Sorghum (Sorghum bicolor [L.] Moench) is the fifth most economically important cereal grown worldwide and is a source of food, feed, fiber and fuel. Sorghum, a C4 grass and a close relative to sugarcane, is adapted to hot, dry adverse environments. Some genotypes of sorghum called stay-green have delayed leaf senescence during grain ripening under drought stress conditions which allows normal grain filling whereas most sorghum lines senesce early under post-anthesis drought. Eight sources of stay-green have been identified in the sorghum germplasm collection, most originating from Sudan and Ethiopia. The diversity of the eight sources of staygreen was analyzed using 55 simple sequence repeats (SSR) markers with genome coverage. This analysis showed that the sources of stay-green are quite diverse and can be divided into five groups based on race or working group. Three sources of stay-green have been used to identify 12 major quantitative trait loci (QTL) that modulate this trait. The origin of favorable alleles for stay-green was traced backward to ancestral lines and forward into breeding materials derived from stay-green germplasm. The analysis of the origin of favorable alleles for stay-green helped explain why subsets of stay-green QTL were identified in different studies and provided evidence that there may be more than one favorable allele in the sorghum germplasm for several of the stay-green QTL. Analysis of stay-green breeding lines from three public sorghum-breeding programs revealed that one of the main QTL identified in mapping studies was not being used in the breeding programs (0/13), most likely due to its association with an allele for lemon yellow seeds. In addition, a subset of the regions containing favorable alleles for staygreen from the genotype BTx642 were over represented in stay-green breeding lines. Nearly isogenic lines containing favorable alleles from BTx642 for Stg1, Stg2, Stg3, and Stg4 in a RTx7000 (senescent) background were characterized and each NIL was shown to exhibit a stay-green phenotype. Based in part on this information, fine-mapping of Stg1 was undertaken by crossing the Stg1 NIL to RTx7000. Overall, these results revealed the origin of favorable alleles for stay-green and the current utilization of alleles for stay-green in public breeding programs. In addition, this study identified additional stay-green sources that could be used for further QTL analysis and highlighted the genetic complexity of the stay-green trait.

The chlorophyll degradation pathway is central to a number of plant processes including senescence and fruit ripening. However, the regulation of the chlorophyll degradation pathway enzymes is not well understood. The aim of this thesis was to elucidate the genetic mechanisms that control changes in pigment composition leading to fruit flesh yellowing in kiwifruit. Actinidia deliciosa and A. chinensis fruit, which are green and yellow, respectively, provide an opportunity to study the regulation of chlorophyll levels. The expression of genes that code for enzymes of the chlorophyll and cytokinin metabolic pathways was measured using qRT-PCR. Candidates for chlorophyll degradation regulatory points were then characterised for functionality by transient transformation in N. benthamiana. The endogenous cytokinin levels were measured in kiwifruit and transient activation assays were carried out with the promoters of key candidate genes. Overall, expression of the chlorophyll degradation genes was elevated in yellow fruit and expression of biosynthetic genes was higher in green fruit. The chlorophyll degradation-associated protein, STAY-GREEN2 (SGR2), was more highly expressed in yellow fruit, and transient over-expression of SGR was sufficient to drive chlorophyll degradation. Expression of isopentenyl transferase (IPT), the rate-limiting step for cytokinin biosynthesis, showed an increase towards maturity in green fruit, but not in yellow fruit. However, both fruit had similar high levels of cytokinin nucleotides and free bases. A gene coding for O-glucosylation was also highly expressed in green fruit. Green fruit contained higher levels of cytokinin O-glucosides and ribosides towards maturity, suggesting differences in cytokinin signalling, which could lead to regulation of chlorophyll levels via activation of the SGR promoter by transcription factors. It is likely that the chlorophyll degradation pathway and cytokinin metabolism are linked. The differential expression of cytokinin response regulators could lead to differential regulation of cytokinin levels in the fruit of the two species, and possibly differential regulation of the chlorophyll degradation pathway. Progress towards elucidation of the control of chlorophyll levels provides knowledge of this key process in kiwifruit and potentially gene-based markers for breeding new kiwifruit cultivars.

