Breeding For Drought Tolerance And High Grain Yield Of Sorghum (Sorghum Bicolor [L.] Moench) In Tanzania

Abstract

ABSTRACT Sorghum is an important staple food crop for semi-arid areas in Tanzania. However, the productivity is very low due to several constraints, particularly drought. The present study was conducted to develop sorghum genotypes with improved drought tolerance and grain yield. The specific objectives were to: i) identify farmers‘ production constraints and traits preferences of sorghum in central Tanzania. ii) Introgress drought tolerant quantitative trait loci (QTL) from donor parents into farmers preferred sorghum varieties and iii) identify traits contributing to drought tolerance of sorghum genotypes. A participatory rural appraisal (PRA) was conducted at Kongwa district in Dodoma region and Ikungi and Iramba district in Singida region to identify constraints facing sorghum production, farmers‘ trait preferences and coping strategies to address drought. Two donor parents B35 and S35 with stay green (STG) QTL 1, 2 and 3 were introgressed to the farmers preferred sorghum varieties to develop F1, BC1F1 and BC2F1 populations. Five BC2F1 (NA307, W82, NA316, NA241 and SE438) populations and their parents were genotyped using 30 Single Nucleotide Polymorphism (SNP) markers. Results of the PRA showed that bird damage, poor soil fertility, drought stress, pests and diseases and lack of improved varieties were the major constraints of sorghum production. Early maturity, drought tolerance, high yield were ranked the highest preference when selecting new sorghum varieties. The coping strategies to address drought stress in sorghum involved early planting and use of drought tolerant varieties. Three (W82, NA241 and NA307) genotypes with heterozygous alleles and two genotypes (NA316 and SE438) with homozygous alleles were selected for selfing to generate BC2F3 populations. Moreover, plants with favourable alleles for either STG 1, 2, or 3 and good agronomic performance in field condition were selected. Eight genotypes namely NA241A, NA241B, NA307, NA316A, NA316B, NA316C, SE408 and SE438 from BC2F3 populations alongside with three parents and one check were selected after phenotyping BC2F2 population. The genotypes were planted in a split plot design under well watered and water stressed environments to determine the performance, correlation, heritability and genotypes by environments interaction. The results from the genotyping study revealed that, 7 out of 30 markers were for STG 1, 2 and 3 after genotyping of BC2F1 population; the remaining 23 markers were for traits contributing to STG in plants such as heat shock domain, programmed cell death triggering, aspartic proteases and chloroplast precursor. Genotyping of BC2F3 population indicated that, 7 SNP markers out of 10 had favourable alleles for STG in sorghum. Seventy one out of 728 BC2F1 samples genotyped were heterozygous. Of these, SNP markers snpSB00075, snSB00102 and snSB00103 were scored as heterozygous allele in seven samples of BC2F1 with B35*Wahi background. Markers snSB00049, snSB00077, snSB00102 and snSB00103 indicated heterozygous allele in 37 samples of BC2F1 with S35*Pato background. The rest (19) SNP markers showed homozygous alleles for BC2F1 population. Eighteen SNP markers indicated favourable alleles among 728 of BC2F1 samples genotyped including nsSB00049 and nsSB00054 for STG 1, snpSB00089 FOR STG 2 and nsSB00102 and nsSB00103 for STG 3. The rest of alleles were favourable for other roles related to STG in sorghum during post flowering drought condition. Similar trend were observed in the genotyping of BC2F3 population which seven SNP markers indicating favourable alleles for STG 1, STG 2 and STG 3. The SNP marker snpSB00089 indicated the highest (729) total number of favourable alleles for STG followed by snpSB00101 (728) and snpSB00102 with 727 after genotyping of BC2F3 population. However, the SNP marker snpSB00103 failed to identiy at least one favourable alleles for STG in this study.The genotype W82 was associated with snSB00102 marker, NA241 (snSB00102 and snSB00103) and NA307 was linked with snSB00101 and snSB00102 markers for STG 3 in BC2F1 population. Grain yield per hectare varied from 1770 kg/ha of donor parent B35 to 3415 kg/ha of BC2F3 genotype NA316C under well watered trial and from 1711 kg/ha of the donor parent B35 to 2652 kg/ha of genotype SE438 under water stressed trial. Significant differences were recorded in plant height, chlorophyll content, panicle length, panicle width, inflorescence exsertion, leaf rolling, grain weight per plant, panicle weight per plant and STG in both environments at P < 0.01. The mean performance of plant height was the highest (142.2 cm) in NA316C under water stressed condition and 143.3 cm under water irrigation. Interaction between genotypes by the environments were directed by majority of traits namely panicle length, panicle width, chlorophyll content, inflorescences exsertion, total number of leaves at physiological plant maturity, total number of green leaves at plant maturity, panicle weight, grain weight per panicle and root biomass. However, the interactions were not differed to plant height, leaf length and grain weight per plant. Panicle weight, panicle width and panicle length were significantly correlated with grain yield. STG and inflorescences exsertion were negatively correlated with grain yield. Chlorophyll content was correlated with total number of green leaves. A negative correlation was detected between traits STG and total number of green leaves. Above 50% heritability estimates were recorded in well watered and water stressed conditions. However, the interaction between genotypes by environments lowered the heritability of traits evaluated. Geometric mean productivity (GMP) and mean productivity (MP) were significantly correlated with yield under well watered environment (YP) and yield under water stressed environment (YS) and each other. There was low correlation between tolerance index and grain yield (0.12) in water stress environment. Stress sustainability index (SSI) showed low correlation with all indices compared thus; SSI is suggested as the best index for screening low yield under stressed environments. The genotypes NA307, NA316C and SE408 produced the highest grain yield per hectare across the environments. Therefore this study provided the bases of new genotypes which are promising for drought tolerance and yield. The genotypes should be advanced to lines and recommended for release after further evaluations in different geographical locations cultivating sorghum in Tanzania.