinity, we compared transcriptome profiles of FL478 and its sensitive parent (IR29) using RNA-seq technique.ResultsA total of 1714 and 2670 genes were found differentially expressed (DEGs) under salt stress compared to normal conditions in FL478 and IR29, respectively. Gene ontology analysis revealed the enrichment of transcripts involved in salinity response, regulation of gene expression, and transport in both genotypes. Comparative transcriptome analysis revealed that 1063 DEGs were co-expressed, while 338/252 and 572/908 DEGs were exclusively up/down-regulated in FL478 and IR29, respectively. Further, some biological processes (e.g. iron ion transport, response to abiotic stimulus, and oxidative stress) and molecular function terms (e.g. zinc ion binding and cation transmembrane transporter activity) were specifically enriched in FL478 up-regulated transcripts. Based on the metabolic pathways analysis, genes encoding transport and major intrinsic proteins transporter superfamily comprising aquaporin subfamilies and genes involved in MAPK signaling and signaling receptor kinases were specifically enriched in FL478. A total of 1135 and 1894 alternative splicing events were identified in transcripts of FL478 and IR29, respectively. Transcripts encoding two potassium transporters and two major facilitator family transporters were specifically up-regulated in FL478 under salt stress but not in the salt sensitive genotype. Remarkably, 11 DEGs were conversely regulated in the studied genotypes; for example, OsZIFL, OsNAAT, OsGDSL, and OsELIP genes were up-regulated in FL478, while they were down-regulated in IR29.ConclusionsThe achieved results suggest that FL478 employs more efficient mechanisms (especially in signal transduction of salt stress, influx and transport of k+, ionic and osmotic homeostasis, as well as ROS inhibition) to respond to the salt stress compared to its susceptible parent.

Rapid identification of a candidate nicosulfuron sensitivity gene (Nss) in maize (Zea mays L.) via combining bulked segregant analysis and RNA-seq.

A candidate nicosulfuron sensitivity gene Nss was identified by combining bulked segregant analysis and RNA-seq. Multiple mutations of this gene were discovered in nicosulfuron-sensitive maize compared with the tolerant. It has been demonstrated that variabilities exist in maize response to nicosulfuron. Two nicosulfuron-sensitive inbred lines (HB39, HB41) and two tolerant inbred lines (HB05, HB09) were identified via greenhouse and field trials. Genetic analysis indicated that the sensitivity to nicosulfuron in maize was controlled by a single, recessive gene. To precisely and rapidly map the nicosulfuron sensitivity gene (Nss), two independent F2 segregating populations, Population A (HB41 × HB09) and Population B (HB39 × HB05), were constructed. By applying bulked segregant RNA-Seq (BSR-Seq), the Nss gene was, respectively, mapped on the short arm of chromosome 5 (chr5: 1.1-15.3Mb) and (chr5: 0.5-18.2Mb) using two populations, with 14.2Mb region in common. Further analysis revealed that there were 43 and 119 differentially expressed genes in the mapping intervals, with 18 genes in common. Gene annotation results showed that a cytochrome P450 gene (CYP81A9) appeared to be the candidate gene of Nss associated with nicosulfuron sensitivity in maize. Sequence analysis demonstrated that two common deletion mutations existed in the sensitive maize, which might lead to the nicosulfuron sensitivity in maize. The results will make valuable contributions to the understanding of molecular mechanism of herbicide sensitivity in maize.

QTL Mapping in Three Connected Populations Reveals a Set of Consensus Genomic Regions for Low Temperature Germination Ability in Zea mays L.

