Advances in Crop Science and Technology
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Sweet potato (Ipomoea batatas (L.) Lam) is a flexible crop of immense potential with its ability to adapt a wider range of environmental conditions. Globally, sweet potato is cultivated in more than 115 countries. In Ethiopia, it is dominantly grown for human consumption in the Eastern, Southern and Southwestern parts of the country. The crop can be consumed as a healthy diet by about 20 millions of people across the country [1].

The importance of Orange Fleshed Sweet Potato (OFSP) is becoming one of the strategic crops for the intervention to nutrition sensitive agricultures as a means to combat problems associated with malnutrition. The crop is grown primarily for its storage roots worldwide; however, both storage roots and leaves can be potential sources of human nutrition.

Currently, encouraging efforts are being made on the leaves of some sweet potato varieties that are found effectively to be consumed as a vegetable in some parts of southern Ethiopia. Besides, research findings elsewhere by various authors indicated that the young sweet potato leaves are rich in polyphenolic, protein, vitamins and minerals and are used as a green vegetable [2-4]. In Ethiopia, most of the communities living in highland areas are expected to have small plots of lands as compared to their family sizes and these lands are occupied relatively with some crops such as Enset, maize and potato. These areas are densely populated and malnutrition which is associated with lack of food diversification is a critical issue. OFSP is known for high nutritive values with wider adaptability including low input requirements thereby it can be considered as a best means for intervention to contribute to the reduction of malnutrition [5]. Thus, evaluation of available released and introduced OFSP varieties in highland areas is a key approach to recommend best adapted varieties for production and promotion for human consumption. Therefore, variety evaluation works was conducted using nine released and introduced OFSP varieties at two locations, namely Bursa and Gedeb districts in Sidama region and Gedeo zone of Southern Ethiopia over the two main growing seasons (2020/21 and 2021/22) with the objective of identifying well adapted varieties for highland environments [6-8].

Description of the study sites

The study was conducted at Gubeta kebele of Gedeb district of Gedeo zone (5°54.056’N, 38°15.384’E, 2350 m.a.s.l) and Bursa kebele of Bursa district of Sidama region (6°32.883’N, 38°38.374’E, 2348 m.a.s.l). Both of the sites were characterized as highland areas with well-drained soil types. The major crops grown in Gedeb areas are Ensete, potato, wheat, highland maze, barley and other root crops in the near vicinity whereas, Ensete, potato, barley and highland maze are among some of dominantly grown crops in Bursa district. In both areas farmers have high interests for the production of sweet potato varieties that are adapted to their environments [9,10].

Experimental materials, field design and procedure

Field trials were conducted using nine released and introduced orange fleshed sweet potato varieties. The study materials were collected from Hawassa agricultural research center (five released in Ethiopia and four introduced from Uganda, Kenya and Mozambique). The description of the varieties is shown in Table 1 [11].

Table 1: List of orange-fleshed sweet potato varieties used for the study.

Experimental design and field management

The experiment was laid out in a Randomized Complete Block Desi gn (RCBD) in 3 replications. The size of plot for each variety was 2.4 m width and 3 m long with an area of 7.2 m². There were four rows/ridges per each plot that accommodated a total of 40 plants/cuttings. The spacing between plants and rows were 0.3 m and 0.6 m, respectively. The spacing between replications was 1.5 meter [12]. One plant (vine cutting) of 30 cm length with 5 nodes-8 nodes was planted in each plot in the prepared holes by burying one-third of the plant in the ground. Replanting was done to replace dead vines after one week of planting and earthing up was done after fourth week of planting and all plots were kept weed free manually by farmers. All plots received the recommended cultural practices uniformly and no fertilizer was applied (HwARC, 2015).

Data collection

Data were collected from the two central rows excluding the two plants grown at both ends of the row and the two border rows. The following yield and yield related parameters were recorded at harvesting:

Sweet Potato Virus Disease (SPVD): Recorded on plot basis, using a scale of 1 to 9, where 1=no virus symptoms, 2=unclear virus symptoms, 3=clear virus symptoms for <5% of plants per plot, 4=clear virus symptoms for 6%^-15% of plants per plot, 5=clear virus symptoms for 16%-33% of plants per plot, 6=clear virus symptoms for 34%-66% of plants per plot (i.e. >1/3 and <2/3), 7=clear virus symptoms for 67%-99% of plants per plot (2/3 to almost all), 8=clear virus symptoms for all plants per plot (not stunted) and 9=severe virus symptoms for all plants per plot (stunted) as per the procedure developed by Gruneberg et al. in 2019.

