Evaluating Drought tolerance indices for selection of drought tolerant Orange Fleshed Sweet Potato (OFSP) genotypes in Ethiopia

The purposes of this study were to assess the effectiveness of drought tolerance indices for selection of drought tolerance in orange fl eshed sweet potato genotypes. In order to assess effi ciency of drought tolerance indices, 10 Orange Fleshed Sweet Potato genotypes (OFSP) were evaluated under Normal or full irrigation and extreme water stress environments. A total of 9 drought tolerance indices including Stress tolerance index (STI), Tolerance (TOL), Mean Productivity (MP), Geometric Mean Productivity (GMP), Stress Susceptibility Index (SSI) Yield Index (YI), Yield Stability Index (YSI), Harmonic Mean (HM), and Stress Intensity Index (SII) were calculated based on yield obtained from the two moisture regimes. Rank mean, standard deviation of ranks and rank sum were calculated to identify well-performed varieties according to all indices. These STI, GMP, YI, SII and TOL showed high and signifi cant correlation under both (Yp) and (Ys) condition. In consideration of all indices a variety MUSG01406521-13, MUSG014065-21-14, and MUSG014019-7-50 exhibited the best mean of ranks and almost low standard deviation of rank. Among 10 genotypes, the combination of nine drought indices and deviation identifi ed MUSG014065-21-13, MUSG014065-21-14, and MUSG014019-7-50 as a three promising and drought tolerant genotypes. Research Article Evaluating Drought tolerance indices for selection of drought tolerant Orange Fleshed Sweet Potato (OFSP) genotypes in Ethiopia Selamawit Abebe Gitore1*, Benjamin Danga1, Sylvia Henga1 and Fekadu Gurmu2 1Department of Dry Land Agriculture and Enterprise Development, Kenyatta University, School of Agricultural Science and Technology, Nairobi, Kenya 2Southern Agricultural Research Center, Hawassa, Ethiopia Received: 30 July, 2021 Accepted: 20 August, 2021 Published: 24 August, 2021 *Corresponding author: Selamawit Abebe Gitore, Department of Dry Land Agriculture and Enterprise Development, Kenyatta University, School of Agricultural Science and Technology, Nairobi, Kenya, Tel: +254702601280; E-mail:


Introduction
Water defi ciency is one of the key abiotic factors affecting crop productivity. Water shortages for agriculture are becoming more common around the world. Drought adaptability in crops like sweet potato is therefore required. Breeding objectives should be pursued for the production of drought tolerant cultivars suitable for stress agriculture to overcome yield reduction under stress and highland environments. The combination of high yield stability and high relative yield under drought has been recommended as a viable selection criterion for assessing genotypic performance under varied degrees of water stress [1]. Genetic manipulation of the crop to improve tolerance is recommended among drought management options because of its sustainability and feasibility, especially in resource-poor areas [2,3]. The development of droughttolerant cultivars with acceptable agronomic and qualityrelated traits is largely dependent on the availability of genetic resources for tolerance, effective screening techniques, identifi cation of genetic traits of tolerance [4], successful genetic manipulation of the desired genetic backgrounds, and fi nally the development of drought-tolerant cultivars with acceptable agronomic and quality-related traits [5]. The relative yield performance of genotypes in drought-stressed Citation: Gitore  Center [20] in Ethiopia were used.

Experimental design and procedures
The experiment was carried out in Simple lattice design with two replications each for stress and non-stress condition in a fi eld at Areka woreda Wolaita Zone, Ethiopia. The study site covers a portion of the zone located between 7o4'N37o42'E, respectively. The yearly average max and min temperatuof the site are 15˚C and 30˚C, respectively. Annual average RF is 1300mm in the highland area and 600mm or less in the low lands. Like the eastern part of Ethiopia, Wolaita zone is also highly affected by drought. Genotypes were arranged in 3 rows per plot with 5 plants per row using 30 cm spacing between plants and 90 cm row spacing, respectively. 30cm length Vine tip cuttings were used as planting material and planted at depth of 4-6 cm.