Improving seed vigor in response to cold stress is an important breeding objective in maize that allows early sowing. Using two cold tolerant inbred lines 220 and P9-10 and two susceptible lines Y1518 and PH4CV, three connected F2:3 populations were generated for detecting quantitative trait locus (QTL) related to seed low-temperature germination ability. At 10°C, two germination traits (emergence rate and germination index) were collected from a sand bed and three seedling traits (seedling root length, shoot length, and total length) were extracted from paper rolls. Significant correlations were found among all traits in all populations. Via single-population analysis, 43 QTL were detected with explained phenotypic variance of 0.62%∼39.44%. Seventeen QTL explained more than 10% phenotypic variance; of them sixteen (94.12%) inherited favorable alleles from the tolerant lines. After constructing a consensus map, three meta-QTL (mQTL) were identified to include at least two initial QTL from different populations. mQTL1-1 included seven initial QTL for both germination and seedling traits; with three explaining more than 30% phenotypic variance. mQTL2-1 and mQTL9-1 covered two to three initial QTL. The favorable alleles of the QTL within these three mQTL regions were all inherited from the tolerant line 220 and P9-10. These results provided a basis for cloning of genes underlying the mQTL regions to uncover the molecular mechanisms of maize cold tolerance during germination.

Research Article Characterization of Genetic Diversity and Linkage Disequilibrium of ZmNAC33 in Maize

The maize NAC gene, encoding a NAC domain transcription factor, plays vital roles in the growth process of maize under abiotic stresses. To investigate the response of ZmNAC33 to salinity stress, we analyzed the expression of the ZmNAC33 gene in roots and leaves of three-leaf stage seedlings among maize inbred lines. The result showed that the expression of ZmNAC33 was up-regulated in the salinity tolerant inbred line Ji853 and down-regulated in the salinity susceptible inbred line Chang7-2. The cDNA and the full sequence from maize inbred line Ji853 were amplified to analyze genetic structure. ZmNAC33 gene comprises three exons and two introns. We compared genetic diversity of ZmNAC33 among 69 elite maize inbred lines, a total of 144 sequence variants, including 113 SNPs and 31 Indels. Neutrality tests were negative selection by Tajima’s D and Fu and Li’s test. Analysis of polymorphism sites showed that at least 28 recombination’s have occurred. This finding provides insight into the development of breeding programs, germplasm management and association genetic studies.

Genetic mapping of QTL for maize leaf width combining RIL and IF2 populations.

Leaf width is an important component of plant architecture that affects light capture during photosynthesis and wind circulation under dense planting conditions. To improve understanding of the genetic mechanisms involved in leaf width at different positions, a comprehensive evaluation using the RIL (Recombinant Inbred Line) and IF2 (Immortalized F2) populations and a subsequent meta-analysis were performed. Forty-seven QTL associated with leaf width at different positions below the tassel were detected. The individual effects of QTL explained 3.5% to 17.0% of the observed phenotypic variation, and ten QTL explained over 10%. The initial QTL were integrated into eight mQTL (meta-QTL) through a meta-analysis. Our results suggested that leaf widths at different positions may be affected by several of the same mQTL and may also be regulated by many different mQTL. These results provide useful information for breeding high density tolerant inbred lines and hybrid cultivars, as well as for using marker-assisted selection for important mQTL.

Genotype by Trait Associations among Drought Tolerant Maize Inbred Lines

Twelve tropical, yellow maize inbred lines identified as drought tolerant were evaluated in multi environments, including managed drought, rain fed and irrigated conditions. The objective was to study genotype-trait associations across environments. A 3 × 4 a-lattice design with two replications was used in each environment. Data were recorded for twenty-one traits. Combined analysis of variance using data from all environments was done for all traits using the GLM procedure in SAS version 9.3. Genotype by trait associations were revealed using the genotype main effect plus genotype-by-environment biplot model in GENSTAT 14th Edition. Inbred lines which were associated with high grain yield and related desirable traits such as a low drought susceptibility index under managed drought were DMR-M-81, DMR-M-88, FA6, GPM36 and M39. Across the diverse environments, DMR-M-84, DMR-M-88, FA6 and GPM36 were associated with grain yield and/or its related traits. The inbred lines associated with desirable traits could be evaluated for combining ability in order to know their desirability in cultivar development. These inbred lines could be used as female parents in seed production programmes since high productivity and drought tolerance are important qualities of female parents in seed production.

