Abstract
Onion (Allium cepa L.) is a vegetable crop that belongs to the genus Allium under the family Alliaceae produced widely throughout the world. Field experiments were undertaken to evaluate the different varieties and plant spacings on the growth and bulb yield performance of onion (Allium cepa L.) in Beyeda district, North Gondar, Ethiopia. Four different levels of spacing (6, 8, 10, and 12 cm) and four different types of varieties (Shallot, Nasik red, Robat, and Nafis) were arranged under randomized complete block design (RCBD) of factorial arrangement with three replications. The experiment was arranged in 4 × 4 factorial combinations, and the total area was divided into three blocks each of which comprised 16 treatment combinations and a total of 48 plots. The highest plant height (54.18 cm), leaf length (45.48 cm), and leaf diameter (1.47 cm) were recorded from the variety of Nasik red while the lowest plant height, leaf length, and leaf diameter were measured from shallot. The highest leaf number was observed from shallot (37.53) while the lowest leaf number was recorded from the Nafis variety. The longest bulb length (4.44 cm) and the widest bulb diameter (4.67 cm) were obtained from the Nafis variety while the shortest bulb length and narrowest bulb diameter were measured from shallot. Inversely, the highest bulb weight (54.50 g/plant), total biomass (94.75 g/plant), and unmarketable bulb yield (1.75 t/ha) were recorded from shallot while the lowest bulb weight, total biomass, and unmarketable bulb yield were recorded from the Robat variety. Intrarow spacing showed a significant effect on onion growth and bulb yield parameters, as the earliest (135 days) and the most delayed (146 days) maturity were recorded at 6 cm and at 12 cm spaces, respectively. The highest unmarketable bulb yield has been seen at 8 cm and the lowest at 12 cm space of the Nasik red variety. Similarly, the highest leaf number, leaf diameter, bulb diameter, bulb weight, total biomass, marketable, and total bulb yield were recorded at 12 cm space under the Nafis variety. Furthermore, under the shallot variety, the widest bulb diameter was recorded at 10 cm while the narrowest one was recorded at 6 cm space. Generally, based on the result of marketable bulb yield performance, Nafis variety at 12 cm and 8 cm spaces, Robat at 6 cm space, and Nafis red at 10 cm space, respectively, have effective performance for onion production. The interaction effects of intrarow spacing with different varieties did not have a significant effect on all of the growth parameters and yield components.
1. Introduction
1.1. Background of the Study
Onion (Allium cepa L.) crop belongs to the genus Allium under the family Alliaceae, which is one of the most important vegetable crops grown in the world commercially as well as at smallholder levels. It is one of the oldest bulb vegetables in continuous cultivation dating back to at least 4000 BC and probably, it originated from Central Asia between Turkmenistan and Afghanistan, South Asia, and Mediterranean region [1]. Most recent studies have reported that there are at least 750 species in the Allium genus. Onions are grown in around 170 countries of the world [2, 3].
The production of onion crop is worldwide because of its wide benefits in our daily food requirements, medicinal/health benefits that fight diseases such as cancer, asthmatic, heart, and diabetic diseases due to the presence of vitamins, minerals, and chemical compounds [4], and it is a source of income [5]. The onion bulbs can be harvested and can either be sold “green” to be used in salads [3], or the mature bulbs are cooked or eaten raw as a vegetable [6]. China is the top producer in the world followed by India and the USA, and countries such as Turkey, Iran, Afghanistan, Brazil, and South Korea are also largely producing and selling to international markets [7]. Egypt is the first country in Africa to produce a large amount of onion per year for home consumption and international markets and is ranked as the fourth world producer [8]. In Ethiopia, as many studies suggested, onion was introduced by foreigners in the early 1970s and is currently produced widely in many parts of the country [9]. Even though onion was a recently introduced bulb crop in Ethiopia, by now, it is rapidly becoming a popular vegetable [10].
