Pre-breeding of wheat (Triticum aestivum L.) for Biomass allocation and drought tolerance.

Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.Bread wheat (Triticum aestivum L., 2n=6x=42) is the third most important cereal crop globally after maize and rice. However, its production and productivity is affected by recurrent drought and declining soil fertility. Wheat cultivars with a well-balanced biomass allocation and improved root systems have better water- and nutrient-use efficiency and, hence, increased productivity under dry-land farming systems. The overall objective of this study was to develop breeding populations of wheat with enhanced drought tolerance and biomass allocation under water-limited conditions. The specific objectives of the study were: (i) to evaluate agronomic performance and quantify biomass production and allocation between roots and shoots in selected wheat genotypes in response to different soil water levels to select promising genotypes for breeding for drought tolerance and carbon (C) sequestration, (ii) to determine variance components and heritability of biomass allocation and grain yield related traits among 99 genotypes of bread wheat and triticale (Triticosecale Wittmack) to optimize biomass partitioning for drought tolerance, (iii) to deduce the population structure and genome-wide marker-trait association of yield and biomass allocation traits in wheat to facilitate marker-assisted selection for drought tolerance and C sequestration, and (iv) to estimate the combining ability of selected wheat genotypes and their progenies for agronomic traits, biomass allocation and yield under drought-stressed and non-stressed conditions for future breeding and genetic advancement for drought tolerance and C sequestration. To achieve these objectives, different experiments were conducted. In the first study, 99 wheat genotypes and one triticale accession were evaluated under drought-stressed and non-stressed conditions in the field and greenhouse using a 10×10 alpha lattice design with two replications. Data on the following phenotypic traits were collected: days to heading (DTH), number of productive tillers per plant (NPT), plant height (PH), days to maturity (DTM), spike length (SL), thousand kernel weight (TKW), root and shoot biomass (RB and SB), root to shoot ratio (RS) and grain yield (GY). There was significant (p<0.05) genotypic variation for grain yield and biomass production. The highest grain yield of 247.3 g m-2 was recorded in the genotype LM52 and the least was in genotype Sossognon with 30 g m-2. Shoot biomass ranged from 830g m-2 (genotype Arenza) to 437 g m-2 (LM57), whilst root biomass ranged between 140 g m-2 for LM15 and 603 g m-2 for triticale. Triticale also recorded the highest RS of 1.2, while the least was 0.30 for LM18. Water stress reduced total biomass production by 35% and RS by 14%. Genotypic variation existed for all measured traits useful for improving drought tolerance, while the calculated RS values can improve accuracy in estimating C sequestration potential of wheat. The following genotypes: BW140, BW141, BW152, BW162, LM26, LM47, LM48, LM71, LM70 and LM75 were selected for further development based on their high grain and biomass production, low drought sensitivity and marked genetic diversity. In the second study, data obtained from the above experiment were subjected to analyses of variance to calculate variance components, heritability and genetic correlations. Significant (p≤0.05) genetic and environmental variation were observed for all the traits except for spike length. Drought stress decreased the heritability of RS from 47 to 28% and GY from 55 to 17%. The genetic correlations between RS with PH, NPT, SL, SB and GY were weaker under drought-stress (r ≤ - 0.50; p70%) for RS observed in this population constitute several bottlenecks for improving GY and RS simultaneously. However, indirect selection for DTH, PH, RB, and TKW, could help optimize RS and simultaneously improve drought tolerance and yield under drought-stressed condition. In the third study, the 99 wheat genotypes and one triticale accession were genotyped using 28,356 DArTseq derived single nucleotide polymorphism (SNPs) markers. Phenotypic and genomic data were subjected to genome wide association study (GWAS). Population structure analysis revealed seven clusters with a mean polymorphic information content of 0.42, showing a high degree of diversity. A total of 54 significant marker-trait associations (MTAs) were identified. Twenty-one of the MTAs were detected under drought-stressed condition and 11% were on the genomic loci where quantitative trait loci (QTLs) for GY and RB were previously identified, while the remainder are new events providing information on biomass allocation. There were four genetic markers, two under each water treatment, with pleiotropic genetic effect on RB and SB that may serve as a means for simultaneous selection. Significant MTAs observed in this study will be useful in devising strategies for marker-assisted breeding to improve drought tolerance and to enhance C sequestration capacity of wheat. Lastly, 10 better performing and genetically diverse wheat genotypes selected during the first experiment were crossed using a half diallel mating design to generate F1 families. The parents and crosses were evaluated using a completely randomized block design with 2 replications under a controlled environment condition. Significant (p<0.05) genotype by water regime interaction effects were recorded for RB, SB, RS and GY. Root and shoot biomass were reduced by 48 and 37%, respectively, due to drought stress hindering biomass allocation patterns and hence C sequestration potential of the tested genotypes. Further, drought stress reduced RS and GY by 18 and 28%, respectively compared with the non-stressed treatment. Analysis of variance showed that both general combining ability (GCA) and specific combining ability (SCA) effects were significant (p<0.05) in conditioning the inheritance of grain yield and related traits and biomass allocation. Non-additive gene effects were more important in controlling the inheritance of the measured traits under drought-stressed and non-stressed conditions. Parental genotypes LM47 and BW140 had significant and positive GCA effects for root and shoot biomass and GY under drought-stressed conditions. These are recommended for recurrent selection programs to improve the respective traits. The crosses BW141×LM48 and LM47×LM75 were good specific combiners for biomass allocation and GY under drought stress, while BW141×LM48 and LM48×LM47 were good combiners under non-stressed condition. These families are selected for advanced breeding to develop pure line cultivars. The preliminary results suggest that simultaneous improvement of grain yield and root biomass can be realized to improve drought tolerance and C sequestration ability in wheat. Overall, the study detected marked phenotypic and genetic variation among diverse set of wheat genetic resources and candidate crosses for drought tolerance and biomass allocation through field and greenhouse based experiments and genomic analyses. The selected parents and novel crosses are useful for wheat breeding to enhance drought tolerance, yield and yield components and biomass allocation for C sequestration. This is the first study that evaluated biomass allocation in wheat as a strategy to improve drought tolerance and carbon sequestration