Doctor of Philosophy / Department of Agronomy / Jianming Yu / Knowledge of the genetic bases of grain quality traits will complement plant breeding efforts to improve the end use value of sorghum (Sorghum bicolor (L.) Moench). The objective of the first experiment was to assess marker-trait associations for 10 grain quality traits through candidate gene association mapping on a diverse panel of 300 sorghum accessions. The 10 grain quality traits were measured using the single kernel characterization system (SKCS) and near-infrared reflectance spectroscopy (NIRS). The analysis of the accessions through 1,290 genome-wide single nucleotide polymorphisms (SNPs) separated the panel into five subpopulations that corresponded to three major sorghum races (durra, kafir, and caudatum), one intermediate race (guinea-caudatum), and one working group (zerazera/caudatum). Association analysis between 333 SNPs in candidate genes/loci and grain quality traits resulted in eight significant marker-trait associations. A SNP in starch synthase IIa (SSIIa) gene was associated with kernel hardness (KH) with a likelihood ratio–based R[superscript]2 (R[subscript]L[subscript]R[superscript]2) value of 0.08. SNPs in starch synthase (SSIIb) gene (R[subscript]L[subscript]R[superscript]2 = 0.10) and loci pSB1120 (R[subscript]L[subscript]R[superscript]2 = 0.09) was associated with starch content. Sorghum is a crop well adapted to the semi arid regions of the world and my harbor genes for drought tolerance. The objective of second experiment was to identify quantitative trait loci (QTLs) for yield potential and drought tolerance. From a cross between Tx436 (food grain type) and 00MN7645 (drought tolerant) 248 recombinant inbred lines (RILs) was developed. Multi-location trials were conducted in 8 environments to evaluate agronomic performance of the RILs under favorable and drought stress conditions. The 248 RILs and their parents were genotyped by genotyping-by-sequencing (GBS). A subset of 800 SNPs was used for linkage map construction and QTL detection. Composite interval mapping identified a major QTLs for grain yield in chromosome 8 and QTL for flowering time in chromosome 9 under favorable conditions. Three major QTLs were detected for grain yield in chromosomes 1, 6, and 8 and two flowering time QTLs on chromosome 1 under drought conditions. Six QTLs were identified for stay green: two on chromosome 4; one each on chromosome 5, 6, 7, and 10 under drought conditions.

<p>Dhurrin is a cyanogenic glucoside (CG), an important compound that can interplay with primary and secondary metabolism in sorghum. Dhurrin metabolism contributes to insect resistance, growth, nitrogen (N) metabolism, drought tolerance, and safety for animal consumption when used as a forage. Through chemical mutagenesis with ethyl methanesulfonate (EMS), a mutation in the gene encoding CYP79A1 (<i>cyp79a1</i>), the first enzyme in the biosynthetic pathway of dhurrin, was discovered that inhibits the production of dhurrin. The acyanogenic phenotype of this mutant could be a major benefit in reducing the risk of hydrogen cyanide (HCN) toxicity within animals; however, understanding the effects of inhibiting dhurrin biosynthesis is important in understanding metabolic tradeoffs that could occur. This dissertation describes research to assess impacts and tradeoffs of the dhurrin-free trait on susceptibility to Fall Armyworm [<i>Spodoptera frugiperda</i> (J.E. Smith)] (FAW) feeding, seedling growth, effects on post-flowering drought tolerance, cold stress and utilization as a forage. Insect susceptibility and seedling growth were examined using near-isogenic lines (NILs) within the greenhouse utilizing non-destructive phenotyping technologies for green plant area and in the field comparing total leaf area and dry weight. Post-flowering drought stress was induced within a greenhouse, growth chamber, and field environments. The <i>cyp79a1 </i>mutation was tested in NILs, a near-isogenic backcross (NIBC) population, and near-isogenic hybrids (NIH), to understand the impacts of the <i>cyp79a1 </i>mutation<i> </i>on the stay-green trait. Palatability as forage was examined by comparing the feeding preference of ruminant animals with multiple conventional hybrids and an experimental hybrid carrying the <i>cyp79a1</i> mutation. This preference was also examined using a set of NILs varying in the <i>cyp79a1 </i>mutation<i>. </i> Safety was assessed in preference trials by testing for HCN release before grazing. To further our understanding of the benefits of sorghum as a forage, the dhurrin-free experimental hybrid was compared to seven conventional hybrids as a dry product. The dry sorghum product was tested for the release of HCN and dhurrin