Climate change is expected to aggravate the effects of drought, heat and combined drought and heat stresses. An important step in developing ‘climate smart’ maize varieties is to identify germplasm with good levels of tolerance to the abiotic stresses. The primary objective of this study was to identify landraces with combined high yield potential and desirable secondary traits under drought, heat and combined drought and heat stresses. Thirty-three landraces from Burkina Faso (6), Ghana (6) and Togo (21), and three drought-tolerant populations/varieties from the Maize Improvement Program at the International Institute of Tropical Agriculture were evaluated under three conditions, namely managed drought stress, heat stress and combined drought and heat stress, with optimal growing conditions as control, for two years. The phenotypic and genetic correlations between grain yield of the different treatments were very weak, suggesting the presence of independent genetic control of yield to these stresses. However, grain yield under heat and combined drought and heat stresses were highly and positively correlated, indicating that heat-tolerant genotypes would most likely tolerate combined drought and stress. Yield reduction averaged 46% under managed drought stress, 55% under heat stress, and 66% under combined drought and heat stress, which reflected hypo-additive effect of drought and heat stress on grain yield of the maize accessions. Accession GH-3505 was highly tolerant to drought, while GH-4859 and TZm-1353 were tolerant to the three stresses. These landrace accessions can be invaluable sources of genes/alleles for breeding for adaptation of maize to climate change.

Maize landrace accessions constitute an invaluable gene pool of unexplored alleles that can be harnessed to mitigate the challenges of the narrowing genetic base, declined genetic gains, and reduced resilience to abiotic stress in modern varieties developed from repeated recycling of few superior breeding lines. The objective of this study was to identify extra-early maize landraces that express tolerance to drought and/or heat stress and maintain high grain yield (GY) with other desirable agronomic/morpho-physiological traits. Field experiments were carried out over two years on 66 extra-early maturing maize landraces and six drought and/or heat-tolerant populations under drought stress (DS), heat stress (HS), combined both stresses (DSHS), and non-stress (NS) conditions as a control. Wide variations were observed across the accessions for measured traits under each stress, demonstrating the existence of substantial natural variation for tolerance to the abiotic stresses in the maize accessions. Performance under DS was predictive of yield potential under DSHS, but tolerance to HS was independent of tolerance to DS and DSHS. The accessions displayed greater tolerance to HS (23% yield loss) relative to DS (49% yield loss) and DSHS (yield loss = 58%). Accessions TZm-1162, TZm-1167, TZm-1472, and TZm-1508 showed particularly good adaptation to the three stresses. These landrace accessions should be further explored to identify the genes underlying their high tolerance and they could be exploited in maize breeding as a resource for broadening the genetic base and increasing the abiotic stress resilience of elite maize varieties.

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Genetic adaptation of maize to the increasingly unpredictable climatic conditions is an essential prerequisite for achievement of food security and sustainable development goals in sub-Saharan Africa. The landraces of maize; which have not served as sources of improved germplasm; are invaluable sources of novel genetic variability crucial for achieving this objective. The overall goal of this study was to assess the genetic diversity and population structure of a maize panel of 208 accessions; comprising landrace gene pools from Burkina Faso (58), Ghana (43), and Togo (89), together with reference populations (18) from the maize improvement program of the International Institute of Tropical Agriculture (IITA). Genotyping the maize panel with 5974 DArTseq-SNP markers revealed immense genetic diversity indicated by average expected heterozygosity (0.36), observed heterozygosity (0.5), and polymorphic information content (0.29). Model-based population structure; neighbor-joining tree; discriminant analysis of principal component; and principal coordinate analyses all separated the maize panel into three major sub-populations; each capable of providing a wide range of allelic variation. Analysis of molecular variance (AMOVA) showed that 86% of the variation was within individuals; while 14% was attributable to differences among gene pools. The Burkinabe gene pool was strongly differentiated from all the others (genetic differentiation values >0.20), with no gene flow (Nm) to the reference populations (Nm = 0.98). Thus; this gene pool could be a target for novel genetic variation for maize improvement. The results of the present study confirmed the potential of this maize panel as an invaluable genetic resource for future design of association mapping studies to speed-up the introgression of this novel variation into the existing breeding pipelines.

