1. Evaluation of second backcross (BC2) population of cassava from wild relatives (Manihot walkerae) for delay post harvest physiological deterioration (PPD) in Nigeria
Favour Ewa1,2, Emmanuel Okogbenin1*, Olalekan Akinbo1, Chiedozie Egesi1, Kenneth Eluwa1, Eunice Ekaette1, Lydia Ezenwaka1, Alex Ogbonna1 Emeka Nwofia2 and Martin Fregene3
1National Root Crops Research Institute (NRCRI), Umudike, Nigeria; 2Michael Opara University of Agriculture (MOUA), Umudike, Nigeria; 3Donald Danforth Plant Centre, St Louis, USA; Corresponding author
ABSTRACT
Cassava (Manihot esculenta Crantz) roots initiate deterioration less than 72 hours after harvest due to bulkiness. Post harvest physiological deterioration (PPD) account for significant annual yield losses globally for fresh cassava roots. Lack of varieties with low delayed PPD has been due to loss of genetic variation in available gene pools, which has resulted for the set back in breeding for tolerance for PPD. Wild relative walkerae is a good source of delay PPD and for improving genetic variability improved varieties. In this study, second backcross introduced from CIAT was introduced to NARs in Africa to evaluate and used the selected varieties to breed for delayed PPD in Africa cassava germplasm. Four hundred and sixty nine in vitro PPD genotypes of BC2 polulation from CIAT from the wild relative Manihot walkerae were multiplied, hardened, weaned, transferred to the field and evaluated for the first year for delayed PPD at the National Root Crops Research Institute (NRCRI), Umudike, Nigeria. Delayed PPD were evaluated at 7, 14 and 30 days after harvest (DAH). Out of the population of the BC2 cassava plants planted on the field, only 53% of the BC2 plants showed good establishment resulting in the evaluation of about 93 BC2 genotypes for delayed PPD. Using specific levels of PPD, result indicate that about 73% of the genotypes were found to have high resistance level of PPD at 7 DAH of which were 49 genotypes while 45 genotypes corresponds to individuals without PPD. Forty-six percent of the genotypes were highly resistant at 14 DAH. Two genotypes maintained 0% at 30 DAH in the half sib populations BIPD280 and BIPD289 respectively. The varieties used are developed from a BC1 population of wakerae as a donor PPD gene that is highly tolerant to PPD. The second backcross to introgress CMD resistance for African NARs evaluation. There is need to further validate these population for the second year to determine stability of genetic basis and if heritable and to estimate the influence of G x E interaction in trait expression.
Table 1: Range of values for agronomic traits of 242 genotypes of cassava second backcross (BC2) population.
Figure 1: PPD evaluation procedure
Variables
Minimum
Maximum
Average
SDa
FRYb
0.50
60.00
13.68
11.76
DRYc
0.11
31.42
5.58
4.82
HId
0.15
0.86
0.51
0.13
DMCe
16.94
58.38
25.38
8.04
CMDf 1 5
4.04
1.40
C-6 X BIPD
7DAH
aStandared deviation; bFresh root yield (ton ha-1); cDry root yield (ton ha-1); dHarvest index(0-1); eDry matter content (%); fCassava mosaic disease (1-5)
C-243 x BIPD
14DAH
RESULTS AND DISCUSSION
The highest fresh root yield was recorded in the genotype CPDCR9A-1 (60 ton ha-1) while the highest dry root yield was recorded in the genotype CPDCR24B-24 (31.42 ton ha-1) (Table 1). Dry matter content ranged from in CPDCR1B-43 to in CPDCR2B-95 (Table 1). Average DMC across the genotype was The range of CMD disease score was between 1-5 (Table 1). Using specific levels of PPD, out of 93 genotypes about 73% were found to have high tolerance level of PPD at 7 DAH. About 46% of the genotypes were highly tolerant at 14 DAH. Two genotypes maintained 0 PPD% at 30 DAH in the half sib populations BIPD280 and BIPD289 respectively (Figure 3). Genotypes that were resistant to CMD were very few. About 10% of the genotypes showed very high level of resistance while the highest population of the genotypes (15-61%) of the genotypes were highly susceptible to CMD (Fig 4) The discovery and use of resistance genes from wild relatives have steadily increased in different crops (Carabali et al., 2010; Akinbo et al., 2012). Result from this study showed a drastical reduction to PPD which indicates the successful introgression delayed PPD from (Manihot walkerae) into cassava. Hybridisation of elite cassava cultivars with M. walkerae, appeared to have also resulted in the introduction of desirable genes for other traits such as yield and dry matter content which will allow for the improvement and selection for these traits. In Africa, the most important disease of cassava is cassava mosaic disease (CMD) caused by a group of begomoviruses. Yield losses caused by cassava mosaic disease is as high as 100% (Thresh et al., 1994). The disease does not occur in the neotropics, and its complex of begomoviruses is unknown to the Americans. The CMD status of the BC2 populations indicated resistance in some of the genotypes meaning that the introgression was equally successful but a lot of the genotypes were susceptible to CMD even though they showed high tolerance to PPD.