Seleksi dan Karakter Sekunder Jagung Inbrida Toleran Cekaman Kekeringan

The availability of drought-tolerant maize inbred materials are useful to develop a hybrid or synthetic drought tolerant maize varieties. This research was aimed to select drought tolerant and determine secondary traits for selection of inbred maize. Selection of 31 maize inbred lines was conducted in nested experimental design in which the replicate was nested into the drought stress selection condition and normal irrigation. Drought stress treatment was applied during flowering to milk phase. In the normal irrigation condition the water was supplied every 10 days. The results showed that the medium tolerant to tolerant to drought stress inbred lines were 1044-30, DTPYC9-F46-3-9-1-1-B, CML161 / NEI9008, DTPYC9-F46-1-2-1-2- B, and Mr14. The yield declined of tolerant/medium tolerant inbred lines of about 17.0-38.3% was lower than sensitive inbred lines of about 42.8-70.7% under stress condition. The tolerant/medium tolerant inbred lines can be used for developing drought tolerant or synthetic varieties. The secondary traits effective for selection of drought tolerant inbred lines were shelling percentage, leaf chlorophyll, leaf rolling scores, number of tassel, the female flowering dates, and anthesis silking interval (ASI). The traits were more effected by genetic factors than the environmental factors and correlated to yield under drought stress conditions.<br /><br />Keywords: anthesis silking interval, environmental condition, inbred lines, tassel number, yield<br /><br />

Genetic analysis of tropical maize inbred lines for resistance to maize lethal necrosis disease

Maize lethal necrosis (MLN) disease is a recent outbreak in eastern Africa and has emerged as a significant threat to maize production in the region. The disease is caused by the co-infection of Maize chlorotic mottle virus and any member of potyviridae family. A total of 28 maize inbred lines with varying levels of tolerance to MLN were crossed in a half-diallel mating design, and the resulting 340 F1 crosses and four commercial checks were evaluated under MLN artificial inoculation at Naivasha, Kenya in 2015 and 2016 using an alpha lattice design with two replications. The objectives of the study were to (i) investigate the magnitude of general combining ability variance (σGCA2) and specific combining ability variance (σSCA2) and their interaction with years; (ii) evaluate the efficiencies of GCA based prediction and hybrid performance by means of a cross-validation procedure; (iii) estimate trait correlations in the hybrids; and (iv) identify the MLN tolerant single cross hybrids to be used as female parents for three-way cross hybrids. Results of the combined analysis of variance revealed that both GCA and SCA effects were significant (P < 0.05) for all traits except for ear rot. For MLN scores at early and late stages, GCA effects were 2.5–3.5 times higher than SCA effects indicating that additive gene action is more important than non-additive gene action. The GCA based prediction efficiency for MLN resistance and grain yield accounted for 67–90% of the variations in the hybrid performance suggesting that GCA-based prediction can be proposed to predict MLN resistance and grain yield prior to field evaluation. Three parents, CKDHL120918, CML550, and CKLTI0227 with significant GCA effects for GY (0.61–1.21; P < 0.05) were the most resistant to MLN. Hybrids “CKLTI0227 × CML550”, “CKDHL120918 × CKLTI0138”, and “CKDHL120918 × CKLTI0136” ranked among the best performing hybrids with grain yield of 6.0–6.6 t/ha compared with mean yield of commercial check hybrids (0.6 t/ha). The MLN tolerant inbred lines and single cross hybrids identified in this study could be used to improve MLN tolerance in both public and private sector maize breeding programs in eastern Africa.

Bulked Segregant RNA-seq Reveals Differential Expression and SNPs of Candidate Genes Associated with Waterlogging Tolerance in Maize.