content. Lastly, the effects of low temperatures and frost were assessed for their effects on the production of dhurrin in cyanogenic and dhurrin-free sorghum genotypes. </p> <p>Overall, the biosynthesis of dhurrin had a significant effect on the deterrence of FAW and on the growth of sorghum seedlings. Dhurrin-free lines were more susceptible to FAW feeding but also exhibited a significantly higher growth rate. Dhurrin-free lines and hybrids only exhibited a slight increase in susceptibility to post-flowering drought stresses with only one dhurrin-free hybrid discovered to senesce faster than its wild-type NIH. Comparisons of the effects of dhurrin biosynthesis on stay-green in a NIBC population in Tx642 (B35), one of the most important sources of the stay-green trait, did not show any variation in chlorophyll concentration (CC) and normalized difference vegetation index (NDVI). Analyses of the impact of dhurrin on palatability as a forage showed that ewes preferred grazing on the dhurrin-free hybrids and NILs, showing that the ewes were able to detect the presence or absence of dhurrin while feeding. Experiments to assess the safety and stability of dhurrin in dried plant material demonstrated that dhurrin content did not change during drying and HCN was released after rehydration. Furthermore, high levels of HCN were immediately released when rumen fluid was added to dried plant materials containing dhurrin; however, no detectable HCN was released from dhurrin-free genotypes. Finally, sorghum plants exposed to freezing temperatures exhibited an increase in dhurrin content in conventional sorghum hybrids while no detectable dhurrin was noted within <i>cyp79a1 </i>mutants. </p> <p>Taken together, these studies demonstrate pleiotropic effects for the <i>cyp79a1 </i>mutation. Dhurrin-free genotypes were more susceptible to insect herbivory and may be slightly more susceptible to post-flowering drought within the hybrids; however, these genotypes exhibited higher seedling growth rates, feeding preference by ewes, no release of HCN in fresh or dry plant material, and frost did not cause an increase in dhurrin content.</p>

No description available.

Doctor of Philosophy / Department of Agronomy / Tesfaye Tesso / Fusarium stalk rot and charcoal rot caused by Fusarium thapsinum and Macrophomina phaseolina respectively are devastating global diseases in sorghum that lead to severe quality and yield loss each year. In this study, three sets of interrelated experiments were conducted that will potentially lead to the development of resistance based control option to these diseases. The first experiment was aimed at identifying sources of resistance to infection by M. phaseolina and F. thapsinum in a diverse panel of 300 sorghum genotypes. The genotypes were evaluated in three environments following artificial inoculation. Out of a total of 300 genotypes evaluated, 95 genotypes were found to have resistance to M. phaseolina and 77 to F. thapsinum of which 53 genotypes were resistant to both pathogens. In the second experiment, a set of 79,132 single nucleotide polymorphisms (SNPs) markers were used in an association study to identify genomic regions underlying stalk rot resistance using a multi-locus mixed model association mapping approach. We identified 14 loci associated with stalk rot and a set of candidate genes that appear to be involved in connected functions controlling plant defense response to stalk rot resistance. The associated SNPs accounted for 19-30% of phenotypic variation observed within and across environments. An analysis of associated allele frequencies within the major sorghum subpopulations revealed enrichment for resistant alleles in the durra and caudatum subpopulations compared with other subpopulations. The findings suggest a complicated molecular mechanism of resistance to stalk rots. The objective of the third experiment was to determine the functional relationship between stay-green trait, leaf dhurrin and soluble sugar levels and resistance to stalk rot diseases. Fourteen genotypic groups derived from a Tx642 × Tx7000 RIL population carrying combinations of stay-green quantitative trait loci were evaluated under three environments in four replications. The stg QTL had variable effects on stalk rot disease. Genotypes carrying stg1, stg3, stg1,3 and stg1,2,3,4 expressed good levels of resistance to M. phaseolina but the combination of stg1 and stg3 was required to express the same level of resistance to F. thapsinum. Other stg QTL blocks such as stg2 and stg4 did not have any impact on stalk rot resistance caused by both pathogens. There were no significant correlations between leaf dhurrin, soluble sugar concentration, and resistance to any of the pathogens.