Aboveground fresh weight biomass (t ha^-1): The weight of above ground parts of the two central rows was taken and converted to t/ha.

Root length (cm): It is the length of storage root that was measured from distal to the proximal end on five randomly taken plants at harvest.

Root girth (cm): It is the diameter from the middle portion of the storage root. The girth of all the storage roots of each of the sampled plants were measured by using caliper and divided by number of storage roots from all plants sampled and expressed in centimeter.

Marketable root yield (t ha^-1): It is the weight of clean, uninfected storage roots that fall in the size range of 100 g-500 g. It was taken by weighing all the storage roots collected from the harvestable plot by using beam balance and expressed as t/ha.

Unmarketable root yield (t ha^-1): It is the weight of infested, under sized (less than 100 g), oversized (more than 500 g) bruised, or cut storage roots. It was taken by weighting all the storage roots collected from the harvestable plot by using beam balance. It was expressed as t/ha.

Total root yield (t ha^-1): It is the sum total of both marketable and unmarketable storage root yields obtained from the harvestable plot and then it was expressed as t/ha.

Yield per hectare: This was obtained from harvestable plot (net plot) and converted in to yield per hectare by using the formula written below and was expressed as ton per hectare.

Data analysis

Analysis of variance: Collected data on root yield and its components were subjected to analysis of variance using SAS package (SAS 9.3). Data were checked for homogeneity of error variance for the two locations over years using F-ma and it was not significant and then, combined analysis was performed [13]. Least Significance Differences (LSD) technique was employed to compare the treatments following the procedures of Gomez and Gomez.

The following statistical model was used for combined analysis of variance over locations:

Yijkl=μ+Gi+Lj+GLij+Rk(j)+Bl(k)+εijkl

Where Yijkl is observed value of variety i in block l and replication k of location j, μ is grand mean, Giis effect of variety i, Ej is location effect, GEij is the interaction effect of genotype i with location j, Rk(j) is the effect of replication k in location j, Bl(k) is the effect of block l in replication k, εijkl is error (residual) effect of genotype i in block l and replication k of location j.

Analysis of variance for root yield and yield related traits

The mean squares of analysis of variance showed the presence of highly significant differences (p<0.001) for root yields and yield related traits between the four environments among nine genotypes (Table 2). Highly significant differences were observed among the varieties for all traits considered (p<0.001). Significant differences were observed with the variations due to locations for most of the recorded traits except unmarketable root yield, suggesting differential response of varieties across locations [14-16].

Highly significant differences (p<0.001) were observed among varieties, locations and their interactions almost for most of the traits under study. The three way interaction effects (variety × location × year) were significant at (p<0.005) for marketable and total root yields and non-significant for the rest of traits studied (Table 2). The existence of significant difference between locations (location main effect) is mainly due to environment, edaphic or management factors prevailing at the specific sites. Similar findings reported the presence of significant difference in root yield of genotypes across various environments. Also, Harrison and Jackson observed yield variation between sweet potato varieties and the authors further explained as these variations might be attributed to differences due to varieties, length of growing season, cultural management and environmental conditions. The results also showed highly significant differences for the interactions of varieties by locations which indicated that there is an opportunity to select the best adapted genotypes for production in highland areas.

Table 2: Analysis of variance for root yield and yield related traits of nine orange fleshed sweet potato varieties evaluated at four environments.