Waterlogging has increasingly become one of the major constraints to maize productivity in some maize production zones because it causes serious yield loss. Bulked segregant RNA-seq (BSR-seq) has been widely applied to profile candidate genes and map associated Single Nucleotide Polymorphism (SNP) markers in many species. In this study, 10 waterlogging sensitive and eight tolerant inbred lines were selected from 60 maize inbred lines with waterlogging response determined and preselected by the International Maize and Wheat Improvement Center (CIMMYT) from over 400 tropical maize inbred lines. BSR-seq was performed to identify differentially expressed genes and SNPs associated with waterlogging tolerance. Upon waterlogging stress, 354 and 1094 genes were differentially expressed in the tolerant and sensitive pools, respectively, compared to untreated controls. When tolerant and sensitive pools were compared, 593 genes were differentially expressed under untreated and 431 genes under waterlogged conditions, of which 122 genes overlapped. To validate the BSR-seq results, the expression levels of six genes were determined by qRT-PCR. The qRT-PCR results were consistent with BSR-seq results. Comparison of allelic polymorphism in mRNA sequences between tolerant and sensitive pools revealed 165 (normal condition) and 128 (waterlogged condition) high-probability SNPs. We found 18 overlapping SNPs with genomic positions mapped. Eighteen SNPs were contained in 18 genes, and eight and nine of 18 genes were responsive to waterlogging stress in tolerant and sensitive lines, respectively. Six alleles of the 18 originated from tolerant pool were significantly up-regulated under waterlogging, but not those from sensitive pool. Importantly, one allele (GRMZM2G055704) of the six genes was mapped between umc1619 and umc1948 on chromosome 1 where a QTL associated with waterlogging tolerance was identified in a previous research, strongly indicating that GRMZM2G055704 is a candidate gene responsive to waterlogging. Our research contributes to the knowledge of the molecular mechanism for waterlogging tolerance in maize.

Cold Stress Evaluation among Maize (Zea mays L.) Inbred Lines in Different Temperature Conditions

Maize (Zea mays L.) is a crop in a tropical region which resists growing under sensitive temperature. This study was conducted to evaluate the performance of Canadian maize inbred lines under controlled cold stress conditions (5 ℃, 10 ℃, and 23 ℃). Data were recorded by measuring germination rate, index, root length, and seed vigour index values. Five higher and three lower tolerant inbred lines were shortlisted. The data were analyzed using analysis of variance, while mean values were compared using Tukey`s Honest Significant Difference Test at α=0.05 and at α=0.01. Using Genstat software, correlation was done. A strong correlation (P＜0.05) was found between germination rate and germination index under all stress conditions. Root length and vigour index were also strongly correlated with germination rate under 5 ℃ stress condition and compared to 10 ℃ and 23 ℃ stress conditions. Our results suggested that five (CO439, CO438, CO450, CO435, and CO445) among 22 maize inbred lines performed better under 5 ℃ cold stress condition and thus had the potential to develop maize hybrids to increase grain yield under environmentally stressful conditions in South Korea.

EVALUATION OF MALE INBRED LINES OF SUNFLOWER (Helianthus annuus L.) FOR RESISTANCE TO DROUGHT VIA CHLOROPHYLL FLUORESCENCE

Sunflower (Helianthus annuus L.) suffers from terminal drought accompanying with high temperature stress since it grows mainly in rain fed areas. Therefore, plant breeders try to improve more drought tolerant varieties and to screen their genetic materials for drought resistance. The present study was conducted to determine drought tolerance levels of sunflower male inbred lines developed by Trakya Agricultural Research Institute (TARI), Edirne, Turkey. Inbred lines grown under controlled environmental conditions were sorted by polyphasic chlorophyll a fluorescence measurements. Drought stress applications were performed at three sunflower growth stages as R-3 (vegetative), R5-1 (head formation) and R-6 (milky seed). Based on applied different JIP-Test (analysis of O-J-I-P fluorescence transient) parameters such as Drought Factor index (-DFI) and Damage index (-DI), 70352 R, 8129 R, 0536 R and 9947 R restorer lines were found more drought tolerant than those of the other examined sunflower inbred lines, whereas TT 317 R and TT 199 R were determined as more drought sensitive than others. The drought tolerant inbred lines will be helpful to improve drought resistance in sunflower breeding programs.