A cultura do milho é uma das atividades agrícolas de maior importância no Brasil. O país é o terceiro maior produtor mundial e a produtividade média passou de 1800 kg ha-1 para 3000 kg ha-1 nos últimos 15 anos. A produção de grãos e a tolerância à seca são caracteres complexos e de difícil seleção e uma possibilidade para se aumentar a eficiência dos programas de melhoramento que visam o incremento da produção de grãos e a tolerância à seca seria através da seleção indireta para caracteres relacionados a elas como o stay-green e os componentes da produção. O presente trabalho teve por objetivo estudar os caracteres stay-green, produção de grãos e seus componentes em milho com a finalidade de reunir informações sobre a herança desses caracteres. Para tanto foi utilizada uma população obtida pelo cruzamento das linhagens L-14-04 B e L-08- 05 F, contrastantes para diversos caracteres. Progênies F2:3 derivadas desses cruzamento foram retrocruzadas com as linhagens parentais formando dois conjuntos com 250 progênies de retrocruzamento cada. Essas progênies foram avaliadas em até seis ambientes segundo o delineamento de látice simples 10x10 para os caracteres produção de grãos (PG), e seus componentes: comprimento de espiga (CE), diâmetro de espiga (DE), número de fileiras por espiga (NFi), número de grãos por fileira (NGF) e peso de 500 grãos (P500) e para o caráter relacionado a tolerância à seca stay-green (SG). Um mapa de ligação obtido com 177 marcadores SSR foi utilizado para mapear QTL para esses caracteres em cada uma das populações de retrocruzamento através da metodologia de mapeamento por intervalo composto expandido para múltiplos ambientes (mCIM). Os efeitos genéticos aditivos e de dominância para cada QTL mapeado foram obtidos por meio de contrastes entre os efeitos dos QTL mapeados nas duas populações de retrocruzamento. Foram mapeados 217 QTL para os sete caracteres avaliados sendo alguns desses de maior efeito, estáveis através dos diversos ambientes e co-localizados com QTL para outras características, o que os tornam bons candidatos para seleção assistida por marcadores moleculares. O grande número de QTL mapeados nesse estudo confirma não só a complexidade desses caracteres como também o poder de detecção do método de mapeamento (mCIM) e do delineamento utilizado nesse estudo (delinemento III). / Maize is one of the most important agricultural activities in Brazil. The country is the third largest world producer and the yield rose from 1800 kg ha-1 to 3000 kg ha-1 in the last 15 years. The grain yield and drought tolerance are complex traits, difficult to select. A possibility to increase the efficiency of breeding programs that aims to improve the grain yield and drought tolerance would be through indirect selection for traits related to them as the stay-green and yield components. The aim of this work was to study the characters stay-green, yield and its components in maize in order to gather information on the inheritance of these characters. For this purpose it was used a population obtained by crossing the lines L-14-04 B and L-08-05 F, contrasting to several traits. Progenies F2:3 from this cross were derived from backcrosses to the parental lines forming two sets of backcross progeny with 250 each. These progenies were evaluated in up to six environments according to the simple lattice design 10x10 for grain yield (PG), and their components: ear length (CE), ear diameter (DE), number of rows per ear (NFI), number of grains per row (NGF), weight of 500 grains (P500) and the drought tolerance related trait stay-green (SG). A linkage map obtained with 177 SSR markers were used to map QTL for these traits in each backcross population through the methodology of composite interval mapping expanded to multiple environments (mCIM). The additive and dominance genetic effects for each mapped QTL were obtained by means of contrasts between effects of QTL mapped in the two backcross populations. Two hundred seventeen QTL were mapped for seven traits evaluated, some of these are of mayor effect, stable through the various environments and co-located with QTL for other characteristics, what make them good candidates for molecular marker assisted selection. The large number of QTL mapped in this study confirms not only the complexity of these characters but also the detection power of the mapping method (mCIM) and of the design used in this study (design III).

