Improvement of maize (Zea mays L.) for heat stress tolerance in Zimbabwe to enhance climate resilience in new genotypes.

Abstract

Maize (Zea mays L.) is an important crop for food security globally. Rising temperatures due to climate change have significantly affected production and productivity of maize in sub Saharan Africa (SSA). Global warming is predicted to increase the frequency, duration, and intensity of heat stress, but the region's ability to adapt to these changes is limited. Given the significant maize yield losses reported due to heat stress, the use of high yielding heat tolerant maize varieties offers a sustainable solution to these challenges. In Zimbabwe and SSA at large, breeding maize for heat stress tolerance is at infancy stage. Therefore, the objectives of this study were to i) determine the genetic potential of exotic heat stress tolerant maize donor lines (HSTDLs) obtained from CIMMYT-India and CIMMYT-Zimbabwe elite lines for potential use in sub-tropical breeding programs, ii) assess the genetic diversity and genetic purity of heat donor lines and elite CIMMYT-Zimbabwe lines using single nucleotide polymorphism (SNP) markers, and iii) determine the performance and yield stability of early maturing commercial hybrids currently on the market in Zimbabwe under heat stress and random drought and heat stress conditions. In the first study, 14 HSTDLs from CIMMYT-India were crossed with 15 locally adapted elite lines from CIMMYT-Zimbabwe using the North Carolina Design II mating scheme. The successful 175 crosses were evaluated together with five commercial hybrids at two locations in the lowveld area of Zimbabwe during the 2020 winter season under managed heat stress and optimal conditions. The parental line trial was planted adjacent the hybrid trial at each location to determine the per se performance of the heat donor lines under heat and optimal conditions. The sentence was restructured and its now reading ― The design II analysis revealed significant (p <0.01) general combining ability (GCA) and specific combining ability (SCA) effects for GYD under heat stress (HTS), optimal conditions (OC), and across locations an indication that GYD is controlled by both additive and non-additive gene action. Three HTSDLs (CAL14138, CAL152, and CAL1440) exhibited significant positive GCA effects for GYD under HTS conditions. The crosses DJ265-15 × VL1018816 and DJ267-9 × CAL1440 revealed positive significant SCA for GYD under HTS. The donor lines (HTSDLs) CAL14138, CAL152 and VL109126 exhibited superior per se performance under HTS, OC and across environments. Thirty five inbred lines were genotyped to assess their genetic diversity, relatedness and purity using 94 single-nucleotide polymorphism (SNP) markers. The identity-by-state (IBS) genetic distance matrix revealed pairwise genetic distance among the inbred lines ranging from 0.04 to 0.64. The widest genetic distance was between inbred line pairs: CZL1112c and CZL16018; CAL14135 and ZL132077; and CZL15153 and CZL16018. The shortest genetic distance was between inbred lines CAL152 and CAL14138, CAL14138 and VL109126; and ZL132077 and DJ611-1. The neighbor-joining algorithm grouped inbred lines into three different main clusters. Some heat tolerant donor lines clustered together with local lines, while the other cluster consisted of either CIMMYT-Zimbabwe or CIMMYT- India lines. Majority (85.78%) of lines assessed were genetically pure with less than 5% heterozygosity. About 54.28% of the inbred lines evaluated exhibited 100% genetic purity. In the third study, 20 early maturing commercial hybrids and 5 experimental hybrids were evaluated across six locations during the 2020/21-2021/22 seasons for adaptability and stability under heat stress, random drought and heat stress conditions. The genotype main effect plus genotype x environment interaction (GGE) biplot showed that commercial hybrids G3 (4.79 t ha- 1), G20 (3.99 t ha-1) and G22 (4.09 t ha-1) were the most adapted under HTS condition while experimental hybrid, G4 (4.31 t ha-1), was the most adapted under HTS conditions. Under random drought and heat stress (RDHS) conditions, the most adapted commercial hybrids were G12 (4.66t ha-1), G14 (4.39 t ha-1), G15 (4.02 t ha-1), G13 (4.17 t ha-1), G21 (3.76 t ha-1) and G25 (3.50 t ha-1). The ‗ranking‘ GGE biplot identified commercial hybrids G16 (4.94 t ha-1) and G3 (4.79 t ha-1)as high yielding and stable across stress and non-stress conditions. The experimental hybrids G6(4.77 t ha-1) and G7 (4.67 t ha-1) were stable across environments. The experimental genotype, G8 (5.47 t ha-1), was overally, the highest performing but was unstable. The exotic lines that exibited significant positive GCA can be exploited for introgression of heat tolerant genes into local maize populations in breeding for heat stress tolerance. The availability of genetic diversity and the relationship observed among the HSTDLs and the local lines shows that there are valuable heat tolerant inbred lines within CIMMYT- Zimbabwe programme that could be used in heat stress tolerance breeding in SSA. Generally the study shows the potential of breeding for heat stress tolerance in SSA.