Bulk segregant analysis (BSA) for improving cold stress resistance in maize using SSR markers.

In this study, eight inbred maize lines with high or low tolerance to cold temperature were investigated using bulk segregant analysis (BSA). Genetic diversity and genetic relationships were investigated using 100 simple sequence repeat (SSR) markers linked to cold tolerance in maize. A total of 319 alleles were identified. Among these, 128 were high coldtolerant-specific alleles and 61 were poorly coldtolerant-specific alleles, while the remaining 130 were shared between the two types. The number of alleles per locus ranged from 2 to 5, with an average of 3.19. The major allele frequency varied from 0.39 to 0.53 with an average of 0.47. The average gene diversity and polymorphic information content among all lines were 0.63 and 0.58, respectively. A dendrogram analysis identified three main clusters and most of the high tolerant inbred lines were clearly distinguished from the poorly tolerant inbred lines. In addition to the BSA, a total of 84 SSR markers were identified as high cold tolerance-specific alleles and 52 SSRs were detected as poorly cold tolerance-specific alleles. Of these, bnlg1273, umc1124, dupssr21, mmc0251, mmc0181, and phi041 have great potential for being molecular markers for cold tolerance in maize. Our results were in agreement with results previously reported for SSR markers linked with cold tolerance in maize. The identification and characterization of high and poorly cold tolerant maize lines based on SSR markers will be useful for future maize breeding studies.

Evaluation of drought tolerance in backcross inbred lines of rice genotypes based on selection indices

Drought tolerant lines of Improved White Ponni (IWP) were developed by marker assisted back crossing of IWP with Apo. Three drought tolerant backcross inbred lines (BILs), developed by introgressing the drought tolerance QTLs from Apo were evaluated for drought tolerance along with their parents based on drought indices. Twenty drought indices were calculated based on grain yield under drought and irrigated conditions. Yield in drought and irrigated conditions were significantly and positively correlated with mean productivity index, harmonic mean, geometric mean of productivity and stress tolerance index. Ranking method and three dimensional plot discriminated IWP-4-2 as the most drought tolerant genotype, which was placed in group A (Genotypes, which performed good under drought and irrigated conditions), and outperformed the drought tolerant parent, Apo. IWP-4-2 possessed medium slender grains similar to the recurrent parent, IWP. Based on its drought tolerance and grain type IWP-4-2 can also be used as donor parent for developing climate resilient rice variety suitable to conditions obtaining in Tamil Nadu.

COMBINING ABILITIES OF GRAIN YIELD AND YIELD RELATED TRAITS IN RELATION TO DROUGHT TOLERANCE IN TEMPERATE MAIZE BREEDING

Temperate maize growing regions are increasingly prone to drought stress which is emerging as a primary constraint to maize productivity. Introgression of tropical sources of drought tolerance into temperate maize breeding may contribute to improving the drought tolerance of the relatively narrow temperate germplasm base that is currently in use. In the present study, seven tropical drought tolerant inbred lines and two well-adapted commercial temperate inbred lines were crossed in a half-diallel design. The 9 parents, 36 diallel hybrids and 2 commercial checks were tested in well-watered and managed water stressed conditions in Antalya, Turkey in 2013 and 2014. Significant differences were detected between genotypes for number of days to anthesis, anthesis-silking interval, plant height, thousand kernel weight, number of ears per plant, number of kernels per ear and grain yield. Tropical inbreds G1 and G5 were comparable to the adapted temperate commercial inbreds in terms of general combining ability. Specific combining ability analysis revealed that tropical x temperate hybrids had higher values compared to tropical x tropical hybrids. Yield potential of hybrid G5 x G9 in particular, a promising tropical x temperate hybrid, was significantly higher under both well watered and drought conditions. Our results suggest that tropical drought tolerant germplasm is likely to have high utility in temperate maize breeding programs. Key Words: maize breeding, combining ability, germplasm, water stress

