Yield evaluation, selection and drought tolerance indices of orange-fleshed sweetpotato (ipomoea batatas lam) under water stress conditions

Abstract

Orange-Fleshed Sweetpotato (OFSP) varieties provides high levels pro-vitamin A and medium amounts of iron and zinc. Their drought susceptibility is perceived as one of the major drawbacks to production and adoption. Experiments were conducted in Kenya at different sites in the years between 2008 and 2010 in order to select drought tolerant, high yielding orange-fleshed sweetpotato genotypes that have acceptable levels of beta carotene. An in vitro screening was conducted in the tissue laboratory at Kenya Plant Health Inspectorate Services Quarantine station, Muguga, Kenya to assess plantlet regeneration of 59 OFSP genotypes. The second experiment was conducted at Kiboko experimental field between September 2007 and January 2008.The objective was to identify 10-20 promising drought tolerant OFSP genotypes for further evaluation, testing and selection. The trial was laid out as randomized complete block design. Susceptible genotype K566632 and drought tolerant genotype Marooko were used as checks. For selection criteria three traits were used: Root- flesh color, root dry matter content and average yield in t/ha. Final selection was based on the ranking of the genotypes based on summation index of these variables. In the third experiment 18 OFSP genotypes selected from the rapid field screening were further evaluated for yields and stability at two sites: Kiboko and Marigat. A split-plot design was used with two water levels ( non-irrigated and irrigated) as the main factor and the genotypes as the sub-factor. The experimental design was randomized complete block design laid out as split-plot. Stress tolerance index was xxv used to identify genotypes with high stress tolerance and high yield potential. In addition a pot experiment was conducted to identify and evaluate traits associated with water stress in sweetpotato genotypes during growth period. The pot experiment was set up in a completely randomized design with five genotypes, two water levels stressed and unstressed replicated three times. One drought tolerant check Marooko was used. Changes in soil water content were evaluated by weighing the pots; Relative water content of the leaves; leaf and stem growth characteristics; morphophysiological responses; the relative parameters and available soil water. Results showed significant variations among the genotypes for water stress tolerance based on plant growth characters. Genotypes 189135.9, 192033.5, 194515.5, 194539.3, 401055, 441724, 440429, 441097, 441538,441768 were observed with outstanding ability to continue root and shoot growth under in vitro stress conditions. Out of the 59 genotypes screened in the field, 21 were found to be dark orange, 12 to be orange, 12 light orange and 14 were found to be either cream or white. The dry matter of fresh storage roots ranged from 15 to 35 % with majority of the dark orange to orange genotypes having less than 30%. Most of the dark orange to orange genotypes recorded high number of roots compared to the cream to white-fleshed genotypes. Overall genotype 440378 was the lowest yielding genotype with a yield of 7.43 t/ha. Genotypes 420027, 187017.1, 420024, 187016.2 and 420014 produced over 43 t/ha. Mean total number of roots were significantly lower in Kiboko (6.15) than Marigat (20.48) under non-irrigated treatment. High numbers of root production under the same treatment in both sites were observed for genotypes 189135.9, 194573.9, 440287 and 441725. In both site genotypes 194573.9, 420014, 440286 and 441725 xxvi showed high stress tolerance and yield potential compared with the check by registering higher stress index values that ranged between 0.37 and 0.96. Genotypes 421066 and 189148.2 had their leaf areas least affected by moisture stress. Genotypes 189148.2 and 194573.9 registered high biomass production. Least biomass production under stress treatment was observed for genotype 421066. Genotypes 194573.9 and 189148.2 had biomass partitioning that favored root system development. Genotypes 421066, 194573.9, 192033.3, 187017.1 and 189135.9 recorded the highest values of STI at both sites, were considered to be tolerant genotypes with high beta-carotene and high dry matter content. Water stress reduced the number of leaves per plant and individual leaf area, the number of storage roots per plant, weight of each storage root and the harvest index. There was variation in tolerance to water stress among the genotypes; example genotypes 189148.2, 194573.9 and 421066 maintained high leaf area under stress conditions, high root: shoot ratios. The genotypes that had a high root: shoot ratio under water stress conditions in both Marigat and Kiboko had storage roots with high beta carotene and high dry matter content. These are desirable traits to meet vitamin A requirement.