GENETIC ANALYSIS OF TOLERANCE TO LOW SOIL NITROGEN IN INTERMEDIATE MATURING MAIZE (ZEA MAYS L.) INBRED LINES - ABSTRACT
Low soil nitrogen (N) is one of the most important constraints to maize production in sub- Saharan Africa (SSA) in general and the Bimodal Humid Forest Zone (BHFZ) of Cameroon. External source of N as nutrient are therefore required for increased yield of the crop. However, nitrogen fertilizer is expensive and inaccessible to most farmers. The development and adoption of maize varieties tolerant to low N soils could reduce the need for nitrogen inputs and significantly contribute to sustainable maize production. The objectives of this research were to:
I. identify maize production constraints and farmers preferred maize characteristics in the
II. Bimodal Humid Forest Zone of Cameroon; ii) identify maize genotypes tolerant to low N soils;
III. estimate the combining abilities of maize inbreds and classify them into heterotic groups and
IV. determine the effect of genotype x environment interaction on grain yield and yield stability of maize hybrids across low N and optimum environments.
A Participatory Rural Appraisal (PRA) consisting of focus group discussions followed by formal surveys was conducted in six villages of the Central Region in the Bimodal Humid Forest Zone of Cameroon. Thirty nine inbreds originating from IRAD, IITA and CIMMYT were crossed to three heterotic testers (87036, Exp1 24 and 9071) in a line x tester scheme to generate 117 F1 hybrids. The 117 F1 hybrids along with 4 checks were evaluated under low N (20 kg ha-1) and optimum N (100 kg ha- 1 N) at two location viz., Mbalmayo and Nkolbisson, during minor season of 2012 and minor and major seasons of 2013. Genotype x environment interaction and grain yield stability of 80 hybrids were assessed across 11 environments under low and optimum N using AMMI and GGE biplot analysis. The study revealed that low soil fertility and high cost of fertilizers were among the most important constraints to maize production in the study area. Farmers cited large grain size, soft grain texture, large ear size, high prolificacy, early maturity, short plants, resistance to lodging, resistance to diseases and reduced post-harvest losses as their preferred characteristics in maize varieties. Across all environments, CLYN246 x 87036, CLWN201 x Exp1 24, J16-1 x Exp1 24, 1368 x 87036, ATP S6-20-Y-1 x Exp1 24 and Cam inb gp1 17 x 87036 were higher yielding than 87036 x Exp1 24, the commercial hybrid used as check in the study. Among these hybrids, CLWN201 x Exp1 24, J16-1 x Exp1 24 and 1368 x 87036 may be candidates for release. Inbreds CLYN246, ATP S6-20-Y-1 and Cam inb gp1 17 could be used as testers to classify lines into heterotic groups in further studies or recombined within groups to develop source populations for new inbred development. For specific areas with low N stress, TL-11-A- 1642-5 x Exp1 24, CLWN201 x 87036 and J16-1 x Exp1 24 may be candidates for release and TL-11-A-1642-5 x 87036, TZ-STR-133 x 87036, CLWN201 x Exp1 24 and J16-1 x Exp1 24 could be proposed for release for optimum N conditions. Both additive and non-additive gene action influenced grain yield under low N with predominance of non- additive genetic effects while additive gene action was predominant under optimum conditions. Hybrid development could therefore be employed to exploit non additive gene action under low N. Based on SCA and yield performance of test crosses under low and optimum N, lines were classified into three heterotic groups for each environment; group A (anti-87036), group B (anti- Exp1 24) and group C (anti-9071). Lines from each group will serve as germplasm for development of the second generation of inbreds. Stability analysis for grain yield revealed highly significant genotype x environment interactions. The GGE biplot analysis divided the study area into three mega environments mainly related to the minor and major growing season. Hybrid 1368 x 87036 was the highest yielding hybrid adapted for minor seasons while TL-11-A- 1642-5 x 87036 was the best for major seasons. Hybrid TL-11-A-1642-5 x 87036 was the most superior hybrid, combining high yield and stability and has the potential for production across environments.