Effects of seed tuber size on growth and yield performance of potato ( Solanum tuberosum L . ) varieties under field conditions

Potato (Solanum tuberosum L.) is an important food and cash crop in Ethiopia; serving, as a food security crop due to its nutrition content, wider adaptability and early maturing behavior. However, the yield of the crop is constrained by a number of factors. Among which unspecified seed tuber size and limited availability and distribution of improved varieties are among the important limiting factors. A field experiment was conducted to study the effects of seed tuber size on growth and yield performance of potato varieties at Agarfa, Agricultural Technical and Vocational Education and Training College experimental field during 2017. The treatments consisted of three potato varieties (Gudenie, Jalene and Kellacho) and three seed tuber sizes (25-34, 35-45 and 46-55 mm). The experiment was laid out in a randomized complete block design with a factorial arrangement and replicated three times. The results showed that, seed tuber size and varieties significantly affected phenological parameters, stem number, leaf area index, stem density, marketable, unmarketable and total tuber yield, shoot fresh and dry weight, underground fresh weight, and medium weight of tuber size. Leaf number, and underground dry weight and tuber size were significantly affected only by seed tuber size. Varieties and seed tuber size interact to influence plant height, leaf area, number of tuber per plant, dry matter concentration and harvest index. Gudenie and Jalene produced the highest total tuber and marketable yields. Large seed tuber size (46-55 mm) produced higher marketable tuber yields than medium and small tuber sizes. In conclusion, Gudenie variety and large seed tuber size showed superior performance both for tuber yield and tuber dry matter concentration compared with the remaining varieties and small tuber size. Therefore, farmers are encouraged to produce Gudenie or Jalene variety with the use of medium to large seed tuber size for potato production.


INTRODUCTION
Potato (Solanum tuberosum L.) belongs to the nightshade or Solanaceae family and originated in South America (Asfaw, 2016).Globally, potato is the third most important food crop in terms of consumption after rice *Corresponding author.E-mail: tednet2011@gmail.com.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License and wheat (De Jong et al., 2011;Birch et al., 2012;Hancock et al., 2012).The global cultivation exceeds 19.34 million hectares of land in more than 158 countries across the world with an estimated annual production of 364 million tons (FAOSTAT, 2014).It is an important tuber crop grown widely in the humid tropics and used as a source of carbohydrates for many people in tropical and sub-tropical regions (Bilate and Mulualem, 2016).
Potato is a high potential food security crop in Ethiopia due to its high yield potential and nutritional quality tuber, short growing period, and wider adaptability (Ayalew et al., 2014).Potato plays a major role in national food security, nutrition, poverty alleviation and income generation (Ayalew, 2014;Gedif et al., 2013).The crop also provides employment opportunity in the production, processing and marketing sub-sectors (Menza et al., 2014).It is grown in over 3.5 million smallholder farmers in the highland areas and known to buffer food deficit during the months of limited food supply (July to August) before the grain crops are being harvested (Tebabal, 2014).Ethiopia is among the top potato producing countries in Africa, with 70% of the arable land suitable for potato production (FAOSTAT, 2008).
Potato production and productivity is low due to several constraining factors such as inappropriate planting materials, prevalence of disease and pests, poor soil fertility (Menza et al., 2014), variability in climatic patterns, shortage of water, shortage of agricultural input, and poor post-harvest handling practices (Tesfaye et al., 2011).Similarly, in Ethiopia absence of improved varieties, dependency on traditional management practices and unproper seed tuber size are among the important factors challenging small scale potato producing farmers.
The productivity of potatoes in Oromia region, Bale zone is less than the national average 11.8 t.ha -1 (CSA, 2014).Among the factors contributing to this low production and productivity is the use of local varieties that are low in yield and susceptible to disease (Asefa, 2015).And also seed tuber size is a limitation for potato production in the area.Therefore, there is an urgent need to determine proper seed tuber size for potato varieties grown in the area.In general, potato seed tuber size and varieties in the South Eastern highland of Ethiopia have not been sufficiently evaluated for increasing higher yields of appropriate seed tuber size.Considering the above limited research work, it is important to identify optimum tuber size and varieties for maximum potato production.Therefore, the objective of this study was to evaluate the growth performance and yield response of potato varieties to variable seed tuber sizes.

