A seca é um dos estresses abióticos mais importantes na cultura do milho, o qual ocasiona reduções significativas na produção de grãos. A arquitetura genética da tolerância à seca é complexa, fazendo-se necessária a melhor compreensão desse caráter. Estudos envolvendo mapeamento associativo são úteis por explorarem a variação genética de caracteres quantitativos e, adicionalmente, levam em conta informações acerca de genótipos, ambientes e interações genótipo por ambiente (G × E). Ao considerar efeitos de G × E em modelos de mapeamento associativo há possibilidade de identificar regiões no genoma associadas à condições e ambientes específicos. Este trabalho teve como objetivo detectar associações relacionadas à tolerância à seca em milho por meio de um modelo de mapeamento associativo para múltiplos ambientes e múltiplos locos, o qual permitiu distinguir associações com efeitos ambiente-específico daquelas com efeitos principais e de interação associação por ambiente (QEI). O painel associativo foi composto por 190 linhagens, classificadas de acordo com os grupos heteróticos quanto ao tipo de grão. Marcadores SNPs (∼500k) foram utilizados para a genotipagem do painel associativo. Duas linhagens (L228-3 e L3) foram usadas como testadores comuns e os híbridos obtidos foram avaliados em duas localidades (Janaúba-MG e Teresina-PI), dois anos agrícolas (2010 e 2011), sob duas condições de tratamento (irrigado e não irrigado). Ao total, consideraram-se seis caracteres: peso de grãos, intervalo de florescimento, florescimento feminino e masculino, altura de planta e de espiga. Consideraram-se dois grupos de mapeamento, agrupados de acordo com os testadores utilizados. SNPs foram úteis para testar associações ao longo do genoma do milho e investigar o relacionamento genético entre indivíduos. O modelo de mapeamento associativo, com inclusão de informações sobre interação G × E, detectou o total de 179 associações, e o maior número de associações foram relacionadas aos caracteres de florescimento. A maioria das associações (168) apresentaram QEI significativo, sendo que o tamanho e a magnitude desses efeitos distinguiram-se de acordo com o ambiente em avaliação. Apenas o caráter florescimento feminino não apresentou associações com efeitos estáveis ao longo dos ambientes em estudo. A detecção de algumas associações em posições próximas do genoma evidenciam possíveis efeitos de pleiotropia. Algumas associações foram co-localizadas em regiões do genoma do milho relacionadas à tolerância à seca, sendo que algumas dessas associações estavam envolvidas a fatores pertencentes à vias metabólicas de interesse. O presente estudo forneceu informações úteis para a compreensão da base genética da tolerância à seca em milho sob os ambientes específicos em avaliação. / Drought is a severe stress factor in maize production and causes significant reduction in grain yield. Genetic architecture of drought tolerance is complex and a better understanding of this trait is required. Association mapping studies are useful to explore quantitative traits and simultaneously account for genetic backgrounds including genotype, environment and genotype-by-environment (G × E) interactions. By accounting for G × E into association mapping models it is possible to identify regions associated with specific environment and conditions. The main goal of this study was detect significant associations related to drought tolerance in maize via multi-environment-multi- locus association mapping model, distinguishing information about specific-environment effects from main and association-by-environment (QEI) effects. Our association panel was composed by 190 inbred lines classified according to heterotic groups. The panel was genotyped with ∼500K SNPs. Two inbred lines (L228-3 and L3) were used as common testers and testcrosses were assessed in two locations (Janaúba-MG e Teresina-PI), two years (2010 and 2011), under two treatment conditions (well-watered and water-stressed). A total of six traits were evaluated including grain yield, anthesis-silking interval, female and male flowering time, plant and ear height. Two mapping groups were considered, grouped by common testers. SNPs were used to test significant association along the maize genome and also to account for population structure and relatedness coefficient. Our mapping model detected a total of 179 associations and the highest number of associations were related to the flowering time measures. The most associations (168) showed significant QEI and the size and magnitude of those effects were distinguished by environment conditions. Only female flowering trait did not show stable effects across all environments. Mapped associations in nearby positions indicate plausible pleiotropy effects. Some associations were co-located in maize genome regions related to metabolic pathway factors. Our study support the detection of significant associations along the maize genome and contributes for understanding of the genetic basis of drought tolerance in maize.