A cultura do milho é uma das atividades agrícolas de maior importância no Brasil. O país é o terceiro maior produtor mundial e a produtividade média passou de 1800 kg ha-1 para 3000 kg ha-1 nos últimos 15 anos. A produção de grãos e a tolerância à seca são caracteres complexos e de difícil seleção e uma possibilidade para se aumentar a eficiência dos programas de melhoramento que visam o incremento da produção de grãos e a tolerância à seca seria através da seleção indireta para caracteres relacionados a elas como o stay-green e os componentes da produção. O presente trabalho teve por objetivo estudar os caracteres stay-green, produção de grãos e seus componentes em milho com a finalidade de reunir informações sobre a herança desses caracteres. Para tanto foi utilizada uma população obtida pelo cruzamento das linhagens L-14-04 B e L-08- 05 F, contrastantes para diversos caracteres. Progênies F2:3 derivadas desses cruzamento foram retrocruzadas com as linhagens parentais formando dois conjuntos com 250 progênies de retrocruzamento cada. Essas progênies foram avaliadas em até seis ambientes segundo o delineamento de látice simples 10x10 para os caracteres produção de grãos (PG), e seus componentes: comprimento de espiga (CE), diâmetro de espiga (DE), número de fileiras por espiga (NFi), número de grãos por fileira (NGF) e peso de 500 grãos (P500) e para o caráter relacionado a tolerância à seca stay-green (SG). Um mapa de ligação obtido com 177 marcadores SSR foi utilizado para mapear QTL para esses caracteres em cada uma das populações de retrocruzamento através da metodologia de mapeamento por intervalo composto expandido para múltiplos ambientes (mCIM). Os efeitos genéticos aditivos e de dominância para cada QTL mapeado foram obtidos por meio de contrastes entre os efeitos dos QTL mapeados nas duas populações de retrocruzamento. Foram mapeados 217 QTL para os sete caracteres avaliados sendo alguns desses de maior efeito, estáveis através dos diversos ambientes e co-localizados com QTL para outras características, o que os tornam bons candidatos para seleção assistida por marcadores moleculares. O grande número de QTL mapeados nesse estudo confirma não só a complexidade desses caracteres como também o poder de detecção do método de mapeamento (mCIM) e do delineamento utilizado nesse estudo (delinemento III). / Maize is one of the most important agricultural activities in Brazil. The country is the third largest world producer and the yield rose from 1800 kg ha-1 to 3000 kg ha-1 in the last 15 years. The grain yield and drought tolerance are complex traits, difficult to select. A possibility to increase the efficiency of breeding programs that aims to improve the grain yield and drought tolerance would be through indirect selection for traits related to them as the stay-green and yield components. The aim of this work was to study the characters stay-green, yield and its components in maize in order to gather information on the inheritance of these characters. For this purpose it was used a population obtained by crossing the lines L-14-04 B and L-08-05 F, contrasting to several traits. Progenies F2:3 from this cross were derived from backcrosses to the parental lines forming two sets of backcross progeny with 250 each. These progenies were evaluated in up to six environments according to the simple lattice design 10x10 for grain yield (PG), and their components: ear length (CE), ear diameter (DE), number of rows per ear (NFI), number of grains per row (NGF), weight of 500 grains (P500) and the drought tolerance related trait stay-green (SG). A linkage map obtained with 177 SSR markers were used to map QTL for these traits in each backcross population through the methodology of composite interval mapping expanded to multiple environments (mCIM). The additive and dominance genetic effects for each mapped QTL were obtained by means of contrasts between effects of QTL mapped in the two backcross populations. Two hundred seventeen QTL were mapped for seven traits evaluated, some of these are of mayor effect, stable through the various environments and co-located with QTL for other characteristics, what make them good candidates for molecular marker assisted selection. The large number of QTL mapped in this study confirms not only the complexity of these characters but also the detection power of the mapping method (mCIM) and of the design used in this study (design III).