Description of the study area
The study was conducted at Agarfa Agricultural Technical Vocational Education and Training (ATVET) College experimental site.The site is located at 7° 17' 0" North latitude; 39° 49' 0" East longitude.The altitude is 2330 m.a.s.l.It is about 458 km away from the capital city of the country.The average annual rainfall is 829.4 mm and has a bi-modal distribution pattern.The mean maximum and minimum annual temperature are 24.75 and 7.10°C, respectively.The soil of the area is a silty loam, black vertisol and clay soil having a pH value of 6.12 (Mesfin et al., 2014).The current experiment was conducted under rain-fed conditions supplemented with irrigation during the 2017 cropping season (April to August).

Description of experimental materials
The experiment was conducted using two improved (Jalene and Gudeine) and one local (kellacho) potato varieties.Jalene and Gudenie were released by Holetta Agricultural Research Centre and Kellacho was a farmers' variety.The varieties were selected based on their adaptability, yielding potential, tolerance to late blight and preference of the producers and consumers.

Treatments and experimental design
The treatments consisted of three potato varieties (Jalene, Gudeine and local) and three tuber sizes (25-34, 35-45 and 46-55 mm in diameter).These tuber sizes 25 -34, 35 -45 and 46 -55 mm were considered as small, medium and large, respectively (Mulugeta et al., 2013).The experiment was laid in a randomized complete block design (RCBD) in a factorial arrangement and replicated three times.Gross plot size was 3 m × 3 m.The spacing between adjacent plots was 1 m and a distance of 1.5 m was maintained between blocks.Spacings between rows and plants were 75 cm × 30 cm, respectively.There were four rows per plot and 10 plants in each row, and a total of 40 plants per plot.Data were collected from the two middle rows.

Experimental procedures and cultural practices
The experimental plots were cultivated to the depth of 15 to 20 cm.The land was leveled and ridges were made by hand based on spacing between rows.Planting was done by selecting well sprouted seed tubers in each seed tuber size category according to the specified treatments for each varieties.Weeding, cultivation and earthing-up and ridging were done at the appropriate time to facilitate root, stolon as well as tuber growth and development.Late Ridomil MZ 63.5% WP at the rate of 2 Kg ha -1 and Mancozeb at the rate 3 Kg ha -1 were applied for late blight (EARO, 2004).

Data collection
Data on growth performance, yield and quality were recorded from five sample plants per plot.

Phenological parameters
Days to 50% emergence: Days to emergence was recorded when 50% of planted tubers were emerged from the soil.
Days to flowering: Days to flowering was recorded when 50% of the plant population in each plot produced flowers.
Days to maturity: Days to maturity were recorded when 90% of the plants in each plot become ready for harvest as indicated by the senescence of the haulms.The days were counted from emergence to maturity.

Growth parameters
Plant height (cm): Was measured from the soil surface to the top most growth point of the plant at physiological maturity.

Number of stems per hill:
Actual number was recorded at 50% flowering.Then an average was taken.Only stems that emerged independently above the soil as single stems were considered.
Leaf number/plant: The number of leaves was counted per plant at 50% flowering and the average was taken.
Leaf area: Was determined by the product of the leaf width (W) and length (L) based on pre-measured width and length from randomly selected five plants of leaves and multiplied by a constant (0.674).
Leaf area index: Was determined, first leaf area was calculated, then multiplied the average leaf area with the respective leaf number of the plant and obtained total leaf area.Then, dividing total leaf area to the respective land area occupied by plants.
Stem density: Was taken by counting stem population per square meter (m 2 ); then calculated the sum of stem density per m 2 .

Yield and biomass components
Total tuber yield (t ha -1 ): The whole tubers harvested from the two central rows were weighed and converted in to t ha -1 .
Unmarketable tuber yield (t ha -1 ): Including diseased, deformed tubers and weighing less than 25 g and converted into t ha -1 .

Number of tubers per plant:
Was recorded as counting of the actual number of tubers collected from five plants at harvest.Shoot fresh weight (t ha -1 ): Leaves, branches and stems were taken.Then weighed soon after harvest and converted to t ha -1 .