Although some studies hypothesized that onion can grow in all types of soils, well-drained sandy loam soils with high fertility and plenty of organic matter (high nutrient availability) are preferable for higher yield. Onion is sensitive to highly acid soils and grows best when the pH is between 5.0 and 6.8 [11]. Related to geographical adaptability, the average altitude range for optimum production of onion is between 700 and 2200 meters above sea level (masl). Similarly, the optimum growth temperature requirement lies between 15°C and 23°C day and 10–12°C at night [12].
Despite the ever-rising demand for onions in all of the country, there were a number of production problems, which makes the farmers not to produce onions in advanced manner. Among those problems, the lack of improved cultivars adapted to different agroecologies of the country, and especially in the north Gondar, Beyeda district, the high cost of seed, inappropriate agronomic practices, and infestation of diseases take the lion’s share of low productivity [13]. Determining the optimum plant spacing for different varieties of onion crops should be helpful for onion growers to produce marketable size and good quality onion bulbs.
Traditionally grown shallots followed by the Bombay red onion variety are the only vegetables as well as spice crops of onion species that are produced poorly by small-scale farmers in the study area. Due to this, to meet the local demands for home consumption, large amounts of onions have been imported from west Gojjam and South Gondar zones [14]. Onion production frequently decreases and price increases due to different diseases and nonrecommended agronomic practices such as inappropriate plant spacing and fertilization management, lack of improved seed varieties, unavailability and high cost of seed, lack of well-organized markets, and shortage of water supply. According to the authors in [13, 14], lack of awareness to use appropriate agronomic practices such as planting spacing, varietal selection, and pest management caused to reduce the productivity of onions, especially in Beyeda district. In this regard, by selecting two factors from the abovementioned chronic problems of onion production, this study determined the optimum intrarow spacing and the best adaptable variety of onion species advisable to boost onion production in the district.
Therefore, the objective of this study was to identify the growth and bulb yield performance of improved onion varieties in different levels of plant spacing and so to select the highest yielding variety and optimum plant spacing in Beyeda District.
2. Materials and Methods
2.1. Description of the Study Area
The research was carried out at Watti kebele, i.e., Abaymesno irrigation site, in Amhara National Regional State (ANRS) north Gondar zone, Beyeda District (Figure 1). Beyeda is found in the northeast direction of ANRS. It is located at 13°6′58″ N latitude and 38°25′18″ E longitude with an altitude of 1800–4620 meters above sea level (masl) from which specifically the study site is 3102 masl. The annual temperature, rainfall, and humidity have been taken for about eight sequential years (2013–2020) data. The annual temperature of the district ranges from a minimum average of 8°C to a maximum average of 27°C. The temperature in the study sites ranges from a minimum average of 7.2°C to a maximum average of 19.5°C. The district’s mean annual rainfall ranges from 23.9 mm to 134.6 mm and humidity ranges from 25–83.5%. The mean annual rainfall and humidity of the study site range from 38.6 mm to 87.6 mm and from 50–57%, respectively (West Amhara Meteorology Service center). In general, Beyeda District is classified into Wurch (alpine), Dega (highland of its altitude), and Woyna-Dega (medium of its altitude) agroclimatic zone, and specifically, the study site is Dega. The district has plateaus, mountains, hills, plains, and valley in physiographical characterization and from which, specifically, the study site is plateau [14].
2.2. Experimental Materials
2.2.1. Onion Varieties
250 g of each improved seed variety of onions, i.e., Nasik red, Robat, and Nafis were obtained from onion seed producing center (Fogera national rice research and training center), and the local shallot was obtained from the farmers and used as control. The seeds were cleaned and made free from extraneous materials.
2.2.2. Fertilizer
NPS (N: P2O5 : S) and urea were available based on the recommended rate of 200 kg/ha NPS and 100 kg/ha of urea [15, 16].
2.2.3. Materials
Meter, balance, ruler, fence, watering bag/water tube, sack, stick, paper and cover plastic, grass, MUAC (mid-upper arm circumference) tape, and other materials were used.
