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• Majority of farmers indicated use of drought tolerant varieties and irrigation were strategies used to cope with drought constraint
2. Evaluation of sweetpotato genotypes for tolerance to drought stress
• Moisture stress did not affect numbers of roots probably because there was sufficient soil moisture during the root initiation period, which is within the first few weeks after they are planted. Significant variation was observed among the genotypes and across environments for all the traits.
• Genotypes 194555.7, Unawazambane06-01, 441725, Tanzania, Chingova did well in both drought and irrigated environment and had DSI <1.
• Genotypes that took few days to permanent wilting point in the rapid box water stress screening also, had low yield across the environment, had DSI of >1, lower HI.
3. Genotype x environment interaction for storage root yield in sweetpotato under managed drought stress conditions in Kenya
• The environmental effects contributed the highest variation of 75.2%.
• Genotype ranking based on DSI, showed that genotype responses to moisture stress were not consistent over environments and thus both DSI and DPWP were vital in discriminating drought tolerance among genotypes.
• The GGE and AMMI biplot showed there was both broad and specific interaction between genotypes and environments,
• Genotype close to the regression line in the regression plot were stable while genotypes far away were not stable across environments
• The biplots showed that both sites (Kiboko or Thika) could be used as test sites for preliminary drought screening
4. Mechanisms of tolerance to drought stress in sweetpotato The drought tolerance mechanisms were found to involve;
• Formation of pencil roots especially when the stress occurred in the first three months after planting but this does not benefit the grower.
• Decrease in the size of storage roots, and number of vine branches
• Reduced vine branches in drought tolerant cultivars resulting in reduced evapo- transpiration.
• Longer pencil roots for genotypes under moisture stress than genotypes under no moisture stress.
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• Increase in shoot and mature leaf pubescence in moisture drought tolerant genotypes.
• Reduced chlorophyll content (CC) in non-tolerant genotypes and in moisture stress regimes.
• Retention of some level of CC by drought tolerant genotypes.
5. Heterosis and combining ability for drought tolerance
• Significant GCA and SCA effects were observed for fresh storage roots (FSR), harvest Index (HI), number of storage roots (NSR) and percent root dry matter (%RDM)
• Ratio of GCA/SCA for traits (FSR, HI, %RDM) indicated additive gene action was predominant over non-additive in the inheritance of the traits.
• For total fresh biomass (BIO) and number of marketable storage roots (NMSR) under drought stress, GCA/SCA indicated non-additive gene action was predominant.
• Parents P3 and P4 combined well with P5 (susceptible) giving the highest root yielding progenies (G15 and G5) under both irrigation and drought environment.
• Most parental crosses with positive significant SCA effects on root yield had negative significant SCA effects on % RDM, however G7 and G8 which had high root yields, had positive SCA effects on % RDM.
• Parental crosses with high significant days to permanent wilting point (DPWP) SCA effects were G7, G8 and G15
• Negative, significant marketable storage roots SCA effects were observed for all the parental crosses under drought.
• Progenies found outperforming their parents were attributed to transgressive segregation which is an indication of drought tolerance of search genotypes.
• Parent P3 was common in these heterotic progenies and probably was the donor of the gene responsible for the improved production under drought environment.
• Parental crosses that had high heterosis (biomass or roots) under irrigation but low or negative heterosis under drought (G10-3, G10-8, G4-10) were unstable across environments.
• Drought tolerant parents with high GCA effects did not always result in crosses with the highest SCA effects.
Breeding implications and the way forward
In general, the findings of this study revealed that capacity building and technology transfers on sweetpotato need to target mostly women farmers aged from 21-40 years. There is also
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a need to further promote beta carotene rich OFSP to benefit the young children and lactating mothers value addition (processing) and promotion of sweetpotato as a cash crop is important since it is a healthy food, thus boosting the income generation of the farmers.
Further, the need to research on use of inorganic fertilisers on sweetpotato to boost production, as well as appropriate technology for seed multiplication and distribution method is required to ensure availability of clean and enough planting materials at onset of rains.
Importantly, an intensified research programme is required towards generating resistant varieties to drought, weevil and virus, which are the major hindrance sweetpotato production.
Results of sweetpotato germplasm screening indicated high levels of genetic variability for various traits. The results also indicated that moisture stress only hindered the development to full size of the roots and not the numbers. Genotype 194555.7, Unawazambane06-01, 441725, Tanzania, Chingova did well in both drought and no drought stress environment and had DSI <1 and thus could be further investigated for confirmation of drought tolerance.
These varieties may further be tested in many sites to validate their tolerance for use in drought areas. They could also be used as parents for breeding programs for improved drought tolerance.
The GGE and AMMI biplot analysis, in this study, showed there was a specific interaction between genotypes and environments, and therefore, genotype for drought tolerance need to be evaluated across environments to select for drought tolerance and thus breeding for specific adaptation for drought tolerance is required.
The study also highlighted a number of drought tolerant mechanisms upon which breeding for drought tolerance may be based. However, further physiological and biochemical studies may be appropriate to substantiate the mechanism at molecular level.
Significant GCA and SCA effects made that both additive gene effects and dominance gene effects were involved in the inheritance of resistance to drought, but additive effects were predominance in both environments. Progenies from families G5, G7, G10, G12 and G15 had good SCA in yield and total biomass, harvest index, in both drought and no drought stress environment an indication they could be having progenies that could be drought tolerant and thus could be further screened for drought tolerance. The GCA/SCA ratios which indicated the predominance of additive gene effects would enable accomplishing of genetic gain for drought tolerance by mass recurrent selection under adequate drought stress selection pressure. The relationship of the crosses and their parents in their yields