Shoot dry weight (t ha -1 ):
Was obtained after drying the samples taken for determination of the shoot fresh weight in open sun for 7 to 9 days and then oven dried at 72°C for 48 h to a constant weight and convert to t ha -1 .
Underground fresh weight (t ha -1 ): Roots, tubers, withered stolons and parts of the stem remaining underground were taken.Then weighed soon after harvest and converted to t ha -1 .
Underground dry weight (t ha -1 ): Was obtained after drying the samples taken for determination of the underground fresh weight in open sun for 7 to 9 days and then oven dried at 72°C for 48 h to a constant weight and convert to t ha -1 .
Harvest index: Was the ratio of tuber dry weight to the total dry weight of the plant after harvest.

Quality parameters
Marketable tuber yield (t ha -1 ): All tubers which were free from Ebrahim et al. 2079 diseases, insect pests and other damages and greater than or equal to 25 g in weight were weighed and converted to t ha -1 .
Tuber size distribution in weight: At harvest, tubers were collected from two central rows in each plot, and categorized into small (25-38 g), medium (39-75 g) and large (>75 g).Then converted to t ha -1 and % for each of the three categories.
Tuber dry matter concentration (%): Five potato tubers were randomly selected from each plot, chopped into small 1-2 cm cubes, mixed thoroughly and two subsamples each weighing at least 200 g was weighed.The exact weight of each sub sample was determined and recorded as fresh weight.Each subsample was placed in a paper bag and put in an oven at 65°C for 72 h until constant dry weight was attained.Each subsample was immediately weighed and recorded as dry weight.Then, percent dry matter concentration for each subsample was calculated following the procedure developed by the International Potato Center (CIP), (CIP, 2006).The dry matter percent was calculated following the formula suggested by William and Woodbury (1968).

Statistical analysis
The collected data were subjected to Analysis of Variance (ANOVA) using the proc mixed Model of the SAS package (SAS version 9.0, 2002).The mean separations were done using the (LSD) test at 5% probability level when there was a significant ANOVA effect.And Pearson correlation coefficients were made in determining association of parameters.

Days to emergence
The analysis of variance showed that seed tuber size had a highly significant effect (P< 0.001) for days to 50% emergence.Also, varieties significantly (P < 0.05) affected days to 50% emergence.However, the two main factor's interaction did not significantly affect the days to 50% emergence.Local variety (Kellacho) took the longest time to emerge from the soil over the two improved varieties (Gudenie and Jalene) (Table 1).
Similarly, seed tuber size affected days to emergence of potato varieties.Larger seed tubers (46-55 mm) took the shorter time to emerge as compared to the smaller seed tubers.This may be due to the fact that large tubers had sufficiently more stored reserve food compared to smaller seed tubers to provide an optimal supply of carbohydrate for the emerging seedling of the larger tuber.

Days to flowering
The analysis of variance showed that seed tuber size had a highly significant effect (P < 0.01) on days to 50%   1).

Days to maturity
The analysis of variance revealed that varieties and seed tuber size both had a highly significantly effect (P< 0.001) on days to maturity.However, the interaction of varieties and seed tuber size did not influence days to maturity.Jalene and Kellacho were late varieties to reach physiological maturity as compared to the Gudenie variety (Table 1).Gudenie matured by about 2.44 and 9.34 days earlier than Kellacho and Jalene, respectively.Jalene and kellacho had similar maturity periods.The perusal of data presented in Table 1 showed that the use of large seed tuber size shortened the time required to reach maturity.Medium seed tuber size (35-45 mm) had a similar effect on maturity of potato varieties (Table 2).Smaller seed tuber size (25-34 mm) required the longest time to mature physiologically.

Plant height
The analysis of variance showed that the interaction of seed tuber size and varieties significantly (P<0.05)influenced plant height.Large tuber size (46-55 mm) of the Gudenie variety resulted in the tallest plants.
Whereas, the shortest plants were recorded from small tuber sizes (25-34 mm) of the local variety (Kellacho).However, small seed tubers from improved varieties (Gudenie and Jalene) did not show a statically significant difference for plant height (Table 2).

