Introgression of desirable traits from temperate maize Zea mays L.)into tropical maize for low phosphorus soils
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Maize (Zea mays L.) is one of the most important cereal crops in the world after wheat and rice. It is the staple food crop of people in Zambia, but phosphorus (P) deficiency in soils has been limiting grain yield. The objective of this study was to introgress desirable yield enhancing alleles from temperate genotypes of maize into tropical maize to increase grain yield on phosphorus deficient soils in Zambia. Twenty-four (24) single-cross hybrids and 9 local checks were evaluated in a 3 x11 α-lattice design with 2 replications at Mutanda station under low P soil condition. Highly significant differences (P ≤ 0.01) among genotypes were found for grain yield (GY), harvest index (HI), 100 grain weight (100GW), shelling %, root biomass (RB), plant height (PH), plant biomass (PB), plant-tissue phosphorus (PTP) and purpling symptom suggesting the presence of genetic variation in maize under low phosphorus soil conditions. Temperate inbred parents gave highly significant (P ≤0.01) GCA variances for GY (27.3**), 100GW (0.00005**) and significant (P ≤ 0.05) GCA variance for HI (0.016*), implying the presence of additive gene action. Similarly, tropical inbred parents gave highly significant (P ≤0.01) GCA variances for; GY (36.19**), HI (0.06**), 100GW (0.00006**), shelling % (171.73**), PH (331.3*), PTP (42543**) and RB (1.01**), implying the presence of sufficient additive gene action. As such, breeding progress under low phosphorus could be achieved through selection of parental genotypes based on the fore-mentioned traits. The single-cross hybrids gave highly significant (P≤ 0.01) SCA variances for; GY (13.89**), HI (0.03**), 100GW (0.00007**), RB (0.37**) PTP (140491**) and significant (P<0.05) SCA variance for PH (246.22*), implying the presence of dominance gene action for effective hybridization under low phosphorus soils. The Baker’s ratio was low for GY, HI, PH, RB, 100GW and PTP indicating the preponderance of dominance gene action over additive, hence justifying the vigour realized in hybrids. However, there was a predominance of additive gene action over dominance gene action for shelling percentage only. There was highly significant positive (P ≤0.01) correlations for GY with; PH (r = 0.40**), HI (r = a0.76**), shelling % (r = 0.60**) and PB (r = 0.43**) and highly significant negative (P≤ 0.01) correlation with purpling of leaves (r = -0.50**). The coefficient of multiple determinations (R2) revealed that harvest index had highly significant (P ≤0.01) influence on grain yield explaining 65.7% of the total variation. Purpling symptom and plant height gave little contribution to grain yield, explaining only 3.3% and 2.9% of the total variation respectively. Temperate inbreds J187 and Mo17 were identified as good sources of desirable yield enhancing alleles for introgression to improve tropical maize on phosphorus deficient soils. Inbred-line J185 was the best general combiner and source of desirable alleles followed by Mo17. Hybrids L151xJ185, L151xMo17, L152xJ185, L1212xMo17, 152xMo17, L1212xJ185 and L5527xJ185 that produced high grain yield ranging from 8.28 tons/ha to 12.77 tons/ha were identified as potential good materials for extracting inbred-lines and developing 3-way cross or double-cross hybrids that are efficient in utilizing phosphorus in soils.