C-4 X BIPD
30DAH
Figure 2: Response of PPD tolerant roots from
BC2 populationon the 7th, 14th and 30 DAH
INTRODUCTION
There is a growing demand for cassava roots (more recently foliage, as well) by the starch, food, animal feed and ethanol industries (Ceballos et al., 2007). However, its starchy storage roots suffer from a rapid post-harvest pysiological deterioration (PPD) which renders them unpalatable and unmarketable within hours of harvest, thereby severely restricting the development of the crop. Cassava breeders are increasingly interested in wild relatives because they offer many opportunities for transfering alien genes. Delay to post harvest physiological deterioration (PPD) has been identified in an interspecific hybrids between cassava and Manihot walkerae.
Extending the shelf life of cassava roots by two to three weeks will be of immense benefit to poor farmers in Africa and elsewhere and it will open up the full potential of this crop. The objective of this work therefore is to evaluate the second back cross populations (BC2) from Manihot walkerae for delay post-harvest physiological deterioration, disease resistance and to assess the effect of such a cross on other yield related traits.
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MATERIALS AND METHODS
The experiment was carried out in National Root Crops Research Institute, Umudike. Four hundred and sixty nine in vitro PPD genotypes of BC2 populations from The Centro Internacional de Agricultura Tropicai CIAT from wild relative Manihot walkerae were multiplied, hardened and taken to the field. Two hundred and fourty two genotypes showed good establishment on the field while ninety genotyes were evaluated for delay to PPD. The experimental design used was randomizeed complete block design (RCBD) of a single row planting of five plant stands per genotype. Disease data (CMD incedence and severity scoring scale 1-5 at 6WAP, 3MAP, 6MAP, 9MAP) and at harvest. At harvest, roots produced by each plant, as well as the above ground biomass (stem and foliage), were weighed. Plot yield was converted to fresh root yield (FRY: tonha-1) Harvest index (HI) was measured as the ratio between fresh root yield and total biomass. Percentage dry matter content of the roots was estimated using the specific gravity methodology Approximately 3kg of root were weighed in a hanging scale (WA). The same sample was weighed with roots sub-merged in water (WW). Dry matter content was estimated as: %DMC = [ x (WA/(WA-WW ) – 142]. Where WA= weight in air and WW= weight in water ( Jaramillo et al., 2005). Dry root yield (DRY) was derived by multiplying FRY with percentage DMC. The different levels for delayed PPD were determined using the method of wheatley et al. (1985). Evaluations were made on the 7th, 14th and 30th day after harvest with 5 roots each. At each evaluation, seven 2-cm thick transversal cut were made all along the root from the proximal end (Figure 2). A score corresponding to the percentage of cut showing discoloration, of 0-10 (0= 0%, 1= 10%, 2= 20%, etc.), was assigned to each slice. The mean PPD score for each root was calculated by averaging the score from 7 slices. Yield data collected were subjected to simple statistics using excel to determine minimum, maximum, average and standard deviation of the individual parameters.
FUTURE PERSPECTIVE
The genotypes tolerant to PPD will be advanced to release in Nigeria and shared with NARs in Africa to improve their germplasm
0-5% highly tolerant; 5-35% partially tolerant; 35-65% Intermediate; 65-95% Susceptible; % highly susceptible
Figure 3: Frequency of BC2 population in PPD evaluation on the 7, 14 and 30 DAH
ACKNOWLEDGEMENT
We acknowledge funding from Generation Challange Programme (GCP) and National Root crops Research Institute (NRCRI), Umudike
REFERENCES
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1 =Highly resistant; 2 =Resistant; 3 =Moderately resistant; 4 =Susceptible; 5 =highly susceptible
Figure 4: CMD score of the BC2 population