Thesis (M.Sc. (Botany))-- University of Limpopo, 2013 / The aim of the study was to identify drought tolerant South African soybean cultivars for cultivation where water is a limited resource. Soybean [Glycine max. (L.) Merr] is one of the most important legumes in the world. A lot of attention has been focused on soybean cultivation in South Africa recently. Soybean production is mainly affected by several biotic and abiotic factors which reduce the yield and quality of the crop. Six South African soybean cultivars (LS 677, LS 678, Mopanie, Sonop, Knap and Pan 1564) and two American cultivars (R01 416 and R01 581) were carefully studied for morphological and physiological markers which contribute to drought tolerance. The study was conducted at the University of Limpopo (Turfloop campus). Soybean plants were grown in a glasshouse in a randomised block design given same amounts of nutrients and differing amounts of water (limited and overwatering). Data was collected at R3 growth stage by measuring several morphological (stem length, leaf surface area, flowers and seeds counts) and physiological (percentage chlorophyll, moisture content, total phenolics, total flavonoids, ureide content and antioxidant activity) parameters. An anatomical study was also carried out on the transverse sections of leaves, roots, leaf stalk and nodules. The different cultivars reacted differently to the three water treatments. LS 678 produced the tallest plants whereas those of Pan 1564 were the shortest. Water stress affected plants by reducing the number of flowers produced, the leaf surface area as well as the relative leaf water content. The moisture content of the growth medium was reduced faster as the plants matured and it was also lowered by the limited water availability. Percentage chlorophyll is another trait which was affected by water limitation. Cultivars with high phenolic and flavonoids content were associated with high antioxidant activity and slightly yielded higher than the others. The anatomical transverse sections of the roots and petioles have shown some secondary growth. The anatomy of the nodules of Mopani has shown some interesting differences in response to the three treatments. Limited water decreased xii the size of the vascular tissue and sclerenchyma as a result altering the functionality of the nodule. The anatomy of Sonop’s petiole had a thickened sclerenchymatous bundle sheath covering the phloem tissue. The sclerenchyma tissue is thought to guard against loss of water. The cross section of the leaf had a double layer of palisade mesophyll (upper surface) and only a single layer of spongy mesophyll (lower surface). In addition, the mesophyll and the epidermal cells of Mopani appeared much thicker. In terms of yield, there was no cultivar which yielded the highest but Mopani yielded the lowest. Since Mopani was low yielding, the main focus of the discussion was on the features (morphological, physiological and anatomical) of Mopani which can be associated with drought susceptibility. Some of these features include reduced stem length, large leaf surface area, low relative leaf water content, low growth medium moisture content and low antioxidant activity.

Thesis (M.Sc. (Soil Science)) --University of Limpopo, 2013 / Large amount of pre-plant nitrogen (N) fertilizer results in low nutrient-use-efficiency due to poor synchrony between soil N supply and maize demand, especially during N sensitive growth stages. Optimum maize production is dependent on adequate N availability to the crop during the critical vegetative and reproductive growth stages. High N fertilizer prices and maize yield decline are the main challenges faced by the Limpopo Province farmers. The objectives of this study were to compare growth and yield of maize under conventional and site-specific N management in a dryland farming system. The study was conducted in Leeukraal, Towoomba, Ga-Marishane and Radium in the Limpopo Province, South Africa. Experimental plots were laid out in a randomized complete block design, with four replications. Phosphorus was applied through band placement using a planter in all plots at a rate of 42 kg P/ha. Hybrid maize SNK 2147 was planted on a 20 by 20 m plot with Inter-row and Intra-row spacing of 0.9 and 0.35 m respectively. Treatments consisted of 3 N management strategies as follows, (i) No N application (N0), (ii) Site-specific N at a rate ranging between 18 and 33 kg N/ha (N1) and (iii) Conventional N application at 58 kg N/ha (N2). Treatment N2 was applied at a uniform rate during maize planting. Sufficiency index as an indication for N deficiency was determined using CCM-200 for treatment N1. The sufficiency index was determined during leaf stage V6, V10 and V14, and thereafter N was applied only when needed. Data were subjected to analysis of variance through Statistical Analysis System package. Mean separation tests were computed using Duncan’s Multiple Range Test. Maize grain yield at Leeukraal of 5.2 t/ha for N1 was higher than 3.2 and 4.0 t/ha of N0 and N2, respectively. There was no difference amongst the three N management approaches on the grain yield at Towoomba. The grain yield at Ga-Marishane for N1 of 2.2 t/ha was significantly higher than 1.7 t/ha of the N0. Conventional management approach, which is a traditional approach used by farmers in the Limpopo Province, had 2.6 t/ha grain yield that was significantly higher than the N0 and N1. The maize growth and yield under N2 and N1 was compared, N1 required between 43 and 69% lesser N fertilizer as compared to N2. Therefore site-specific nutrient management approach sustains and improves growth and yield of maize using minimal inputs of N compared to conventional approach. This therefore saves input costs and avoids unnecessary environmental consequences. Key words: maize yield, nitrogen management, site-specific approach / Vlaamse Interuniveritatire Raad and Limpopo Department of Agriculture