A informação sobre o tipo de herança de um caráter considerado para fins de seleção é de extrema importância para o sucesso dos programas de melhoramento. O caráter senescência retardada, usualmente chamado de stay-green, tem sido relacionado em diversas culturas à tolerância a estresses abióticos, principalmente ao estresse devido à seca. Embora a maioria dos híbridos de milho comerciais sejam stay-green, as informações sobre o seu tipo de herança são muito limitadas. Assim, este trabalho teve como objetivo estudar a herança do caráter stay-green em milho tropical. O material genético utilizado incluiu 55 linhagens de diversas origens, a fim de representar a variabilidade genética em milho tropical. Foram realizados cruzamentos dialélicos parciais, onde 50 linhagens foram cruzadas com outras 5 linhagens utilizadas como testadoras, originando 250 cruzamentos. Os 250 cruzamentos e seis híbridos comerciais foram avaliados em 8 ambientes no delineamento de látice simples 16x16 com duas repetições. O caráter stay-green foi avaliado em cinco plantas competitivas por parcela, 120 dias após a semeadura, através de uma escala de notas visual de 1 a 5, onde a nota 1 se referia às plantas verdes e a nota 5 às plantas secas. Foi necessário tomar dados de florescimento feminino para utilizá-los como covariável nas análises estatísticas e corrigir as diferenças de maturação entre os cruzamentos. A análise de variância dialélica foi realizada de acordo com o método 4 do modelo 1 de Griffing (1956), adaptado para dialelos parciais em múltiplos ambientes. A capacidade geral de combinação (CGC), tanto para as linhagens como para os testadores, e a capacidade específica de combinação (CEC) foram altamente significativas )01,0(<=P, mostrando que tanto a CGC como a CEC contribuíram significativamente para a expressão do caráter. Porém a contribuição da CGC foi de 69,06% e a da CEC foi de 30,94% para a variação entre cruzamentos, indicando que os efeitos aditivos, relacionados à CGC, são mais importantes que os efeitos não aditivos (dominância e epistasia), que são relacionados à CEC, na variação dos cruzamentos. Tanto a CGC como a CEC interagiram significativamente com o ambiente, evidenciando que estes parâmetros não são consistentes nos diversos ambientes. Então, a seleção para o caráter stay-green deve ser baseada em médias de experimentos avaliados com repetições em diversos ambientes. / Information on the inheritance of traits to be selected is of paramount importance for the success of breeding programs. The trait delayed senescence, usually named \"stay-green\" trait, has been related to tolerance to abiotic stresses, mainly drought stress, in several crop species. Although the majority of commercial maize hybrids are \"stay-green\", limited information are available on its inheritance. Thus, this research was conducted to study the inheritance of the stay-green trait in tropical maize. The genetic material included 55 inbred lines from several sources to represent the genetic variation of tropical maize. Fifty inbred lines were crossed to 05 inbreds as testers following the partial diallel cross design, giving rise to 250 single crosses. The crosses and six commercial hybrids, 256 entries, were evaluated at eight environments using a 16 x 16 lattice design with two replications per environment. The stay-green trait was recorded 120 days after sowing, in five competitive plants per plot, following a visual note scale, i.e., from 1 to 5, where 1 refers to green plants and 5 to no-green plants. Also, the trait days to mid-silking was recorded and used as covariate to correct for differences of maturing among crosses. The analysis of variance of the diallel crosses was computed following the method 4 model 1 of Griffing (1956) extended to multiple environments. The general combining ability (GCA) for both the inbreds and the testers, and the specific combining ability (SCA) were all highly significant (P<=0.01), showing that GCA as well as SCA contribute significantly for the expression of the trait. However, the contribution of the GCA was 69.06% and of the SCA was 30.94% for the variation among the crosses, indicating that the additive effects, which are related to GCA, are more important than the non-additive effects (dominance and epistasis), which are related to SCA, for the variation of the crosses. Both GCA and SCA interacted significantly with the environments, showing that these parameters were not consistent across the environments. Thus, selection for the stay-green trait should be based on the means of experiments evaluated in several environments.

