4.3 Results
4.3.2 General combining ability effects meaned over two sites
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In Set 1, the GCAf effects for MRN were not significant (P>0.05) with those of Semanda and Silk Omupya, negative and those of Kidodo and Bwanjule, positive. All male parents, except parent Araka Red, had highly significant (P<0.01) GCA effects for MRN. Dimbuka and NASPOT 1 had negative GCA effects whereas NASPOT 2 and NASPOT 4 had positive GCA effects. In Set 2, female parent Shock had significant positive GCA effects (0.16) for MRN (Table 4.8). Male parents, New Kawogo and Magabali had negative GCA effects while Silk Luwero and Budde had positive but non-significant GCA effects for MRN (Table 4.8). For this trait, parents with positive, significant GCA effects are desirable because they contribute to an increase in the number of roots in their progeny while parents with negative effects contribute to a reduction.
In Set 1, the GCA effects for UMRN were significant (P<0.05) for Bwanjule among the females, and NASPOT 2 and Araka Red among the males (Table 4.7). Importantly, Bwanjule and Araka Red had negative GCA effects, which is desirable as these parents will contribute towards a reduction in UMRN in their progeny. Only Tanzania had highly significant (P<0.01) GCA effects for UMRN in Set 2; however, they were positive which is not desirable for this trait (Table 4.8).
4.8 Performance and general combining ability effects of Set 2 parents for four traits meaned over two sites
* = Significant at P≤0.05; ** = significant at P≤0.01; AUDPC = area under disease progress curve for Alternaria blight;
SPVD = sweetpotato virus disease (scores 1-9: 1 = no SPVD and 9 = SPVD causing stunted growth); MRN = number of marketable storage roots per plant; UMRN = number of unmarketable storage roots per plant; GCA effects meaned over two sites were considered
No parent had significant GCA effects for DM% in either set (Tables 4.9 and 4.10).
Parents AUDPC SPVD MRN UMRN
Mean GCA Mean GCA Mean GCA Mean GCA
Females
Tanzania 194.6 7.59 2.90 -0.06 0.24 -0.10 1.21 0.32**
Mbale 178.4 -8.65 3.52 0.56 0.28 -0.06 0.69 -0.20
Shock 185.9 1.06 2.47 -0.49 0.49 0.16* 0.78 -0.12
SE 12.7 13.41 0.13 0.45 1.96 0.07 0.32 0.12
Males
New Kawogo 210.8 26.74 2.78 -0.18 0.33 -0.01 0.73 -0.18 Silk Luwero 173.2 -13.84 3.03 0.07 0.37 0.04 0.87 -0.03
Magabali 194.2 7.19 3.12 0.16 0.29 -0.05 0.90 0.12
Budde 167.0 -20.10 2.91 -0.05 0.35 0.02 0.99 0.09
SE 14.6 14.90 0.16 0.28 2.27 0.03 0.36 0.11
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The best parents to be used when breeding for large TRN are those with large positive GCA effects for the trait. In Set 1, female parents Semanda and Kidodo (Table 4.9) had highly significant (P<0.01), positive GCA effects for TRN. Similarly, male parents NASPOT 2 and NASPOT 4 had significant (P<0.05), positive GCA effects for the trait. Since Bwanjule had highly significant (P<0.01), negative GCA effects, it is not a desirable general combiner for this trait.
Among the parents of Set 2, only female parent Tanzania had significant (P<0.05) GCA effects for TRN (Table 4.10); however, it is not a good general combiner for this trait due to its negative GCA effects.
No parent in Set 1 (Table 4.9) had significant GCA effects for TRY. However, Kidodo had the highest positive GCA effect (0.35) for the trait. In this set, NASPOT 4 and NASPOT 2 had positive but non-significant GCA effects. In Set 2 (Table 4.10), Tanzania and Shock had significant (P<0.05) GCA effects for TRY, but Tanzania had negative effects which are not desirable for this trait.