Bibliography: leaves 201-228. Widespread deficiencies of mineral nutrients in soils along with limited moisture supply are considered major environmental stresses leading to yield losses in chickpea. This study was conducted to determine the zinc requirement of chickpea and the effect on plant water relations. Critical zinc concentration was estimated. It was found that high and low moisture regimes had no effect on critical zinc concentration and that the value remained almost the same in all chickpea genotypes. Sensitivity of 28 chickpea genotypes were evaluated at two zinc levels. Field studies on zinc fertilization in both Australia and Pakistan showed that the application of zinc increased grain yield in all chickpea genotypes. It was found that plants grown under zinc deficiency could not exploit available soil moisture and water use and water use efficiency was reduced, and concluded that high zinc availability may enhance the ability of plants to endure periods of drought by promoting osmotic adjustment.

Hemiepiphytes are important components of tropical forests and are attractive to scientists due to their unique epiphytic growth habit during some period of their life cycle. Unique characteristics in plant water relations and carbon economy have been found in hemiepiphytic plants; however, to further understand this group of species on an evolutionary basis it is necessary to carry out comparative studies between hemiepiphytes and their close relatives. In this dissertation I conduced a comparative study in a suite of functional traits related to plant water relations and photosynthesis between hemiepiphytic and non-hemiepiphytic tree species from a single genus-Ficus. Great differentiation in functional traits has been found between species of the two growth forms both during juvenile and adult stages. Seedlings of hemiepiphytic Ficus species (H) had significantly lower xylem hydraulic conductivity, stomatal conductance, net light saturated CO2 assimilation, and higher water use efficiency than congeneric non-hemiepiphytic species (NH), which are adaptive to a drought-prone epiphytic growth conditions under natural conditions. The conservative water use adaptation in H species is likely crucial to the drought tolerance and survival in the forest canopy but is related to much lower growth rates than NH species. Species of the two growth forms both showed relatively large plasticity in responding to variation in light level as in typical light-demanding species. Surprisingly, the NH species showed characteristics related to higher light demand than H species, which is opposite from the prediction that H species are more light-demanding than NH species. Thus, although commonly accepted, it is likely that light was not the selective pressure for the evolution of hemiepiphytism in Ficus. Using adult trees grown in a common garden, I found that H species showed characteristics of more conservative water use even after they established connections to the soil. Moreover, H species showed significantly different traits in photochemistry compared to NH species due to hydraulic-photosynthetic coordination. The evolution of an epiphytic growth habit during the juvenile stage of a life cycle in the hemiepiphytic Ficus species thus involved changes in a suite of functional traits that persist during their terrestrial growth stages.