A informação sobre o tipo de herança de um caráter considerado para fins de seleção é de extrema importância para o sucesso dos programas de melhoramento. O caráter senescência retardada, usualmente chamado de stay-green, tem sido relacionado em diversas culturas à tolerância a estresses abióticos, principalmente ao estresse devido à seca. Embora a maioria dos híbridos de milho comerciais sejam stay-green, as informações sobre o seu tipo de herança são muito limitadas. Assim, este trabalho teve como objetivo estudar a herança do caráter stay-green em milho tropical. O material genético utilizado incluiu 55 linhagens de diversas origens, a fim de representar a variabilidade genética em milho tropical. Foram realizados cruzamentos dialélicos parciais, onde 50 linhagens foram cruzadas com outras 5 linhagens utilizadas como testadoras, originando 250 cruzamentos. Os 250 cruzamentos e seis híbridos comerciais foram avaliados em 8 ambientes no delineamento de látice simples 16x16 com duas repetições. O caráter stay-green foi avaliado em cinco plantas competitivas por parcela, 120 dias após a semeadura, através de uma escala de notas visual de 1 a 5, onde a nota 1 se referia às plantas verdes e a nota 5 às plantas secas. Foi necessário tomar dados de florescimento feminino para utilizá-los como covariável nas análises estatísticas e corrigir as diferenças de maturação entre os cruzamentos. A análise de variância dialélica foi realizada de acordo com o método 4 do modelo 1 de Griffing (1956), adaptado para dialelos parciais em múltiplos ambientes. A capacidade geral de combinação (CGC), tanto para as linhagens como para os testadores, e a capacidade específica de combinação (CEC) foram altamente significativas )01,0(<=P, mostrando que tanto a CGC como a CEC contribuíram significativamente para a expressão do caráter. Porém a contribuição da CGC foi de 69,06% e a da CEC foi de 30,94% para a variação entre cruzamentos, indicando que os efeitos aditivos, relacionados à CGC, são mais importantes que os efeitos não aditivos (dominância e epistasia), que são relacionados à CEC, na variação dos cruzamentos. Tanto a CGC como a CEC interagiram significativamente com o ambiente, evidenciando que estes parâmetros não são consistentes nos diversos ambientes. Então, a seleção para o caráter stay-green deve ser baseada em médias de experimentos avaliados com repetições em diversos ambientes. / Information on the inheritance of traits to be selected is of paramount importance for the success of breeding programs. The trait delayed senescence, usually named \"stay-green\" trait, has been related to tolerance to abiotic stresses, mainly drought stress, in several crop species. Although the majority of commercial maize hybrids are \"stay-green\", limited information are available on its inheritance. Thus, this research was conducted to study the inheritance of the stay-green trait in tropical maize. The genetic material included 55 inbred lines from several sources to represent the genetic variation of tropical maize. Fifty inbred lines were crossed to 05 inbreds as testers following the partial diallel cross design, giving rise to 250 single crosses. The crosses and six commercial hybrids, 256 entries, were evaluated at eight environments using a 16 x 16 lattice design with two replications per environment. The stay-green trait was recorded 120 days after sowing, in five competitive plants per plot, following a visual note scale, i.e., from 1 to 5, where 1 refers to green plants and 5 to no-green plants. Also, the trait days to mid-silking was recorded and used as covariate to correct for differences of maturing among crosses. The analysis of variance of the diallel crosses was computed following the method 4 model 1 of Griffing (1956) extended to multiple environments. The general combining ability (GCA) for both the inbreds and the testers, and the specific combining ability (SCA) were all highly significant (P<=0.01), showing that GCA as well as SCA contribute significantly for the expression of the trait. However, the contribution of the GCA was 69.06% and of the SCA was 30.94% for the variation among the crosses, indicating that the additive effects, which are related to GCA, are more important than the non-additive effects (dominance and epistasis), which are related to SCA, for the variation of the crosses. Both GCA and SCA interacted significantly with the environments, showing that these parameters were not consistent across the environments. Thus, selection for the stay-green trait should be based on the means of experiments evaluated in several environments.