CHAPTER 5: EVALUATION OF F 1 CASSAVA PROGENY PERFORMANCE FOR
5.4 Discussion
5.3.8 Agronomic related trait correlations
A positive, significant (P<0.001) correlation was observed between plant height and branch height (Table 5.13). Similarly a positive significant (P<0.001) correlation was observed between fresh root yield and root dry matter content. Plant height and root dry matter content were weakly and positively correlated. The fresh root yield was also significantly (P<0.001) and positively correlated with plant height. There was weak correlation between fresh root yield and harvest index. Similarly, there was weak correlation was observed between harvest index and root dry matter content.
Table 5.12: Correlation coefficients for agronomic related traits on 800 genotype of a seedling evaluation trial, 2011
BH -
RDMC -0.048 -
HI 0.038 -0.012 -
LR -0.037 -0.0013 0.0009 -
FRY 0.0059 0.37*** 0.051 0.023 -
PH 0.21*** 0.086* 0.0078 -0.058 0.16*** -
BH RDMC HI LR FRY PH
BH (branching height, cm); RDMC (root dry matter content); HI (harvest index); LR (leaf retention); FRY (fresh root yield, kg plant-1); PH (plant height, cm)*, *** significant at P<0.05, P<0.001, two-sided test of correlations different from zero
temperature fluctuations (Ellis and Roberts, 1979). Temperatures were kept constant at 36oC and relative humidity at 80%. High temperatures (35oC) have been reported to enhance seed germination (Pujol et al., 2002). Ellis and Roberts (1979) observed high germination rates at constant temperature of 35oC.
Tall plants are required by the farmers (Chapter 2) as they are able to obtain more cuttings for planting. Noteworthy is that some plants had reduced growth points (Figure 5.4); however, it was not possible in this study to determine whether the cause was genetic or environmental.
Significant variation in leaf retention was observed among the crosses. Although, leaf longevity has been reported to contribute to high yields (El-Sharkawy, 2003), there was no correlation between leaf retention and fresh root yield in this study. Some clones within families retained most of their leaves at seedling stage. Lenis et al. (2006) has suggested selecting for stay green trait as an alternative to harvest index; however, in this study it was not possible to select for stay green as irrigation might have caused the plants to retain leaves longer.
Fresh root yield varied significantly across families. In some crosses the root yield was low and some clones were without storage roots. The differences could also be due to differences in genetic make-up and seedling growth rates. Since the seeds germinated at different times this could have contributed to differences in storage root mass for individual progenies. The overall mean yield (1.6 kg plant-1) recorded in this study 11 MAP was similar to that reported by Mtunda (2010). At seedling stage, the tap root tends to dominate other roots, creating variability in root mass. Rajendran et al. (2004) reported an increase in the mass of storage roots from cassava seedlings from which tap roots were removed. Supplementary irrigation might have also played a part in improving storage root mass and other yield components as the crop was watered during most of the growing period. The relatively high root yield in some families (11 MAP) indicates the potential of the developed clones to bulk early and suggests that selection can be made at the seedling stage. Early bulking is an important trait as indicated by most small scale farmers in Zambia (Chapter 2), Kenya (Kamau, 2006), and Nigeria (Nweke et al., 1996).
Root dry matter content was also significantly different among the crosses. For individual clones the mean dry matter ranged from 10.3 to 69.6%. In some clones, the tap root below the soil line bulged and was considered as part of the root. This could also mean that some clones had high
dry matter content and low yields and can be used to develop cultivars with high dry matter content.
Harvest index ranged from 0.01 to 0.80 with an overall mean of 0.33. These values are in agreement with that (0.05 to 0.9) obtained by Ojulong et al. (2006) at seedling trial stage using a diallel cross. Selecting clones with high harvest index has been reported to be more effective in identifying high yielding genotypes than using root yield (Kawano et al., 1978). Harvest index is a high heritable and consistent trait at all stages of selection (Kawano et al., 1987; Kawano et al., 1998; Kawano 2003). Selection based on fresh root yield is affected by the environment, whereas harvest index is not (Jaramillo et al., 2005).
To conclude, selection for yield and other yield related traits can be made at the seedling stage.
Bangweulu x Nalumino performed better in terms of fresh root yield than all the other crosses.
Bangweulu in combination with TMS3001 also performed consistently better for plant height and branch height. The results also demonstrate that F1 clones can be produced with sufficient planting materials in areas experiencing short cold winters at high elevation and at high altitude.
The variation observed in the segregating populations can be exploited to improve cassava.
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Appendices
Appendix1: Agro-ecological regions of Zambia
Appendix 2: Monthly rainfall distribution for 2008, 2009 and 2010 seasons, Mount Makulu
Mpika
Solwezi
Sesheke Kaoma
Serenje
Kalabo
Chama
Mkushi
Mumbwa Kasempa
Lukulu
Chinsali
Mw inilunga
Kalomo Senanga
Mufumbw e Zambezi
Lundazi Kaputa
Kazungula
Isoka
Kabompo
Mansa
Mongu
Mbala
Nyimba
Itezhi-Tezhi
Shangombo
Samfya Kasama
Chibombo Chongw e
Mungwi
Kapiri Mposhi
Luwingu Mporokoso
Petauke
Kafue
Choma Lufw anyama
Mpongwe
Mpulungu
Chipata Mwense
Kaw ambwa
Milenge
Monze Mazabuka
Mambw e Chilubi
Namwala
Katete
Chavuma Masaiti
Chiengi
Nakonde
Gwembe
Luangwa
Siavonga
Sinazongwe Nchelenge
Chadiza
Kabwe
Livingstone
Lusaka Urban
Chi l il abomb w e
Muf ul i ra Ch ing ol a
Kalul ushiKi tw e
Luan shy a Ndo la
200 0 200 400 Kilometers
N
E W
S
Agro-Ecological Regions
District boundary
KEY
Source: Soil Survey, Mt. Makulu Chilanga December 2002
Scale 1: 2,500,000
Regions I IIa IIb III
LEGEND
0 50 100 150 200 250 300 350 400
Rainfall (mm)
Months
2008 2009 2010
Appendix 3: Average maximum and minimum temperature for 2008, 2009 and 2010 seasons, Mount Makulu
0 5 10 15 20 25 30 35 40
Temperature( oC)
Months
Max 2009 Max 2008 Max 2010 Min 2008 Min 2009 Min 2010
Appendix 4: Cassava crossing block based on a 4 x 6, reduced to 4 x 5 NCII design A X G A x H A x I A x J
B X G B x H B x I B x J
C X G C x H C x I C x J
D X G D x H D x I D x J
E X G E x H E x I E x J
F X G F x H F x I F x J
10 m (2 m between plants within rows)
2 m between rows
3 m (path)
Males;
A = Mweru B = TME2 C= Nalumino D= TMS3001 E= TMS190 F= Tanganyika Females;
G= Chila
H= Kampolombo I= Chikula J= Bangweulu
CHAPTER 6: COMBINING ABILITY ANALYSIS OF CASSAVA GERMPLASM FOR