Combining ability, genetic gains and path coefficient analyses of maize hybrids developed from maize streak virus and downey mildew resistant recombinant inbred lines.

Master of Science in Plant breeding.Farmers in SSA continue to obtain low yields (less than two tonnes per hectare) despite the high potential yield (about 14 tonnes per hectare) that can be achieved. The development of improved and high yielding hybrids can help to reduce this gap significantly. Characterisation of maize inbred lines is crucial for developing high yielding maize hybrids. A line x tester analysis involving 38 crosses generated by crossing 19 maize inbred lines with two tropical testers was conducted for different agronomic traits. The maize inbred lines used in this study were sampled from a bi-parental inbred population developed by a shuttle breeding program at University of KwaZulu Natal. The objectives of the study were to estimate combining ability of inbred lines and hybrids, to evaluate the performance of the hybrids in agronomic traits and grain yield, to calculate breeding gains achieved through selection and to deduce the relationship between secondary traits and grain yield. In total 50 hybrids, including control hybrids were evaluated in the trial. The hybrids were planted in the summer season of 2014/15 under rainfed conditions at three sites, Cedara, Dundee and Ukulinga in five metre row plots and replicated twice in 5X10 alpha lattice design under recommended agronomic practices for maize. Data was collected using a CIMMYT protocol and subjected to statistical analyses using ANOVA and REML packages in GENSTAT 14th edition and PATHSAS macros in SAS 9.3 computer software. The results showed varying performances between the lines, crosses and control hybrids at the different sites. Inbred lines DMSR-8, DMSR-13, DMSR-30 and DMSR-35-5 were shown to have good combining ability while DMSR-21 and DMSR-73 showed positive specific combining ability. Selection across sites improved grain yield by 9.32% over the population mean and by 10.22% and 12.73% at Cedara and Dundee, respectively over commercial hybrids. Ranking by mean yield identified hybrids 15XH16, 15XH20 and 15XH28 at Cedara, Dundee and Ukulinga respectively, as the highest yielding hybrids for that particular environment. GGE biplot and AMMI analyses revealed that hybrids 15XH10, 15XH13, 15XH20, 15XH25, 15XH28, 15XH34 and 15XH39 were the most stable hybrids. Secondary traits were found to be associated with grain yield potential of hybrids. Ear prolificacy had the most important relationship with grain yield and was recommended for selection in grain yield improvement programs

