The influence of abiotic stress on CIMMYT provitamin A elite maize germplasm
Micronutrient malnutrition, including vitamin A deficiency, affects more than half of the world population, having a major effect on children less than five years old, pregnant and lactating women. The problem is significant in sub-Saharan Africa (SSA) where people subsist mostly on white maize which lacks vitamin A. Vitamin A deficiency is responsible for a number of health disorders that include poor vision and reproduction, and supressed growth and immunity. Biofortification of staple food crops such as maize with β-carotene can be a sustainable approach to address dietary vitamin A deficiency. Orange maize contains high levels of β-carotene, making it an important crop for combating vitamin A deficiency. The SSA region is also prone to various abiotic stresses that impact negatively on maize productivity. To ensure food security in the region, there is a need to breed highly nutritious maize cultivars adapted to the major abiotic stresses experienced in the region. To breed increased provitamin A hybrids, it is important to understand the mode of gene action affecting grain yield and β-carotene expression, and the heritability of β-carotene concentration under the prevailing stresses. There is also a need to determine the stability of provitamin A germplasm for grain yield and nutritional traits such as β-carotene under these stress conditions. In this study, 22 elite provitamin A inbred lines and five yellow drought tolerant inbred testers were crossed following a line × tester crossing design. Thirty hybrids had sufficient seed for replicated trials out of a potential of 110. The 30 hybrids and five checks were evaluated in Zimbabwe under optimum conditions, random drought stress, managed drought stress, combined drought and heat stress, low N stress and low P stress in 2014 and again in 2015. There was significant variation between hybrids for grain yield for all environments, except grain yield under low nitrogen stress. There was a significant interaction between year, environment and genotype for grain yield but no interaction was observed for grain texture. Inbred lines were highly heterotic for grain yield, especially under stress conditions. Narrow sense heritability for grain yield was more than 50% under optimal conditions, managed drought stress, combined and drought and heat stress and low P stress. AMMI and GGE analyses showed that genotype by environment interaction (GEI) was a very important source of maize grain yield variability. The environments were grouped into one mega-environment. The highly significant correlations between the environments suggest that testing can be done in only one environment. Hybrid environment, year and GEI effects for β-carotene were highly significant. Beta-carotene concentration was higher under optimum than under stress conditions and was highly significantly correlated with grain yield. Heritability for β-carotene was very high; 97% and 90% under optimum and 70% and 94% under managed drought stress in 2014 and 2015 respectively. General combining ability for β-carotene was significant and specific combining ability was not, emphasising the importance of additive gene action in the expression of the trait. Provitamin A hybrids had β-carotene concentration in the expected range (5-12 μg g-1) for first generation medium to high provitamin A maize genotypes. Lines 6, 7 and 8 can be used for breeding hybrids suitable for all environments except for managed stress conditions. Testers 1 and 2 were ideal for breeding for optimum conditions, managed drought stress, tester 2 for random drought stress and tester 3 for low P stress. Line 8 contributed consistently positively to grain yield, line 3 was favourable under managed drought stress and combined drought and heat stress, lines 6, 7, 8 and 9 were desirable under low N, 6, 7 and 8 under optimum conditions, 4, 6, 7, 8, and 10 under random drought stress, and 3, 8 and 10 under managed drought. The best performing and most stable genotypes for both grain yield and β-carotene can be distributed to SSA farmers for production. These hybrids will go a long way to alleviate vitamin A malnutrition among resource poor households in the region.