2.3. Experimental Layout, Design, and Treatment
The field experiment was done on a total land of 168 m2 (24 m7 m) that was selected from the study site. The experimental land was plowed and harrowed frequently with oxen. The experiment consisted of three types of onion varieties (Nasik red, Robat, and Nafis) that were taken from Fogera National Rice Research and Training Center, and a local shallot which serves as control was collected from the farmers with four different levels of intrarow spacing (6, 8, 10, and 12 cm). This is based on [17, 18] where 10 cm is recommended as optimum standard among 5, 7.5, 10, and 12.5 cm intrarow spacing for onion production. Similarly, the authors in [19] recommend that 10 cm is optimum for bulb yield in garlic among 10, 15, and 20 cm intrarow spacing.
Hence, the experiment was arranged in 4 × 4 factorial combinations [20]. The treatments were laid down in a randomized complete block design (RCBD) with three replications (Figures 2 and 3). The total area was divided into three blocks each of which comprised 16 treatment (Table 1) combinations and a total of 48 plots. The size of each experimental plot was 1.1 m × 1.3 m (1.43 m2) with 20 cm height above the surface with six rows where each row contained 21, 16, 13, and 11 plants per row for 6, 8, 10, and 12 cm intrarow spaces (spacing between plants), respectively. In all treatments, interrow space (spacing between rows) was constantly 20 cm. In addition to this, the net plot area of each treatment was 0.58, 0.53, 0.48, and 0.43 m2 for 6, 8, 10, and 12 cm intrarow spacing, respectively. Growth and yield parameters of onion were collected from the central two rows of each net plot area to avoid border effects (Figure 3).
2.4. Experimental Procedures
Three different onion variety seeds were raised in well-prepared three seedbeds after this nursery land was plowed frequently. Each bed was constructed with 3 m × 2 m (6 m2) width and 20 cm height above the surface area as indicated by [15]. Seedling rows at a distance of 10 cm were prepared and seeds were sown at 2 to 3 cm distance from each other in seedbeds. Cultivation, weeding, and irrigation of seedlings in the nursery were done uniformly as described by [15]. Similarly, the plots have been covered by grasses to protect against solar radiation and maintain moisture [15]. Seedlings were ready for transplanting when they produced two pairs of true leaves and attained averagely the height of 12–15 cm which was taken about 58 days after sowing.
After the experimental field was plowed 3-4 times, the soil was categorized and laid out in rectangle shape and then plots/beds were made by maintaining 40 cm distance between them and 75 cm distance between blocks to facilitate cultural practices. The experimental beds were irrigated sufficiently before seedlings were transplanted to make the soil suitable for seedlings. After preparing the experimental site based on the layout, three varieties (Nasik red, Robat, and Nafis) of onion seedlings were transplanted to the experimental field. Before seven days of transplanting, the cloves of local shallot were sown into plots based on the experimental spacing. Seedlings were irrigated soon after transplanting. Hence, transplanted seedlings were irrigated by showering irrigation system in 3-4 days intervals at early growth stages, but after establishment, it was extended up to 6-7 days intervals [15].
120 g of NPS was applied for each plot uniformly as a source of phosphorous, nitrogen, and sulfur on the seedbed based on the recommendation rate of 200 k/ha, and also in the permanent field for each plot, 28.6 g of NPS have been incorporated into the soil before transplanting [16]. A total of 0.686 kg urea as a main source of nitrogen was applied into two equal splits. The first half was applied at the time of transplanting while the remaining half was applied 45 days after transplanting based on the recommendation rate of 100 k/ha [13, 16, 21]. Other cultural practices such as weeding, hoeing, and irrigation were performed uniformly for all treatments.
2.5. Soil Sample
To assess the fertility status of the experimental soil, samples were taken randomly from the experimental field at five spots at the depth of 0–20 cm before transplanting and then mixed to make a soil composite. Then, the chemical and physical properties of soil (organic matter, pH, N, P, and soil texture) were analyzed at Amhara Design and Supervision Works Enterprise Soil Laboratory.
2.6. Growth Parameters
At maturity, which means that before the leaf became yellow, dry, and collapsed or shrunk at the neck region, the measurement of growth parameters was performed (Figure 3) [17].
