GENETIC STUDIES OF MAIZE (Zea mays L.) FOR HIGH PLANT DENSITY TOLERANCE
The use of high plant density tolerant hybrids was one of the major interventions that increased maize yield in the developed world. However, not much has been done on the improvement of high plant density tolerance of maize in the developing world. Therefore, this study was undertaken to (1) identify promising inbred lines for high density planting, (2) evaluate the performance and stability of maize hybrids under different plant densities and identify promising hybrids for high density planting, (3) determine the relationship of the per se performance of inbred parents and hybrids on high plant density tolerance, (4) determine the association between maize traits and high plant density tolerance, (5) determine the gene action that controls different traits of maize under high plant density. Forty inbred lines were evaluated under three plant densities (low =53,333, medium = 66,666 and high = 88,888 plants ha-1) at the research farm of West Africa Centre for Crop Improvement (WACCI) in 2017. Ten inbred lines were selected based on their yield performance and other agronomic traits which are associated with high density tolerance. The selected ten inbred lines were crossed among each other in a half diallel mating design to produce 45 F1 hybrids. These 45 F1 hybrids together with other two F1 hybrids and one three-way cross commercial hybrid (PAN53) were evaluated under the three different plant densities in four different environments that is, Fumesua, Nyankpala, and twice in Legon in 2018. The experimental design was alpha lattice with split plot arrangement and the experiment was replicated twice. The ten inbred parents were also included in the evaluation but in separate blocks. Mixed model was used for data analysis where block and replication were considered random and genotypes, plant density, environments and their interactions were fixed. Six inbred lines (87036, EXP124, M131, TZDEI501, TZEI1, and TZEI87) performed better under the high or medium plant densities in terms of yield compared to the low plant density and four of them were included as parents in the hybrid production. Among these four parents, three of them (87036, M131, and TZEI1) also performed better under the high or medium densities compared to the low density during the evaluation of the inbred parents alongside the hybrids. Moreover, two additional inbred parents (CML16 and ENT11) also performed better under the high or medium densities compared to the low density during the evaluation of the inbred parents alongside the hybrids. Similarly, the performance of the hybrids in terms of yield was dependent on plant density and on the growing environments. The highest yield of 9.5, 9.2 and 8.6 t ha-1 were obtained from the high plant density in Legon (minor season), Fumesua, and Legon (off-season) respectively and it was 26.7, 22.7 and 30% increase in comparison to the respective yields under the low density. However, the highest yield (7.9 t ha-1) in Nyankpala was recorded under the medium density. The most superior at the same time most stable hybrids under the high plant density were CML16 x TZEI1 and M131x CML16 followed by CML16 x 87036, TZEI387 x CML16 and ENT11 x 87036 in descending order. On the other hand, CML16 x 87036 was the most superior and at the same time stable hybrid under the medium density followed by M131x CML16, ENT11 x 87036, TZMI740 x CML16, and TZEI1 x 87036 in descending order. The average per se yield performance of the inbred parents was moderately correlated with the yield performance of the hybrids under the high plant density but the correlation was not significant for the other densities. Hybrids with longer ears and filled ear lengths, wider ear diameter and at the same time with high chlorophyll content were more tolerant to high plant density. Both additive and non-additive gene effects were important for yield and selected yield components with additive gene action being more important than non-additive gene action for all traits tested under the high plant density. Finally, it was shown from this study that there is a potential for high density planting of maize in Ghana.