Table 4.9 Performance and general combining ability effects of Set 1 parents for four traits meaned over two sites
* = Significant at P≤0.05; ** = significant at P≤0.01; DM% = percentage dry mass composition; TRN = total number of storage roots per plant; TRY = total storage root fresh mass (t ha-1); HI = harvest index; GCA effects meaned over two sites were considered
Parents DM% TRN TRY HI
Mean GCA Mean GCA Mean GCA Mean GCA Females
Semanda 31.12 0.32 2.47 0.22** 14.39 -0.26 0.35 0.03 Silk Omupya 30.75 -0.05 2.26 0.01 13.06 -0.01 0.35 0.02 Kidodo 30.34 -0.46 2.46 0.21** 16.86 0.35 0.36 -0.02 Bwanjule 30.95 0.15 1.81 -0.44** 16.54 -0.08 0.30 -0.03
SE 0.51 0.34 0.27 0.04 3.35 1.29 0.02 0.02
Males
Dimbuka 31.38 0.58 2.10 -0.16 12.04 -1.95 0.32 -0.01 NASPOT 2 29.88 -0.92 2.61 0.34* 17.79 1.59 0.36 0.03*
NASPOT 1 30.89 0.09 2.00 -0.25 12.47 -2.41 0.31 -0.02 NASPOT 4 31.58 0.78 2.56 0.32* 18.83 1.86 0.36 -0.03*
Araka Red 30.23 -0.57 1.99 -0.26 14.94 0.92 0.34 0.01
SE 0.57 0 .71 0.31 0.14 3.74 1.41 0.02 0.01
The HI expresses the economic yield as a proportion of the total biomass; therefore a genotype that produces a high proportion of storage root mass in relation to the total biomass is more desirable. Only NASPOT 2 and NASPOT 4 of Set 1 had significant (P<0.05) GCA effects for HI (Table 4.9). As a parent, NASPOT 2 with a positive GCA effect would be preferred to NASPOT 4 which had a negative GCA effect. In Set 2, female parents Mbale and Shock had highly significant (P<0.01) GCA effects for HI (Table 4.10). The GCA effect for Shock was positive while that of Mbale was negative, thus Mbale was not a very good general combiner for the trait. Again, New Kawogo and Budde had significant (P<0.01) GCA effects but only Budde’s was positive and would therefore be a preferred general combiner.
Table 4.10 Performance and general combining ability effects of Set 2 parents for four traits meaned over two sites
Parent DM% TRN TRY HI
Mean GCA Mean GCA Mean GCA Mean GCA
Females
Tanzania 31.60 -0.50 2.06 -1.45* 7.16 -2.95* 0.31 -0.01
Mbale 32.62 0.52 1.53 -0.28 8.22 -1.88 0.26 -0.06**
Shock 31.92 -0.18 1.84 0.02 14.94 4.83** 0.39 0.07**
SE 0.57 0.80 0.31 0.21 3.74 1.36 0.02 0.02
Males
New Kawogo 32.52 0.42 1.76 -0.05 9.32 -0.78 0.27 -0.05**
Silk Luwero 31.04 -1.06 1.72 -0.10 12.25 2.15 0.31 -0.01 Magabali 33.22 1.12 1.74 -0.07 8.77 -1.34 0.30 -0.01
Budde 31.40 -0.70 2.03 0.21 10.09 0.03 0.39 0.07**
SE 0.66 0.80 0.35 0.17 4.32 1.58 0.02 0.02
* = Significant at P≤0.05; ** = significant at P≤0.01; DM% = percentage dry mass composition; TRN = total number of roots per plant; TRY = total storage root fresh mass (t ha-1); HI = harvest index; GCA effects meaned over two sites were considered
It is important to note that some genotypes with high HI have weak or underdeveloped foliage which makes propagation and maintenance of such genotypes difficult. Furthermore, HI on its own is obviously not a direct indicator of absolute yield and should therefore be used to evaluate genotypes in combination with other yield-related traits, in particular overall marketable root yield.