Mean root yield performance of orange fleshed varieties at highland areas

The combined mean values of the varieties for marketable root yield ranged from 5.70 tons to 13.15 tons per hectare with an overall mean of 8.05 t ha^-1. Variety designated as V9 (Kulfo) gave better marketable root yield of 13.15 t ha^-1 while variety V4 (NASPOT-8) produced the lowest marketable root yield of 5.70 t ha^-1 as compared to the others (Table 3). All the studied varieties showed the lowest unmarketable root yield which was less than 1 ton per hectare. The high and low temperature extremes play a key role in sweet potato storage root initiation and bulking at its critical phinophase. Although the obtained root for most of the tested varieties was found between acceptable ranges, the characteristics of low temperature in highland areas (at high altitude) expected to have an influence on sweet potato root development and bulking ability. The size of the roots is very important since roots with less than 100 g are considered under sized and those with more than 500 g are labeled as oversized and not preferred by the consumer. The under sized roots are considered as unmarketable. Varieties with low mean values for unmarketable root yield (t ha^-1) can be considered as better varieties with less wastage. The highest total root yield of 13.53 t ha^-1 and the lowest total root yield of 6.03 t ha^-1 were recorded for variety V9 and variety V4, respectively. In addition to other traits, root yield can be used as a means for selecting the best adapted variety to recommend for production in the target agro ecological areas. The result of this study suggested that it can be used as an entry point to diversify the production of sweet potato in high land areas (up to 2400 m.a.s.l). The three varieties such as V9 (Kulfo), V2 (Kabode) and V7 (Alamura) produced total root yield of 13.53 t ha^-1, 10.71 t ha^-1 and 10.40 t ha^-1, in that order, implying that they can be potential varieties for further production in the study areas and other areas with similar agroecologies (Table 3). Although the yield obtained seemed not comparable with yields of the low and midlands areas, the study is the first of its kind in evaluating sweet potato adaptation to high altitude areas and can be considered as a promising result. However, there were some research reports by Vinaj and Babu and Moussa et al., on variability for yield and yield components in sweet potato can be attributed to genetic and environmental factors. These authors explained that altitude differences reasonably influence genotypes’ performance for root yield and yield components.

Table 3: Mean performance of sweet potato genotypes for root yield and yield related traits evaluated across two locations over two cropping seasons (2020/21 and 2021/22).

Reaction of OFSP varieties to SPVD

Highly significant differences existed within the nine varieties, between the two locations and their interactions over years for SPVD reaction. However, the two way interaction (V × Y) and the three way interaction (V × L × Y) effects were non-significant. Although significant differences were observed between the tested varieties, the obtained result showed resistance/tolerance of the varieties to SPVD with low scores of <3.0. Various reports indicated that SPVD is the most destructive disease that can cause root yield reduction by over 90% on sweet potato world, especially in East Africa [17]. In this study, mean values for SPVD scores of the nine varieties varied from 1.58 to 2.67 corresponding for varieties V6 (Mayayi) and V1 (Birtukane), respectively. These low scores for the reaction to SPVD in tested areas might be attributed to low level of virus diseases pressure in the highland conditions (highland areas) Varieties with better root yield performance and low SPVD severity scores can be considered as a selection means to recommend for production in highland areas.

Performance of varieties for above ground biomass, root length and root girth

Combined analysis of variance showed the existence of a highly significant difference (p<0.05) between varieties, locations and most of their interactions. The mean performance of the varieties for above ground biomass ranged from 8.0 t ha^-1 to 20.5 t ha^-1 with an overall mean of 15.83 t ha^-1. Two varieties V7 (Alamura) and V6 (Mayayi) produced above ground biomass yield of 20.22 t ha^-1 and 20.50 t ha^-1, respectively. This might suggest that those varieties with good above ground biomass can reasonably be used for dual purpose (for human consumption and feed for livestock) [18]. The root length of the tested sweet potato varieties was significantly varied. The long root length of 13.47 cm was obtained from variety V1 (Birtukane) whereas the shortest length of 10.00 cm was recorded for V5 (Kyoyabwerer). The largest and smallest root girth was recorded for varieties V9 (Kulfo) and V7 (Alamura) with values of 5.70 cm and 3.13 cm, respectively with an average mean value of 4.22 cm. The variations in the length and diameter of the roots of sweet potato have been reported by various authors. The authors explained that such variation in sweet potato may be attributed to soil types, soil fertility and productivity, cultivation techniques adopted, environmental factors and genetic makeup of the variety [19].

Adaptation study of improved OFSP varieties in highland environments is a key approach for contributing to food and nutrition security of the communities that are mainly based on cereal farming. Evaluation of nine OFSP varieties for root yield and yield related traits was conducted at two areas that can be characterized as highland environments. Out of the nine tested varieties, two varieties (Kabode and Alamura) showed relatively good performance at the target areas. These varieties can be recommended for production in the study areas and areas that share similar agro-ecologies.

The authors would like to express their gratitude to the South Agricultural Research Institute (SARI) and the Ethiopian Institute of Agricultural Research (EIAR) for their financial support. The Sweet GAINS project of CIP, funded by the Bill and Melinda Gates Foundation, is especially acknowledged for the financial support to run the field works.

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