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days to flowering. Internodes formation stop during floral initiation consequently early flowering maize varieties are usually shorter in plant heights.
Previous studies showed highly significant variability in ear height in maize genotypes (Nazir et al., 2010). Ear height has been described to be one of the most important selection criteria in maize breeding especially for root and stock lodging resistance and increased grain yield (Nazir et al., 2010; Bello et al., 2012; Zheng and Liu, 2013). High ear position could be susceptible to root and stock lodging, therefore most breeders usually prefer selecting for lower ear position in maize (Bello et al., 2012).
Mean NTB varied from 1.9 (TL2012-54) to 19.9 (TL2012-29) (Table 3.4). Tassel size is positively correlated with pollen production and consequently of seed set therefore has great implications in breeding programmes. Few tassel branches implies less pollen production this in turn can affect controlled pollination process in breeding programmes. For example, if a pollen parent has low pollen production ability it will not sufficiently pollinate the desired number of female parents therefore lowering the desirable number of crosses to be generated. However a desirable female parent could have a lesser number of tassel branches to avoid assimilates being invested in excessive pollen production than grain yield (Bello et al., 2012).
Principal component analysis
The principal component analysis measures important characters which have significant contributions to the total explained variation (Sinha and Mishra, 2013). The first few principal components with eigenvalues of >1 are often of most important in reflecting the variation pattern among study materials and differentiation of their associated characters (Sinha and Mishra, 2013). In this study, the first four principal components (PCs) captured about 67.9% of the total variation hence were considered as the most important components. This result was comparable to that of Lopez-Morales (2014) who reported 54% of the total variation which was attributed to three components, when studying the morphological diversity of native maize in the humid tropics of Puebla, Mexico.
Variability of inbred lines based on qualitative characters
The results presented in Table 3.5 reflect the variability of studied inbred lines based on 27 qualitative characters used. Knowledge on these variables can be useful for several applications including site-specific crop management in precision agriculture (Shrestha, 2013). The present result shows that about 40% and 50% of these inbred lines had narrow leaf width and small average angle of inclination (< 250), respectively. Narrow leaves and small leaf angle of
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inclination are associated with low interception to sun light thereby reduced photosynthesis and subsequently of decreased yields (Torres et al., 2011). Inbred lines with small angle of inclination (< 250) and narrow leaves may not be grown in places where light is limiting. Plants with large leaf width and angle of inclination are preferable. Because they are efficient in light interception, provide good ground surface cover to minimize loss of moisture due to evaporation and to supress weeds. In the current result, 70% of the lines had upright or straight stems desirable for mechanical harvesting. Genotypes with curved stems do not show uniformity and good physical appearance. Often qualitative morphological characters are used to differentiate morphotypes. However, these traits may not have direct contributions to yield. Some characters such as length of peduncle of the ear, length of ear, number of ear rows and length of husks off the tip of the ear, shapes of ears, grain type and colour have direct implications for breeding and end users preferences.
Genetic relationships among 80 maize inbred lines used in the study
The classification and expression of inbred lines using dendrogram provide visual assessment of genetically unrelated individuals for use in maize breeding. Inbred lines within the same cluster are genetically related in one or several traits and should not be sampled for cross formation. The UPGMA cluster analysis (Figure 3.2) generated a dendrogram of 80 inbred lines germplasm using 27 morphological qualitative data revealing nine different clusters. This suggests that the tested lines showed considerable genetic diversity. Similar result was reported by Azad et al. (2012). The major clusters identified in this study were III, IV and VI, consisting of 77.5% of the inbred lines evaluated (Table 3.6). Crosses involving parents belonging to the maximum divergent clusters are expected to manifest maximum heterosis and also wide genetic variability on agronomic traits. Thus, cross combinations of genotypes TL2012-53 and TL2012- 61 (from cluster II) with TL2012-20, TL2012-70, and TL2012-78 (cluster IV) may provide considerable degree of heterosis or novel recombinants for further breeding and genetic analysis. Also inbred lines such as TL2012-1 and TL2012-11 from cluster VIII and TL2012-5, TL2012-6, TL2012-55 and TL2012-56 (cluster III) could be considered as potential parents for breeding owing to their genetic divergence.
Limitations of morphological descriptors
Assessment of genetic diversity using morphological characterization is relatively a cheaper option where genomic tools are underdeveloped or not readily available such as in Tanzania (Mbuya et al., 2012; Semagn et al., 2012; Khan et al., 2014). In conventional breeding, this method has been extensively used as an important tool to aid identification and selection of
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diverse parents suitable for hybrid combinations (Subramanian and Subbraman, 2010; Mbuya et al., 2012; Semagn et al., 2012; Parasanna, 2012; Fischer et al., 2014; Lopez-Morales et al., 2014) despite its limitations. Morphological characterization is greatly limited by several factors such as seasons and growth stage and results can be unrealistic especially when working with quantitative traits because they are influenced by environments. Therefore in order to increase the efficiency of genetic diversity characterization molecular characterization techniques should compliment the weakness of the conventional approach.