The Use of Stress Tolerance Indices for the Selection of Tolerant Inbred Lines and their Correspondent Hybrids under Normal and Water-stress Conditions

Drought is a major cause of yield loss for many important crops including maize. Therefore, the development and release of drought tolerant varieties that will yield well under a broad range of environmental conditions is an important breeding goal. Several stress indices have been developed, aiming to assist identification and selection of stable, high-yielding, drought tolerant genotypes. A novel equation (B value) that predicts crop yield potential of a genotype at normal farming densities has been also suggested1. The estimation of B value relies on plant yield potential at ultra-low densities and on the coefficient of variation (CV) of individual plants. The present study aimed to investigate the tolerance of inbred lines and their correspondent hybrids using stress tolerant indices and compare them with the B values. Therefore, we used thirty one maize inbred lines and their correspondent hybrids under ultra-spaced and highly dense and under two different water regimes (normal and drought conditions) in three different areas in Greece during the 2012 and 2013 growing season, respectively. The inbred lines and the hybrids were evaluated using grain yield and for each genotype, B value and nine other stress indices based on their yield under normal and water stress conditions were calculated, including stress susceptibility index (SSI), mean relative performance (MRP), stress tolerance (TOL), mean productivity2 (MP), relative efficiency index (REI), stress tolerant index3 (STI), geometric mean of productivity (GMP), yield index (YI) and harmonic mean4 (HM). It was found a strong and positive correlation (P<0.001) of B values with all indices, except SSI, for all experimental locations. These results suggest that B value matches the ability of the other stress indices to identify drought sensitive and tolerant genotypes and is an effective selection criterion for high yielding genotypes with stable performance under variable environmental conditions.

Genome-Wide Expression Profile of Maize Root Response to Phosphorus Deficiency Revealed by Deep Sequencing

Phosphorus (P) is one of the three primary macronutrients that are required in large amounts for plant growth and development. To better understand molecular mechanism of maize and identify relevant genes in response to phosphorus deficiency, we used Solexa/Illumina's digital gene expression (DGE) technology to investigate six genome-wide expression profiles of seedling roots of the low-P tolerant maize inbred line 178. DGE studies were conducted at 6, 24 and 72 h under both phosphorus deficient and sufficient conditions. Approximately 3.93 million raw reads for each sample were sequenced and 6 816 genes exhibited significant levels of differential expressions in at least one of three time points in response to P starvation. The number of genes with increased expression increased over time from 6 to 24 h, whereas genes with decreased expression were more abundant at 72 h, suggesting a gradual response process for P deficiency at different stages. Gene annotations illustrated that most of differentially expressed genes (DEGs) are involved in different cellular and molecular processes such as environmental adaptation and carbohydrate metabolism. The expression of some known genes identified in other plants, such as those involved in root architecture, P metabolism and transport were found to be altered at least two folds, indicating that the mechanisms of molecular and morphological adaptation to P starvation are conserved in plants. This study provides insight into the general molecular mechanisms underlying plant adaptation to low-P stress and thus may facilitate molecular breeding for improving P utilization in maize.

Screening of Tomato Inbred-Lines for Drought Tolerance at Germination and Seedling Stage

This investigation was carried out to screen 45 tomatoes inbred-lines’ response to drought at germination and seedling stage. These inbred-lines which were collected from different countries were screened with osmotic concentration of 12.5% polyethylene glycol 6000 while their respective control treatments were treated using distilled water for twelve days at germination stage. Relative germination energy, relative germination rate, relative germination index, relative vitality index and the relative PEG injury rate were computed to identify the most tolerant inbred-lines on germination stage. Inbred-lines TTI1103B-2, A0911, Roma, L12, and TTD210B were found to be the most tolerant. While inbred-lines L9, TTI1109B-1-2-1, A4, B6 and A7 were the most sensitive to drought. Moreover, analysis of variance revealed significant differences among the inbred-lines studied. The experiment was repeated for confirmation and the result was similar. An additional study was conducted on 30 days old seedlings to observe their water stress response at their seedling stage. A total of 30 inbred-lines, 10 inbred-lines each from tolerant, moderate and sensitive; were selected based on the results found at germination stage. After proper watering for 30 days, watering was stopped for the following 10 days. After that, the dead and alive was counted. The seedlings were then re-watered for the following one week and those recovered from dead were counted again. Finally, the dead percentage and recovery percentage was computed as an indicator of water stress tolerance. And the result was very similar to the experiment done on germination stage.

