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

Evaluation of bread wheat (Triticum aestivum L.) genotypes for yield and related traits under drought stress conditions

Drought is a major factor threatening crop production worldwide. Developing wheat varieties that are adapted to drought prone environments is a sustainable strategy to improve wheat production and productivity. The aim of this study was to evaluate and select bread wheat genotypes for yield and yield components, and for stability under drought stress and non-stress conditions. One hundred and twenty genotypes were evaluated at five test sites in the 2018/19 cropping season using a 10 x 12 alpha lattice design with two replicates. The level of drought stress was imposed using different sowing dates (early planting representing non-stressed, while late planting as drought stressed conditions) following the onset of the main rain at each site. Grain yield and yield components were recorded, and drought indices were calculated for each genotype. Among the drought tolerance indices, GMP, MP, HM, STI and YI were found to be the most suitable for predicting drought tolerance because they had significant and positive correlations with yield under drought stress and non-stress conditions. Rank sum analysis identified the most drought tolerant genotypes as ‘YS-34', ‘YS-85' and ‘YS-82’. The selected wheat genotypes are useful genetic resources for future drought tolerance breeding programmes in Ethiopia or similar agro-ecologies

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

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

Genetic Improvement of Wheat for Drought Tolerance: Progress, Challenges and Opportunities

Wheat production and productivity are challenged by recurrent droughts associated with climate change globally. Drought and heat stress resilient cultivars can alleviate yield loss in marginal production agro-ecologies. The ability of some crop genotypes to thrive and yield in drought conditions is attributable to the inherent genetic variation and environmental adaptation, presenting opportunities to develop drought-tolerant varieties. Understanding the underlying genetic, physiological, biochemical, and environmental mechanisms and their interactions is key critical opportunity for drought tolerance improvement. Therefore, the objective of this review is to document the progress, challenges, and opportunities in breeding for drought tolerance in wheat. The paper outlines the following key aspects: (1) challenges associated with breeding for adaptation to drought-prone environments, (2) opportunities such as genetic variation in wheat for drought tolerance, selection methods, the interplay between above-ground phenotypic traits and root attributes in drought adaptation and drought-responsive attributes and (3) approaches, technologies and innovations in drought tolerance breeding. In the end, the paper summarises genetic gains and perspectives in drought tolerance breeding in wheat. The review will serve as baseline information for wheat breeders and agronomists to guide the development and deployment of drought-adapted and high-performing new-generation wheat varieties

Morphological variations of wheat (Triticum aestivum L. em. Thell.) under variable ethyl methanesulphonate mutagenesis

Genetic gains in wheat yield have stagnated over the years due to genetic drift. There is a need to create new genetic variation for yield using various methods including ethyl methanesulphonate (EMS). The use of EMS mutagenesis is limited by variations in lethality, efficiency and effectiveness, which confound to selection. The objective of this study was to evaluate morphological variation in wheat after EMS mutagenesis. Wheat genotype LM43 was subjected to EMS mutagenesis under the following conditions: 0.1% v/v for 1 h at 25 °C, 0.1% v/v for 1 h at 30 °C and 0.7% v/v for 1.5 h at 25 °C. Some mutant plants in M1 had significantly (p < 0.05) higher spikelets per spike and kernels per spike. The number of tillers and kernels per spike increased significantly at M2 generation. EMS treatment with 0.1% v/v for 1 h at 30 °C was the most effective and efficient with the lowest biological damage. Macro-mutations were observed in spike, peduncle, awn and flag leaf morphology. The study identified early generation mutants that could be exploited for improving drought tolerance, yield and biomass, or for genetic analysis to identify quantitative trait loci in wheat

Phenotypic Divergence Analysis in Pigeonpea [Cajanus cajan (L.) Millspaugh] Germplasm Accessions

Pigeonpea (Cajanus cajan (L.) Millspaugh) is an important source of grain protein for low-income countries such as Malawi. Knowledge of the genetic diversity in pigeonpea is essential for an effective breeding program. The study objective was to assess the genetic diversity among diverse pigeonpea accessions to select complementary and unique genotypes for breeding. Eighty-one pigeonpea accessions were evaluated in six environments in Malawi using a 9 &times; 9 alpha-lattice design with two replications. The cross-tabulation analysis revealed a significant genotype variation on plant growth, flower, and seed traits. The combined analysis of variance identified genotypes MWPLR 14, ICEAP 01170, ICEAP 871091, and ICEAP 01285 as early maturing varieties, while Kachangu, MWPLR 16, TZA 5582, No. 40, and MWPLR 14 were identified as high-yielding genotypes. The correlation analysis revealed a significant positive correlation between grain yield and a hundred seed weight (HSWT) (r = 0.50, p &lt; 0.01), suggesting the usefulness of this trait for selection. The nonlinear principal component analysis identified grain yield (GDY), days to 50% flowering (DTF), days to 75% maturity (DTM), number of pods per plant (NPP), number of racemes per plant (NRP), 100 seed weight (HSWT), leaf hairiness (LH), and number of seeds per pod (NSP) as the most discriminated traits among the test genotypes. The cluster analysis using morphological traits delineated the accessions into three clusters. The selected high-yielding and early-maturing genotypes may be recommended as parental lines for breeding and grain yield improvement in Malawi or similar agro-ecologies

Genotype-by-environment interaction and stability analyses of grain yield in pigeonpea [Cajanus cajan (L.) Millspaugh]

Genotype-by-environment interaction (GEI) analysis is fundamental in crop improvement programmes to guide selection and for recommendation of high performing and stable genotypes for breeding or production. The objectives of this study were to quantify the GEI effects and determine grain yield stability among diverse pigeonpea genotypes to select candidate lines for breeding in Malawi. Eighty-one pigeonpea genotypes were tested under six environments using a 9 × 9 alpha-lattice design with two replications. Data collected were subjected to additive main effects and multiplicative interaction (AMMI) analysis and genotype plus genotype-by-environment interaction (GGE) biplot analysis. Genotype, environment, and genotype × environment interaction (GEI) accounted for 16.4, 33.5, and 49.6%, respectively, of the total variation for grain yield. The test environments were delineated into three mega-environments, based on site and seasonal variability. The AMMI and GGE analyses indicated that the top-yielding and stable genotypes across the test environments were MWPLR 24, ICEAP 01155, MWPLR 14, TZA 5582, and MWPLR 4. The selected genotypes are recommended as parental lines for grain yield improvement in Malawi or similar agro-ecologies