2.6.1. Plant Height (cm)
Plant heights of ten randomly selected plants grown in the net plot area were measured from the soil surface to the tip of the longest leaf using a ruler at physiological maturity, and the mean values were computed for further analysis.
2.6.2. Leaf Length (cm)
The longest leaves of ten randomly selected plants were measured from the point of their emergence to the tip of leaves using a ruler in centimeter (cm).
2.6.3. Leaf Number
The number of leaves of ten randomly selected plants from the middle rows of the net plot was counted.
2.6.4. Leaf Diameter (cm)
The mid-diameter of the widest leaves of ten randomly selected plants from the net plot was measured.
2.6.5. Days of Maturity
Days to maturity were the actual number of days from the transplanting to the time when 80% of plants’ foliage collapsed and became yellow as well as shrinked at the neck region.
2.7. Yield Components and Bulb Yield
2.7.1. Bulb Length (cm)
The length of matured bulbs of ten randomly selected plants in each plot was measured by using a ruler.
2.7.2. Bulb Diameter (cm)
The bulb diameter was measured at the widest circumstance of the bulb of ten randomly selected plants from each net plot by using MUAC measurement (Figure 3).
C = 2πr and D = 2r. Then, D = C/π = C/3.14 (π = 3.14).
C = circumference, D = diameter, and r = radius.
2.7.3. Average Bulb Weight (g)
The average bulb weights of ten randomly selected plants from the net plot were taken and calculated as the mean fresh bulb weight measured by sensitive balance after harvesting and used for further analysis [17].
2.7.4. Total Biomass (g)
The total biomasses of ten randomly selected plants were measured, including the aboveground biomass with bulb and so weighed at harvesting time and the average weight was computed and used for further analysis.
2.7.5. Marketable Bulb Yield (t/ha)
Marketable yield was categorized by weight into large (100–160 g), medium (50–100 g), and small (21–50 g) and finally calculated in t/ha. The total weight of clean, disease, and damage-free bulbs greater than 21 g in weight was considered as marketable bulb yield.
2.7.6. Unmarketable Bulb Yield (t/ha)
It was determined by bulbs smaller than 3 cm in diameter [20], split, rotten, damaged, and discolored bulbs, and undersized or oversized bulbs (<21 g or >160 g) were categorized as unmarketable.
2.7.7. Total Bulb Yield (t/ha)
The yield that includes both marketable and unmarketable bulb weight was expressed as kg per plot and converted into t ha−1 (Figure 3).
2.8. Data Analysis
The collected experimental data, displayed as mean and standard deviation (Table 2), were statistically analyzed by using Statistical Analysis System (SAS) version 9.1 for the analysis of variance (ANOVA). If the analysis of variance shows a statistically significant difference in growth and yield parameters, mean separation was carried out using 1% or 5% level of significance [22].
3. Results and Discussions
3.1. Soil Analysis: Physicochemical Properties of the Experimental Soil
The experimental soil had a high percentage of clay, which might disagree with [23] where it was stated that for higher yield, well-drained friable sandy loam soils are preferable. The mean pH value is relatively acidic (5.91), and this soil pH was relatively suitable for the production of onion based on the studies showed that the optimum pH for onion production ranges between 6 and 8 [24] and on the other study results, onion grows best when the pH is between 5.0 and 6.8 [11]. The organic carbon content (0.49%), as well as total nitrogen content (0.05%) of the soil, are categorized as the lower level which has its own limitation in onion crop growth while the soil has medium available phosphorus content as listed in the following (Table 3).
3.2. Growth Parameters and Yield Components of Onion
3.2.1. Vegetative Growth Parameters
Most of the growth parameters (plant height, leaf number, leaf length, and leaf diameter) were significantly different among the varieties ( < 0.001). The most significant difference of plant height performance was between shallot (36.12 cm) and the varieties such as Nasik red (54.18 cm), Nafis (51.17 cm), and Robat (48.84 cm), respectively. This result was similar to the finding of [25, 26] where it was indicated significant differences among onion varieties in plant height. There was a highly significant difference ( < 0.001) between the highest number of leaf in shallot (37.53) and the lowest number in Naffis (8.32). However, there was no significant difference between Nasik red (8.73), Robat (8.34), and Nafis (8.32) varieties (Table 4).