Leaf area per hill
The analysis of variance showed that interaction of seed tuber size and varieties significantly (P<0.05)influenced the leaf area.Jalene variety with large (46-55mm) seed tuber size produced the highest leaf area per hill.Whereas, planting the Kellacho variety with small (25-34mm) seed tuber size produced the numerically lowest leaf area per hill (Table 2).Gudenie variety with large (46-55mm) and medium (35-45mm) seed tuber size relatively produced the highest leaf area per hill.On the other hand, the local variety (Kellacho) with medium and large seed tuber size gave a similar response as that obtained for an improved variety with small seed tuber size.

Stem number per hill
The analysis of variance showed that two main factors (the seed tuber size and varieties) had highly significantly (P< 0.01) effects on stem number.However, the interaction did not influence this parameter.Gudenie variety produced maximum stem number per hill compared with Jalene and Kellacho varieties (Table 3).This is probably due to a greater number of sprouts observed in Gudenie variety during emergence that might have resulted from its genetic potential for sprouting capacity.The perusal of data presented in Table 3 showed that use of large seed tuber size (46-55 mm) gave maximum stem number per hill.Whereas, using small seed tuber size (25-34 mm) resulted in only a limited stem number per hill.The general trend indicated by these data is that as the tuber seed size increased the number of stems per plant also increased.

Leaf area index
The analysis of variance showed that two main factors the seed tuber size and varieties had a highly significantly (P< 0.01) effect on leaf area index.However, the interaction did not influence this parameter.Local variety (Kellacho) had the minimum leaf area index per hill as compared to the Gudenie and Jalene varieties (Table 3).Jalene and Gudenie varieties had similar leaf area index values.With regard to seed tuber size, smaller tuber size (25-34 mm) produced the lowest leaf area index compared to large (46-55 mm) and medium (35-45 mm) sized seed tubers.However, use of larger (46-55 mm) and medium (35-45 mm) seed tuber sizes resulted in maximum leaf area index values and their effect was similar (Table 3).

Leaf number per hill
The analysis of variance showed that seed tuber size had a highly significant (P < 0.01) effect on leaf number.However, varieties and the interaction did not significantly influence leaf number.Smaller tuber size (25-34 mm) resulted in the lowest leaf number per hill compared to larger (46-55 mm) and medium (35-45 mm) seed tuber sizes.Larger (46-55 mm) and medium (35-45 mm) seed tuber sizes gave similar leaf numbers per hill (Table 3).Thus, increasing seed tuber size from 25-34 to 46-55 mm resulted in the production of maximum leaf number per hill.

Stem density
The analysis of variance revealed that varieties and seed tuber size were highly significant (P< 0.01) in effects on stem density.However, the interaction did not significantly affect stem density.Local variety (kellacho) produced minimum stem density or number of stem population per meter square whereas improved varieties (Gudenie and Jalene) produced the maximum stem density or number of stem population per meter square (Table 3).Seed tuber size significantly affected stem density such that the maximum stem densities were recorded from larger seed tuber size (46-55 mm), while the minimum stem densities were recorded from smallest seed tuber size (25-34 mm).

Total tuber yield
The analysis of variance showed that seed tuber size and varieties had a very highly significant (P< 0.001) influence on total tubers yield in tonnes per hectare.However, the interaction did not to influence the total tuber yield produced in tonnes per hectare (Table 1).The improved varieties (Gudenie and Jalene) produced the higher total tuber yield than local variety (Kellacho).Gudeine and Jalene varieties gave similar total tuber yield ton per hectare (Table 4).With regard to seed tuber size, medium to large sizes resulted in higher total tuber yield than the small tuber size (25-34mm) (Table 4).

Unmarketable tuber yield
The analysis of variance revealed that seed tuber size and varieties had highly significantly (P<0.01)effects on unmarketable tuber yield per hectare.However, there were no significant interaction effects.Local variety (Kellacho) produced maximum unmarketable tuber yield compared to the improved (Gudenie and Jalene) varieties.And both improved varieties were similar in producing unmarketable tuber yield (Table 4).With regard to seed tuber size, seed tuber size significantly affected unmarketable tuber yield.The maximum unmarketable tuber yields were recorded from smaller seed tuber size (25-34 mm), while the minimum unmarketable tuber yields were recorded from larger seed tuber size (46-55 mm) (Table 4).