Characterization of maize inbred lines for drought and heat tolerance

Drought and high temperature are two major environmental factors that severely limit plant productivity in the United States and worldwide, often causing extensive economic loss to agriculture. As global climate change progresses, agricultural production worldwide faces serious threats from frequent extreme weather conditions. Integrated approaches that improve the efficiency of agricultural water use and development of plant varieties that can alleviate the negative impacts of environmental stresses to maintain yield stability are essen- tial to sustain and increase agriculture production. Maize (Zea mays L.) is a major crop in the United States and worldwide. Its production and yield stability are greatly affected by drought and high temperature stresses. Improving drought and heat tolerance in maize has become one of the top priorities for maize breeding programs in both private and public sectors. Identification of maize germplasm with superior drought and/or heat tolerance is essential and prerequisite for such propose. In this report, we evaluated a selection of maize inbred lines for drought and heat stress tolerance under field conditions in 2009 and 2010 and identified several inbred lines that showed high tolerance to drought. Tolerant inbred lines (Tx205, C2A554-4, and B76) were able to maintain relatively high leaf relative water content when subjected to drought stress, while sensitive lines (B73 and C273A) showed a rapid reduction in leaf relative water content at very early stage of drought. The tolerant lines also showed significantly greater ability to maintain vegetative growth and alleviate damage to reproductive tissues under drought conditions compared to the sensitive lines. Maize inbred lines and hybrids were also evaluated for tolerance to high temperature under well-watered conditions through field observations following the occurrence of major heat events. Maize inbred lines of distinct heat tolerance phenotype were identified. Furthermore, genetic and phenotypic analysis showed that maize hybrids made from inbred lines with superior heat tolerance inherited an enhanced tolerance to elevated temperatures. The tolerant germplasm accessions, like those identified in this study, are essential materials for breeding drought- and/ or heat-tolerant maize hybrids. Study for the potential use of such materials to produce maize hybrids that are able to alleviate the negative impacts of drought and heat stress on the growth and development of maize plants is underway.

Evaluation of drought tolerance in different growth stages of maize (Zea mays L.) inbred lines using tolerance indices

In order to find the best drought tolerant inbred lines, experiment was performed at the Agricultural College of Islamic Azad University, Shoushtar Branch, Iran during 2010. Experiment treatments were compared in a split-plot design by a randomized complete block design with 3 replications. Main factors included non-stress, drought stress in 6 to 7 leaves (vegetative) stage, drought stress in pollination stage and drought stress in grain filling stage. Sub factors were 7 inbred lines. Five stress tolerance indices, including mean productivity (MP), stress tolerance (TOL), stress susceptibility (SSI), stress tolerance index (STI) and geometric mean productivity (GMP) were used in this study. Drought stress in grain filling stage had the minimum grain yield that showed severe effects of drought stress at this stage of growth. Data analysis revealed that the MP, GMP and STI indices were the more accurate criteria for selection of drought tolerant and high yielding inbred lines. The positive and significant correlation of STI and grain yield under all conditions revealed that this index is more applicable and efficient for selection of parental inbred lines in producing hybrids tolerant to drought in vegetative, pollination and grain filling stages and high yielding under non-stress and stress conditions. Based on the STI, GMP and MP indices, K166B proved to be the most drought tolerant line. Biplot analysis allowed us to distinguish groups of tolerant and sensitive inbred lines. Based on the results of this study, the inbred line K166B can be recommended in future breeding programs for production of drought tolerant hybrids. Key words: Biplot, drought stress, maize, tolerance indices.