Similarly, the lengths and diameters of onion leaves showed a highly significant difference ( < 0.001) between a shallot and the rest three varieties of onion (Table 4). The longest and the widest mean (45.48 cm, 1.47 cm) of leaves, respectively, were obtained from Nasik red variety while the shortest length (28.84 cm) and the narrowest diameter (0.69 cm) leaves were observed from the control variety shallot. In addition, there was a significant difference ( < 0.05) in both leaf length and diameter among Nasik red (45.48, 1.47), Nafis (42.50, 1.32), and Robat (40.43, 1.30), respectively. Therefore, this result showed that the difference in varieties in leaf length and diameter might be due to their differences in genetic makeup or the environment where it has been grown as stated in [27]. There was no significant difference ( > 0.05) in days of maturity among the onion varieties (Table 4). This result disagrees with the observation of [28] who reported that there were significant differences among eight onion varieties in days of bulb maturity.
Related to intrarow spacing, most of the growth parameters not showed a significant difference ( > 0.05) but showed a numerical difference. Nasik red variety at 6 cm intrarow spacing was matured at approximately 135 days, significantly earlier than other treatments. Onions of intrarow spacing at 8 cm matured at 141 days, while those at 10 cm and 12 cm intrarow spacing matured at approximately 146 days (Table 4). This result is consistent with the finding in [23] where it was reported that maturity in response to the increased rate of nitrogen application and wider intrarow spacing might be attributed to enhancing plant biochemical processes, which in turn extends vegetative growth and delay maturity as well.
Similarly, under Nafis variety of four levels of intrarow spacing, there was a slightly significant difference ( < 0.05) in leaf diameter. The highest width/diameter of leaves was recorded in 12 cm spacing (1.45 cm) while the lowest width of leaves was recorded in a space of 6 cm (1.23 cm). This result was inversely related to [19] where it was noted that increased leaf width was observed at narrower row spacing in garlic. Other treatments such as plant height and leaf length were not significantly different with different plant spacings. On the contrary, the experiment in [19] resulted in a larger plant height as the plant spacing gets narrower in garlic. The highest number of leaves was counted in space at 12 cm (9.13/individual), while the lowest number of leaves was counted in space at 10 cm (7.37/individual). In general, even though there was a numerical difference, there was no significant difference in the interaction effect (varieties with spacing) among the whole tested growth parameters.
3.2.2. Yield and Yield Components of Onion
(1) Bulb Length and Diameter. There were significant differences among varieties ( < 0.05) in onion bulb length and diameter, even though intrarow spacing and their interaction with varieties did not make significant differences in bulb length (Table 5). The highest bulb length and the widest bulb diameter were observed from Nafis (4.44 cm, 4.67 cm), followed by Nasik red (4.41 cm, 4.51 cm) and Robat (4.13 cm, 4.38 cm) varieties, respectively. This result might be due to either genetic or environmental effects as studied by [27]. Similarly, it was related to [18] where it was stated that the production of quality seed yield of onion depends on the genotype, locality, season, and method of seed production that have a vital role in crop yield production.
Related to plant spacing for shallot, the highest bulb diameter was 2.63 cm at 10 cm intrarow spacing, followed by 2.28 cm at 12 cm spacing and 2.13 cm at 8 cm spacing while a significantly minimum bulb diameter was obtained from the closer spacing of 6 cm. The widest bulb diameter of Nafis was 5.33 cm at the intrarow spacing of 12 cm, followed by 8 cm intrarow spacing, while the minimum bulb diameter was 4.25 cm at 6 cm intrarow spacing (Table 5). This indicates that the closer intrarow spacing could decrease bulb diameter Reference [29], reported that there was a significant increase in the diameter of onion bulbs as the plant population decreased.