Information on combining ability and heterotic patterns of multiple stress-tolerant inbred lines are fundamental prerequisites for devising appropriate breeding strategies for the development of climate-resilient maize hybrids. In the present study, we evaluated 150 single cross hybrids derived from the North Carolina Design II (NCD II) along with six commercial checks under terminal drought stress (TDS), heat stress (HS), and combined drought and heat stress (CHDS)conditions. The objectives of the study were to: (i) determine the combining ability of the inbred lines and identify the best testers across the stresses; (ii) classify the inbred lines into heterotic groups (HGs) based on the general combining ability of multiple traits (HGCAMT) and sequencing-based diversity array technology (DArTseq) and (iii) assess the performance and stability of the lines in hybrid combinations. The inbred lines showed significantly (p < 0.01 and p < 0.05) positive and negative general combining ability (GCA) and specific combining ability (SCA) effects for grain yield (GY) and most other measured traits. The inbred line TZEI 135 displayed relatively larger positive GCA effects for GY when mated either as male or female and was identified as the best tester. TZEI 135 × TZEI 182 was identified as the best single-cross tester across environments. Results of the assessment of the relative importance of GCA and SCA effects revealed the predominance of additive gene action over the non-additive. Six HGs of inbreds were identified using the HGCAMT and three, based on the DArTseq marker genetic distance method, were the most efficient. The best hybrids in this study significantly out-yielded the best checks by 21, 46, and 70% under CHDS, HS, and TDS, respectively. These hybrids should be extensively tested in on-farm trials for possible commercialization in sub-Saharan Africa.

Drought, heat, and combined drought and heat are important abiotic stresses constraining the production and productivity of maize ( Zea mays L.) in sub-Saharan Africa (SSA). In the face of climate change, these stresses are likely to occur simultaneously and put at risk food and economic security in SSA. This review describes maize breeding activities conducted by the International Institute of Tropical Agriculture (IITA) in partnership with national scientists under the Drought Tolerant Maize for Africa (DTMA) and Stress Tolerant Maize for Africa (STMA) projects, which together sought to develop and deploy multiple stress tolerant hybrids, and open-pollinated varieties. Emphasis was on (i) developing a reliable methodology for screening maize for tolerance to drought stress (DS), heat stress (HS), and combined drought and heat stress (CDHS) using key secondary traits and grain yield, (ii) use of appropriate breeding techniques for tailoring maize for tolerance to DS, HS and CDHS, (iii) exploring diverse sources of germplasm for genetic enhancement of maize, (iv) extensive multilocational evaluation to identify genotypes with stable performance under the stresses, and (v) application of genomic tools to accelerate genetic gains in maize breeding at IITA. At IITA, the performance of maize hybrids under stresses of DS, HS and CDHS have been improved using conventional breeding techniques/procedures. These techniques/ procedures have led to accelerated genetic gains in yield that were 26–49% higher than the best commercial hybrid checks under CDHS and DS. Additive gene action has been consistently found to be more important than the non-additive among early maize under DS and CDHS while both the additive and non-additive have been reported to be important for the extra-early maize. The most reliable secondary traits for selecting for improved grain yield under the stresses include anthesis-silking interval, ears per plant, and plant and ear aspects. Several early and extra-early landraces have been identified as potential sources of tolerance to DS, HS, and CDHS. Several quantitative trait loci (QTLs) associated with grain yield and key secondary traits have been identified via genome-wide association studies in landraces and inbred lines. Those desirable QTLs, upon validation, could be invaluable for genomics-enabled breeding.

Abstract:Kinetin is an important growth hormone used for in vitro propagation, but its dynamic and temporal effects on Dioscorea alata have not been thoroughly evaluated. In this study, surface response models were developed to better elucidate the effects of kinetin on D. alata propagated in vitro. Nodal segments were obtained from Akaaba, an important D. alata cultivar in Ghana, and propagated in vitro under five kinetin rates (0, 2.5, 5, 7.5 and 10 µM). The models were developed using segmented multiple regression with time and kinetin as the predictors. The effects on plant height, the number of leaves, shoots and roots were assessed with three-dimensional figures for better observation of temporal trends. The model fit was very good with normalized root mean squared error (NRMSE) = 0.1, R-squared = 0.83 and adjusted R-squared = 0.82, averaged across the different growth parameters. Different kinetin levels elicited the maximum shoot, leaf and root formation, as well as the growth rates over time. Moderate kinetin levels (2-4 µM) provided better growth at early culturing period. Higher kinetin levels (5-10 µM) suppressed the growth of the plantlets at early stages, but the plantlets recovered from the stress and resumed normal growth thereafter. After 4-5 weeks, the growth rates of the moderate kinetin levels (2-4 µM) declined much faster and were lower compared to the higher kinetin levels, except plant height and the number of roots which were still higher at the moderate kinetin level even after eight weeks of culturing. Thus, kinetin requirements vary depending on the growth parameters of interest.