Number of tubers per plant
The analysis of variance results showed that the interaction between seed tuber size and varieties was highly significantly (P< 0.01) affect number of tubers per plant (Table 5).Gudenie variety produced the highest number of tubers per plant when planted from large seed tuber size (46-55 mm).While minimum number of tubers per plant were produced when Kellacho variety was planted using small seed tuber size (25-34 mm) (Table 5).

Harvest index
The analysis of variance showed that the interaction of Means followed by the same letter within a column are not significantly different at the 5% level of significance.LSD = Least significant difference and CV = coefficient of variation.
two main factors such as tuber size and varieties factors highly significantly (P<0.01)affected the harvest index values (Table 2).The interaction of Gudenie and Jalene varieties with small seed tuber size (25-34 mm) produced numerically highest harvest index values; whereas, Jalene variety with large seed tuber size (46-55 mm) produced numerically the lowest harvest index value (Table 5).

Shoot fresh weight
The analysis of variance result showed that seed tuber size highly significant (P< 0.001) affected shoot fresh weight; and variety significantly (P < 0.05) affected shoot fresh weight.However, the two main factor interaction did not significantly influence shoot fresh weight.Gudenie variety produced the numerically maximum shoot fresh weight; whereas, Jalene and Kellacho varieties produced minimum shoot fresh weight, but the difference was not statistically significant (Table 6).The results also showed that improved Gudenie variety recorded the highest vegetative growth compared to the other varieties.Seed tuber size significantly affected shoot fresh weight per hectare.Medium to large seed tuber size resulted in larger shoot fresh weight.Whereas, small seed tuber size (25-34 mm) attributed to only a small fresh weight.Medium seed tuber size (35-45 mm) had a similar impact as the larger seed tuber size (46-55 mm) on production of shoot fresh weight (Table 6).

Shoot dry weight
The analysis of variance revealed that the main factors had a highly significant (P< 0.01) influence on shoot dry weight.However, the interaction did not significantly influence this parameter.Kellacho variety produced the minimum shoot dry weight per hectare compared to Gudenie and Jalene varieties.The Gudenie and Jalene varieties produced maximum shoot dry weight per hectare (Table 6).
Seed tuber size significantly influenced shoot dry weight per hectare.The maximum shoot dry weights were recorded from larger seed tuber size (46-55 mm), while the minimum shoot dry weight were recorded from the smaller seed tuber size (25-34 mm) (Table 6).The use of medium seed tuber size as a planting materials showed a statistically significant effect in producing shoot dry weight.

Underground fresh weight
Seed tuber size and varieties had highly significant (P<0.01)effects on underground fresh weight (Table 1).However, the interaction factors of two factors did not affect underground fresh weight.The highest underground fresh weights were recorded from Gudenie and Jalene varieties compared to the local variety (kellacho).Kellacho variety produced the lowest underground fresh weight.Gudenie and Jalene had similar underground fresh weights (Table 6).The lowest underground fresh weight (tonnes per hectare) was recorded from small seed tuber size (25-34 mm).Larger and medium seed tuber size produced the highest underground fresh weight.Medium seed tuber size (35-45 mm) had a similar impact as the larger seed tuber size (46-55 mm) on producing underground fresh weight (Table 6).

Underground dry weight
Seed tuber size had a highly significant (P<0.01)effect on underground dry weight.However, varieties and the interaction of two factors did not significantly affect underground dry weight.The larger and medium seed tuber size produced the highest underground dry weight; whereas, the lowest underground dry weight were recorded from small seed tuber size (25-34 mm) (Table 6).

Marketable tuber yield
The analysis of variance results showed that seed tuber size and varieties had a highly significant (P< 0.001) effect on marketable tubers yield (tonnes per hectare).However, the two factors interaction was not statistically significant.Improved varieties (Gudenie and Jalene) produced higher marketable tuber yield as compared to the local variety (Table 7).The large seed tuber size (46-55 mm) scored highest marketable tuber yield; whereas, the small seed tuber size (25-34 mm) scored the lowest marketable tuber yield (Table 7).