Data collected
Drought tolerance indices: The storage root yield data were recorded for each genotype at both environment (nonstress and stress) and were subjected to calculate and analyze different drought selection indices using following formulas.  For screening drought tolerant genotypes a rank sum (RS) was calculated by using the following relationship formula: and non-stressed conditions can be utilized to discover drought-resistant cultivars in breeding programs for droughtprone areas [6,7]. Several drought indices based on genotype drought resistance or susceptibility have been proposed and calculated between yield under stress and optimal conditions in this regard. Indicators have been used to screen drought tolerant genotypes because they provide a measure of drought based on yield loss under drought conditions compared to normal conditions. Drought indices have been used to screen drought-tolerant genotypes because they provide a measure of drought based on yield loss under drought conditions compared to normal conditions [8]. Different approaches have been employed by different studies to assess genetic differences in drought resistance. Drought resistance is defi ned as a genotype's relative yield compared to other genotypes treated to the same drought stress [9]. Drought susceptibility of a genotype is frequently assessed as a function of yield reduction under drought stress [10], although the results are muddled by genotype yield potential differences [11]. Several screening process for genotypes based on their performance in stress and non-stress contexts have been presented. According to [12],  [14]. The Stress Susceptibility Index (SSI) was proposed by Fischer and Maurer [15] as a method of determining yield stability that took into account variations in both prospective and actual yields in a variety of settings. Clarke, et al. [16] employed SSI to assess drought tolerance in wheat genotypes and discovered year-toyear variance in SSI for genotypes, as well as the ability to rank their patterns. Guttieri et al. [17] used SSI to imply that an SSI > 1 indicated above-average susceptibility to drought stress in spring wheat cultivars. The Yield Index (YI) proposed by [18] and the Yield Stability Index (YSI) proposed by Bouslamaand Schapaugh [19] were used to assess genotype stability under stress and non-stress situations. The Stress Tolerance Index (STI) was developed as a tool for identifying genotypes' high yield and stress tolerance potential [13].

Ranking of genotypes
The goal of this study was to determine the selection criteria for determining drought resistance in orange fl eshed sweet potato varieties in Ethiopia so that acceptable cultivars could be selected for cultivation in Ethiopia's drought-prone areas.

Statistical analysis
The mean storage root yield data were recorded for each genotype at both environment (non-stress and stress) and were subjected to calculate and analyze different drought selection indices using following using MS Excel. Rank mean, standard deviation of ranks and rank sum were calculated to identify well-performed varieties according to all indices.
Correlation among indices and grain yield in two conditions were performed SPSS Version 20 software.

Comparing cultivars based on the resistance/tolerance indices
Different drought tolerant indices were derived based on storage root yield of genotypes under non-stress (Yp) and stressed (Ys) environments (Table 1).

Stress susceptibility index (SSI)
The mean yield of genotypes under stress and nonstress conditions are Ys and Yp, respectively and genotypes with the lowest value (SSI 1) are more droughts tolerant.
The results showed that all genotypes with the lowest SSI

Mean Productivity Index (MPI)
Under stress conditions, genotypes with a high value of this index are thought to be more desirable. Vita and MUSG014065-21-14 genotypes had a higher value and were considered tolerant, whilst

Stress Intensity Index (SII)
The SSI > 1 value suggested a higher-than-average susceptibility to drought stress. The result showed that all varieties had SII<1 score, indicating that they are suitable and drought tolerant under stress condition. closely followed by Naspot-12 (8.1) were identifi ed as the most sensitive genotype ( Table 2).

Correlations of drought tolerance indices and storage root yield
The correlation coeffi cients between Yp, Ys, and other drought tolerant criteria were used to determine the most acceptable drought tolerant criterion. Drought tolerance indicators were calculated quantitatively (Table 3). In other terms, a correlation study between two or more variables.  (Table 3).

Discussion and conclusion
Storage root yield of cultivars under both non-stress and stress situations were measured for computing different sensitivity and tolerance indices to examine potential stress resistance indices for screening of cultivars under drought conditions (Table 1)