Genome-wide association analysis of bean fly resistance and agromorphological traits in common bean

The bean fly (Ophiomyia spp) is a key insect pest causing significant crop damage and yield loss in common bean (Phaseolus vulgaris L., 2n = 2x = 22). Development and deployment of agronomic superior and bean fly resistant common bean varieties aredependent on genetic variation and the identification of genes and genomic regions controlling economic traits. This study’s objective was to determine the population structure of a diverse panel of common bean genotypes and deduce associations between bean fly resistance and agronomic traits based on single nucleotide polymorphism (SNP) markers. Ninety-nine common bean genotypes were phenotyped in two seasons at two locations and genotyped with 16 565 SNP markers. The genotypes exhibited significant variation for bean fly damage severity (BDS), plant mortality rate (PMR), and pupa count (PC). Likewise, the genotypes showed significant variation for agro-morphological traits such as days to flowering (DTF), days to maturity (DTM), number of pods per plant (NPP), number of seeds per pod (NSP), and grain yield (GYD). The genotypes were delineated into two populations, which were based on the Andean and Mesoamerican gene pools. The genotypes exhibited a minimum membership coefficient of 0.60 to their respective populations. Eighty-three significant (P<0.01) markers were identified with an average linkage disequilibrium of 0.20 at 12Mb across the 11 chromosomes. Three markers were identified, each having pleiotropic effects on two traits: M100049197 (BDS and NPP), M3379537 (DTF and PC), and M13122571 (NPP and GYD). The identified markers are useful for marker-assisted selection in the breeding program to develop common bean genotypes with resistance to bean fly damage

Assessment of genetic diversity among Bambara groundnut lines using SSR markers

Abstract Evaluating available genetic diversity is important for crop improvement especially in neglected crops such as Bambara groundnut. The objective was to characterise genetic variation among 19 Bambara groundnut lines using 20 simple sequence repeat (SSR) markers for initiating Bambara groundnut improvement in South Africa. Genomic DNA was extracted from leaf samples of each genotype and genotyped with diagnostic SSR markers. The SSR fragments were scored numerically as codominant before the genetic parameters were calculated using GenAlex software. The markers exhibited an average polymorphic information content (PIC) of 0.57. Similarly, the observed heterozygosity among the lines was 0.58, showing some level of outcrossing. The first two principal components explained 65.2% of the variation among the genotypes and grouped the genotypes into three clusters. The shortest genetic distance was zero between genotypes Brown and Light brown while Tiganecuru and S19 were farthest related genotypes (0.60). Lines such as IITA686, Cream and Uniswa Red-R that had good performance for multiple agronomic traits occurred within 0.40 genetic distance making them genetically divergent enough for developing breeding populations for Bambara groundnut improvement. The most divergent genotypes with favourable agro-morphological characteristics will be recommended as genetic resources for Bambara improvement and variety development programs.</jats:p

Farmers’ preferred traits and perceived production constraints of bread wheat under drought-prone agro-ecologies of Ethiopia

