|dc.description.abstract||Sweetpotato (Ipomoea batatas, L.) is one of the important sources of carbohydrates and
economic income in Mozambique. As with most of the food crops in Mozambique, it is
usually produced by small-scale farmers under dryland conditions. Despite the importance
of the crop, the storage root yields are still low and it is difficult to keep planting material
(vines) for the next planting season. One of the major challenges to production is drought
stress. Drought stress affects sweetpotato by retarding aboveground growth, reducing total
root yield, percentage of dry mass, and reducing the quality of the roots as a result of the
increase in damage caused by the sweetpotato weevil (Cylas formicarius).
The objective of this study was to identify sweetpotato genotypes tolerant to drought
particularly amongst the orange fleshed types which can be used in breeding programmes
to improve the drought tolerance of genotypes grown in Mozambique. To this end, 48
genotypes were evaluated in both field and greenhouse studies conducted at Umbeluzi
Research Station (26º 03’ S, 32ºC 23’ E; 12 masl) located about 30 km from Maputo city.
The field trial was a three replicate, α-design with split-plots. Genotypes were the wholeplot
treatment factor and irrigation levels were the sub-plot treatment factor. The three
irrigation levels imposed were: nonstressed plants irrigated from planting to 120 DAP;
moderately stressed, plants irrigated until 60 DAP; and severely stressed, plants irrigated
until 30 DAP. In the greenhouse trial the 48 genotypes were grown in wooden boxes
arranged in a two replicate, randomized complete block design. The plants were exposed to
water stress from 10 DAP to the end of experiment at 60 DAP.
Genotypes were significantly different for all traits, namely: survival %, vine vigour,
aboveground biomass, total and commercial root yield, total fresh biomass, harvest index, β-carotene content, % dry mass, dry mass yield, incidence of sweetpotato virus disease,
and incidence of weevil damage. Irrigation levels were significant for the traits: survival
%, vine vigour, aboveground biomass, total and commercial root yield, total fresh biomass,
harvest index, β-carotene content, % dry mass, and dry mass yield. Irrigation levels were
not significant for incidence of sweetpotato virus disease and incidence of weevil damage.
The genotypes x irrigation levels interaction was significant for: total and commercial root
yield, and incidence of weevil damage; and not significant for: survival %, vine vigour,
aboveground biomass, total fresh biomass, harvest index, β-carotene content, % dry mass
composition, dry mass yield and incidence of sweetpotato virus disease.
The mean dry mass yields across irrigation levels of the national breeding lines and
introduced genotypes were higher than the landrace genotypes. Most of the national
breeding lines had higher β-carotene content than the introduced and landrace genotypes.
The landrace genotypes had relatively higher % dry mass composition compared to the
national breeding lines.
The stress tolerance index (STI) separated the 48 sweetpotato genotypes evaluated in the
field trial into three groups: drought tolerant (high STI); moderate drought tolerant
(intermediate STI); and drought sensitive (low STI). Under moderate stress, yield potential
(Yp) and yield in a stress environment (Ys) were highly significant, positively correlated
with Mean productivity (MP), Geometric mean productivity (GMP), Stress tolerance index
(STI) and Tolerance index (TOL). Under severe stress the same correlations were reported.
Under moderate and severe stress, the correlation between stress tolerance index (STI) and
Stress susceptibility index (SSI) was significant and negative. In the greenhouse trial, differences between genotypes in vine length increment, vine
diameter increment, leaf width increment and number of nodes vine-1 were significant
(P<0.05). Vine length, vine diameter, leaf width and length increments either increased or
were reduced due to water stress. Less than 10% increment in vine length (between 25 and
50 DAP) was recorded in MGCl01, Atacama, Cordner, Beauregard, and CN1448-49.
Higher than 40% vine length increment was recorded in Jonathan and UNK-Malawi,
Naspot, MUSG0614-24, Resisto, K566632, Tainung64, Ejumula and MUSG0623-09. Vine
diameter decreased in Manhissane and MUSG0616-18. No change in leaf length in Tacna
and Jonathan and in leaf width in Xihetamakote and Resisto-Nairobi was recorded.
The longest petiole length at 30 DAP was recorded by Tacna and the shortest by
Nhacutse4. The longest internode length was recorded in 199062.1. Similar to petiole
length, Nhacuste4 reported the shortest internode length. The highest number of primary
vines was recorded by MUSG0608-61 and lowest by Beauregard. Of the 48 genotypes
exposed to water stress, 18 survived until the end of the greenhouse experiment at 60 DAP
and were therefore considered to be drought tolerant.||en