The economic feasibility of producing biomass-based fuels requires high-yielding feedstocks to supply biomass to biorefineries. Populus trees are a potential biomass feedstock due to their high yield, but their high water requirement limits productivity under drought conditions. The number of genes controlling drought tolerance, and the long generation time for perennial species, slows cultivar development. Accelerated domestication proposes using the sequenced Populus genome to quickly incorporate target traits into productive clones by transgenesis. Six putative drought tolerance transcription factors: DREB2A, DREB2B, AtMYB, AREB1/ABF2, MYB, and NAC, had been previously identified and manipulated in eastern cottonwood (Populus deltoides). Three constructs of each gene were transformed into a P. deltoides background clone, including constitutive overexpression (OE), drought inducible OE, and knockdown. This greenhouse study examines the effect of these previously transformed constructs on drought tolerance by characterizing leaf abscission, leaf water potential, and growth under drought and well-watered conditions. AREB1/ABF2 constitutive OE lost significantly fewer leaves under drought than the Vector control, and had one of the lowest rates of leaf loss overall. Both DREB2A inducible OE and AREB1/ABF2 constitutive OE plants were more productive than the Vector control under drought conditions. MYB inducible OE was a productive construct and initially appeared to be drought tolerant. It is possible that this construct experienced xylem cavitation early on due to the severity of drought experienced by the large trees containing this construct. DREB2A inducible OE, AREB1/ABF2 constitutive OE, and MYB inducible OE were the most productive constructs as well as being likely to confer drought tolerance. Field trials would be the next step, providing a clearer picture of how these constructs would perform under natural conditions.

The growth and development of two sorghum drought tolerant genotypes BTx642 (post-flowering drought tolerant, “stay green”) and RTx7000 (pre-flowering drought tolerant) were characterized and compared. Differences in phenology and the growth and development of leaves and stalks were identified that could contribute to variation in shoot biomass, grain yield and response to water deficit. An F12 recombinant inbred line (RIL) population derived from the two parents was genotyped using the Illumina Genome Analyzer II platform and the information used to generate a genetic map useful for analysis of quantitative trait loci (QTL). Seventy-two different traits were measured in the RIL population at anthesis and at grain maturity. Plants were grown in well-watered environments in greenhouse conditions and in field conditions near College Station, TX in 2008-2010. QTL mapping was used to analyze the genetic basis of trait variation in the population and to detect associations between traits. A total of 477 QTL were identified that in combination modulate leaf size (length, width, and area), shoot biomass accumulation (shoot, stalk, stem, leaf, and leaf sheath), panicle weight, root size and architecture (length, surface area, and volume, number of tips, forks and nodal roots, and root biomass), stalk and stem length, and flowering time. Six flowering time QTL were identified and variation in time to anthesis affected the expression of several other traits including leaf size and biomass accumulation. However, QTL infrequently had an impact on traits associated with different organs. The specificity observed is consistent with independent genetic control of traits associated with leaves, stems and roots. Nine QTL that modulated shoot biomass accumulation were detected that were not affected by flowering time. Of these, four shoot biomass QTL co-localized with leaf size traits. Eight QTL for panicle biomass were detected with two coincident with QTL for upper leaf size. A QTL for leaf width at anthesis was found to co-localize with a stay green locus.

No description available.

Thesis (Ph.D.)--University of Hawaii at Manoa, 1994. / Includes bibliographical references (leaves 97-114). / Microfiche. / xiii, 114 leaves, bound ill. 29 cm

Bibliography: leaves 201-228. / xvii, 228 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Widespread deficiencies of mineral nutrients in soils along with limited moisture supply are considered major environmental stresses leading to yield losses in chickpea. This study was conducted to determine the zinc requirement of chickpea and the effect on plant water relations. Critical zinc concentration was estimated. It was found that high and low moisture regimes had no effect on critical zinc concentration and that the value remained almost the same in all chickpea genotypes. Sensitivity of 28 chickpea genotypes were evaluated at two zinc levels. Field studies on zinc fertilization in both Australia and Pakistan showed that the application of zinc increased grain yield in all chickpea genotypes. It was found that plants grown under zinc deficiency could not exploit available soil moisture and water use and water use efficiency was reduced, and concluded that high zinc availability may enhance the ability of plants to endure periods of drought by promoting osmotic adjustment. / Thesis (Ph.D.)--University of Adelaide, Dept. of Plant Science, 1999