Abstract Background Bread wheat (Triticum aestivum L.) is a valuable commodity crop for local, regional and global markets. In Ethiopia, wheat ranks third after maize (Zea mays L.) and tef (Eragrostis tef Zucc.) in terms of total production, and fourth after maize, tef and sorghum (Sorghum bicolor L.) in areas of cultivation. The major wheat-producing areas are mainly found in the mid-altitude (1900 to 2300 m above sea level) and high-altitude (2300 to 2700 m above sea level) regions of the country that are regarded as high-potential environments due to their high and reliable rainfall. However, wheat is widely adapted and grows in diverse environments. It is produced mainly under rainfed condition by small-scale farmers. The country is ranked first in wheat production in sub-Saharan Africa (SSA) followed by South Africa, Sudan and Kenya. However, the average productivity of the crop is 2.4 tons ha–1, which is lower than the global (3.4 tons ha–1) average, which is due to low adoption of new improved varieties. The objective of this study was to assess farmers’ preferred traits of bread wheat variety, factors influencing their adoption for new improved variety and perceived production constraints of wheat under drought-prone agro-ecologies of Ethiopia. Methodology The study was conducted in selected districts of Arsi Zone in the Oromia Regional State of Ethiopia during 2018. A multistage random sampling was employed to arrive at household level. Data were collected based on primary and secondary sources. Relationships were examined through frequency, percentages and Chi-square values within and between districts for variables considered. Kendall’s coefficient of concordance (W) analysis was used to identify the varietal attributes that are most preferred by the farmers. Binary logistic regression model was used to determine the factors influencing farmers’ adoption of improved varieties. Rank Based Quotient (RBQ) was computed to identify the most important production constraints perceived by the farmers in the study areas. Results High grain yield was the most preferred trait as perceived by the farmers in the study areas followed by stress adaptation (drought and heat stress tolerance), disease resistance and early maturity. The binary logistic regression model showed socio-demographic characteristics, such as education had positive and significant (p &lt; 0.01) effect on adoption of new improved bread wheat varieties. Gender and access to extension officers affected the adoption negatively and significantly (p &lt; 0.05). Varietal attributes, such as early maturity (p &lt; 0.01) and plant height (p &lt; 0.05), had positive and significant effects on adoption of new improved varieties, while adaptation and baking quality had negative and significant (p &lt; 0.05) influences on the acceptance of the new improved varieties. Moisture stress, disease (especially rust) and the high cost of fertilizers were, in order, first-, second- and third-ranked production constraints in the study areas. Conclusions Farmers had different variety-specific trait preferences. Grain yield, rust resistance, adaptation to drought and heat stresses, and early maturity were the most farmer-preferred traits. Socio-demographic factors, such as gender, education level and access to extension officers, influenced variety adoption by the farmers. Early maturity, plant height, baking quality and stress adaptation were the major varietal characteristics contributing towards adoption of new improved bread wheat varieties. Drought stress, disease (especially rust) and the high cost of fertilizers were among the major constraints of wheat production identified by the farmers. This study can serve as a guide for future wheat breeding programmes incorporating farmer-preferred traits, including stress adaptation (drought and heat tolerance). This will enhance adoption of newly developed improved varieties and for sustainable production and food security of smallholder farmers in drought-prone areas. Future wheat improvement and extension programmes of the National Agricultural Research System (NARS) should involve marginal wheat-growing environments to boost adoption of new improved varieties and the production potential of the crop. </jats:sec

What crop type for atmospheric carbon sequestration: Results from a global data analysis

International audienceSequestration of atmospheric carbon (C) into soils is a strategy to compensate for anthropogenic emissions of carbon dioxide. The response of SOCs to crop types is yet to be determined under different environments. The objectives of this study were to elucidate the impact of crop type on the allocation of atmospheric C to shoots and roots, and ultimately to the soils and to determine its association with soil carbon stocks. Three hundred and eighty-nine field trials were compared to determine allocation of biomass and C in plants and SOCs under fields of different crop types. Grasses had the highest plant biomass production (19.80 ± 1.16 Mg ha−1 yr−1), followed by cereals (9.44 ± 0.45 Mg ha−1 yr−1), fibre (7.90 ± 1.00 Mg ha−1 yr−1), legumes (3.29 ± 0.63 Mg ha−1 yr−1), and oil crops (3.05 ± 1.16 Mg ha−1 yr−1) showing significant differences (p −1 yr−1) had the highest plant C amongst summer crops, while wheat (2.2 ± 0.35 Mg C ha−1 yr−1) had the highest plant C amongst winter crops. In all the studies, crops allocated more C to their shoots than roots yielding root C: shoot C (Rc/Sc) ratios below magnitude. The greatest C allocation to roots was in grasses (Rc/Sc = 1.19 ± 0.08), followed by cereals (0.95 ± 0.03), legumes (0.86 ± 0.04), oil crops (0.85 ± 0.08), and fibre crops (0.50 ± 0.07). There was evidence that high plant C stocks were found in crops grown under carbon rich clayey soils of tropical humid areas. Natural grasses and cereals should be promoted as they appeared to yield greater potential for atmospheric carbon sequestration in plants and soils. Overall, the study evaluated the relative potential of the main crop types to sequester atmospheric C useful in screening of crop types for carbon efficiency and for development of plant C models