Tuber size distribution
The analysis of variance showed that variety had a significant (P< 0.05) effect on small and large tuber size parameters.However, seed tuber size had no significant effects for the overall tuber size distribution.Variety had a Means followed by the same letter within a column are not significantly different at the 5% level of significance.LSD (5%) = Least significant difference and CV (%) = coefficient of variation (%).
significant difference in response to tuber size.Gudenie produced the highest small size tuber whereas Jalene and Kellacho (local variety) produced the lowest small sized tuber weight (Table 8).Local variety produced a larger proportion of the large sized tuber weight compared to Gudenie (Table 7).Gudenie variety produced the highest proportion of small size tubers due to the fact that it produced a larger number of tubers per plant.Although Gudenie produced a larger proportion of small tubers and a lower proportion of large tubers, the variety also had highest marketable yield that still makes it preferable.

Tuber dry matter concentration (%)
The analysis of variance showed that interaction of varieties and seed tuber size had a highly significant (P<0.01)effect on tuber dry matter concentration.Gudenie variety with large seed tuber size (46-55 mm) produced the highest tuber dry matter concentration.Potato varieties grown from large (46-55 mm), medium (35-45 mm) and small (25-34 mm) seed tuber sizes across three varieties produced a similar response for tuber dry matter concentration, except Gudenie variety with large seed tuber size (Table 8).However, numerically Kellacho variety with small tuber size produced the lowest dry matter concentration.

Conclusion
The results of this experiment revealed that planting any of the two improved varieties (Jalene and Gudenie), using any of large (46-55 mm) or medium (35-45 mm) seed tuber sizes resulted in the highest total tuber yield of potato.Similar results were also observed for marketable tuber yield, with the exceptions that large seed tuber size resulted in better results.Therefore, it is wise to advice for farmers in the study area, and areas having similar agro-ecology, to use improved varieties Gudenie and/or Jalene with medium to large seed tuber sizes so as to boost the yield and quality of the potato crop.

Table 1 .
Days to emergence, flowering and maturity of potato as influenced by variety and seed tuber size.
flowering.Moreover, varieties significantly (P < 0.05) affected days to flowering.However, interaction did not influence the days to 50% flowering.Jalene and Kellacho required a longer time to reach flowering as compared to Gudenie (Table1).Gudenie flowered earlier by 3.56 and 5 days than Kellacho and Jalene, respectively.It is known that earliness in flowering is controlled by many factors including genetic and environmental factors.With regard to seed tuber size, smaller tuber sizes (25-34 mm) required significantly more number of days to reach flowering stage whereas, larger seed tuber sizes (46-55 mm) required significantly smaller number of days to reach flowering stage (Table

Table 2 .
The interaction effect of variety with seed tuber size on plant height and leaf area of potato at Agarfa during 2017 cropping season.

Table 3 .
Stem number, Leaf number, Stem density and Leaf area index as influenced by variety and seed tuber size.
Means followed by the same letter (s) within a column are not significantly different at 5% level significance.LSD = least significant difference, CV = coefficient of variation.

Table 4 .
Total and unmarketable tuber yield as affected by variety and seed tuber size.

Table 5 .
The interaction effect of variety and seed tuber size on number of tuber per plant and harvest index of potato at Agarfa during 2017 cropping season.

Table 6 .
Shoot fresh weight, Shoot dry weight, Underground fresh weight and Underground dry weight of potato as affected by variety and seed tuber.Means followed by the same letter within a column are not significantly different at the 5% level of significance.LSD =Least significant difference and CV = coefficient of variation.

Table 7 .
Marketable tuber yield (t ha-1), Small, Medium and Large size as influenced by varieties and seed tuber size.
Means followed by the same letter within a column are not significantly different at the 5% level of significance.LSD = Least significant difference and CV = coefficient of variation.

Table 8 .
The interaction effect of varieties and seed tuber size on dry matter concentration of potato varieties at Agarfa during 2017 cropping season.