(2) Bulb Weight and Total Biomass. As the analysis of variance showed that both mean bulb weight and total biomass were highly affected ( < 0.001) by varieties and less significantly influenced ( < 0.05) by intrarow spacing (Table 5). The highest bulb weight was 54.50 g/plant and the total biomass was 94.75 g/plant for shallot. On the other hand, the least mean bulb weight was 39.25 g/plant and the total biomass was 74.67 g/plant for Robat (Table 5).
Regarding intrarow spacing for Nafis variety, the greatest bulb weight was 52.67 g/plant and total biomass was 104.33 g/plant at12 cm spacing, followed by 45.00 g/plant, 78.33 g/plant, respectively) at 8 cm spacing (Table 5). This result showed that when plant spacing increased from 6 cm to 12 cm, similarly bulb weight and total biomass also increased. This might be due to the fact that closer spacing between plants resulted in competition for nutrients, water and light, thus reducing the amount of assimilating stored in the bulbs, which reduced their bulb weight and total biomass. This result is similar to [30] who reported that plants spaced at the narrowest gave the lowest bulb weight for a single onion bulb.
(3) Marketable, Unmarketable, and Total Bulb Yield. Marketable, unmarketable, and total bulb yield of onion were not significantly affected by both intrarow spacing and their interaction effect (Table 5). Marketable and total bulb yield were not significantly affected by different varieties.
There were significant differences in unmarketable bulb yield among the varieties. The highest unmarketable bulb yield (1.75 t/ha) was obtained from the control variety shallot followed by Nasik red (1.19 t/ha) and Nafis (1.12 t/ha), while the least values were recorded from Robat with (0.79 t/ha). This result might be due to genetic trait differences that resulted in, split, rotten, damaged, discolored bulbs or a few large-sized or small-sized bulbs classified as unmarketable bulbs.
There was a significant difference in marketable and total bulb yield among four levels of intrarow spacing for Nafis variety (Table 5). The highest marketable and total bulb yield was 16.8 t/ha and 17.6 t/ha, respectively, at 12 cm spacing, followed by 15.87 t/ha and 17.20 t/ha, respectively, at 8 cm spacing.
4. Conclusion and Recommendation
The different intrarow spacings and varieties had different significant impacts on the onion yield. Newly tested varieties such as Nafis (12 and 8 cm), Robat (6 cm), and Nasik red (10 cm) varieties were the best varieties with optimum plant intrarow spacing for the production of greater marketable bulb yields than local shallot.
The major constraints of onion production in the study area are the absence of reliable and well-adaptable improved seed supply. The local shallot produced by farmers is poor in its marketable productivity through irrigation under varieties of intrarow spacing. Hence, the growers/agricultural office should purchase seed varieties such as Nafis, Robat, and Nasik from well-known improved seed producers as well as certified sellers to cultivate in the district. The frequency and amount of irrigation should be based on the requirement levels of the plants. Related to yield and yield components, the variety Nafis was the best variety in the way of producing the highest bulb length and diameter, while the control variety shallot produced unmarketable bulb yield.
It can be recommended that Nafis, Robat, and Nasik red at 8–12 cm, 6 cm, and 10 cm intrarow spacing, respectively, are the best option for optimum onion production in Beyeda district, North Gondar, Ethiopia [31].
Data Availability
The data sets used and/or analyzed in the study are available from the text.
Conflicts of Interest
The authors declare that they have no competing interests.
Authors’ Contributions
Wubetie Adnew Wassie conceptualized the study, collected and analyzed the data, and administered the study. Gashaw Fenta Assegahegn conceptualized the study, curated and analyzed the data, and administered the study. Berhanu Abraha Tsegaye curated and analyzed the data, and investigated the study. Amare Bitew Mekonnen conceptualized the study, collected and analyzed the data, and investigated the study. All the authors read and approved the final manuscript.
Acknowledgments
This work was supported by Bahir Dar University College of Science Research office (grant nos. 00112 and 2020).