Wheat production in the highlands of Eastern Ethiopia: opportunities, challenges and coping strategies of rust diseases

Ethiopia is the primary wheat producer in Sub-Saharan Africa (SSA) owing to the suitable agro-ecological conditions. Despite wheats economic potential for food security, the actual yield under smallholder farmers conditions is low due to various production constraints. Thus, the objectives of this study were to assess the present wheat production opportunities and constraints and identify farmer-preferred traits to guide variety design with stem rust-resistance and economic traits in eastern Ethiopia. Data on production constraints and trait preferences were collected using structured questionnaires involving 144 wheat-producing farmers. Wheat rust (reported by 97.3% of respondents), small land size (90.4%) and a lack of improved varieties (75.6%) were identified as the major constraints. About 41.7% of respondents in the West-Hararghe and 27.8% in the East-Hararghe zones did not use crop protection strategies to control rusts. Substantial respondent farmers used cultural practices (18.8%), rust-resistant cultivars (13.2%) or, a combination of these (10.4%) to control rust diseases. The essential farmer preferred traits in a wheat variety were rust resistance, high yield potential and good quality grain for bread making. Therefore, there is a need to breed new varieties with high grain yield and quality and durable rust resistance for sustainable wheat production in eastern Ethiopia

Are There Wheat Cultivars Allowing Enhanced Carbon Allocation to Soils?

The transfer of atmospheric carbon (C) in soils is a possible strategy for climate change mitigation and for restoring land productivity. While some studies have compared the ability of existing crops to allocate C into the soil, the genetic variations between crop genotypes have received less attention. The objective of this study was to compare the allocation to the soil of atmospheric C by genetically diverse wheat genotypes under different scenarios of soil water availability. The experiments were set up under open-field and greenhouse conditions with 100 wheat genotypes sourced from the International Maize and Wheat Improvement Centre and grown at 25% (drought stressed) and 75% (non-stressed) field capacity, using an alpha lattice design with 10 incomplete blocks and 10 genotypes per block. The genotypes were analyzed for grain yield (GY), plant shoot and root biomass (SB and RB, respectively) and C content, and stocks in plant parts. Additionally, 13C pulse labeling was performed during the crop growth period of 10 selected genotypes for assessing soil C inputs. The average GY varied from 75 to 4696 g m−2 and total plant biomass (PB) from 1967 to 13,528 g m−2. The plant C stocks ranged from 592 to 1109 g C m−2 (i.e., an 87% difference) under drought condition and between 1324 and 2881 g C m−2 (i.e., 117%) under well-watered conditions. Atmospheric C transfer to the soil only occurred under well-drained conditions and increased with the increase in the root to shoot ratio for C stocks (r = 0.71). Interestingly, the highest transfer to the soil was found for LM-26 and LM-47 (13C/12C of 7.6 and 6.5 per mille, respectively) as compared to LM-70 and BW-162 (0.75; 0.85). More is to be done to estimate the differences in C fluxes to the soil over entire growing seasons and to assess the long-term stabilization of the newly allocated C. Future research studies also need to identify genomic regions associated with GY and soil C transfer to enable the breeding of “carbon-superior” cultivars.</jats:p