Effect of inter and intra row spacing on the yield and yield components of Onion ( Allium cepa L . ) at Mehoni District , Northern Ethiopia

Onion is one of the most important high market value bulb crops cultivated commercially in most parts of the world. It is the most cultivated and high market value of vegetable crops In Tigray Northern Ethiopia. However, bulb yield of onion is limited due to improper use of plant spacing. Therefore, field experiment was conducted to determine the optimum planting spacing of Onion to maximize its productivity in 2014 and 2015 under irrigation conditions. The trial contains treatment of singe planting pattern of 20 x 10, 40 x 10, 30 x 5, 30 x 7.5, 30 x 10 and 30 x 12.5 cm inter and intra spacing respectively and double planting pattern combinations of four intra plant spacing (5, 7.5,10 and 12.5 cm) and two levels of inter spacing (40 and 50 cm) by means of total fourteen treatments were carried out in Randomized Complete Block Design (RCBD) with three replications. The current findings showed that plant height, bulb size, bulb weight, total bulb yield and marketable bulb yield were significantly influenced by planting spacing. Accordingly, the tallest plant height (37.70 cm) was obtained at a spacing of single planting of 40 cm inter row and 10 cm intra row while the maximum total bulb yield of 27.01 ton ha -1 ) was recorded at a spacing of double planting pattern of 40 cm inter and 5 cm intra row in both years. Therefore, 40 cm inter row and 5 cm intra row spacing in double row planting manner is recommended for the growers to improve onion productivity in the study area.


INTRODUCTION
Onion (Allium cepa L.) belongs to the genus Allium of the family Alliaceae (Hanelt, 1990;Griffiths et al., 2002).Onion is the most important bulb crops cultivated commercially in most parts of the world.The crop is grown for consumption both in the green state as well as in mature bulbs.Onions exhibit particular diversity in the *Corresponding author.E-mail: gebremeskel12@gmail.com.
Author(s) agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License eastern Mediterranean countries, through Turkmenistan, Tajikistan to Pakistan and India, which are the most important sources of genetic diversity and believed to be center of origin (Brewster, 2008).Onion is one of the most popular and the most cultivated vegetables in Ethiopia in general and in Tigray region in particular (Hailu et al., 2015).So far, research in the country was mainly focused on the identification of superior cultivars of onions and adopting improved management practices.Different cultural practices and growing environments are known to influence yield and quality of dry bulbs.Spacing has effect on different varieties as their root and leaf growth habits differ (Yemane et al., 2013).
Onion is considerably important in the daily Ethiopian diet, mostly used as seasonings or as vegetables in stews (MoARD, 2009).It is one of the richest sources of flavonoids in the human diet and flavonoid consumption has been associated with a reduced risk of cancer, heart disease and diabetes.In addition, it is known for anti bacterial, antiviral, anti-allergenic and anti-inflammatory potential.Onions are spread throughout the country being cultivated under both irrigated as well as rain fed conditions in different agro-climatic regions due considerably increasing its important in the daily Ethiopian diet.Onions are spread throughout the country being cultivated under both irrigated as well as rain fed conditions in different agro-climatic regions due considerably increasing its important in the daily Ethiopian diet (Lemma and Herath, 1992) According to Geremew et al. (2010), yield and quality of dry bulbs can be influenced by cultural practices and growing environments.So far, research in the country was mainly focused on the identification of superior cultivars of onions and adopting improved management practices for better yield.The control of plant spacing is one of the cultural practices to control bulb size, shape and yield.The higher yield and better control of over or under bulb size could be obtained if plants are grown at optimum density.Bulb neck diameter, mean bulb weight and plant height decreased as population density increased.Total bulb yield can be increased as population density increases (Kantona et al., 2003).
Although the focus of research in the country was mainly on the identification of superior cultivars of onions and adopting, yield and quality of dry bulbs influenced by different cultural practices and growing environments.Spacing has effect on different varieties as their root and leaf growth habits differ (Yemane et al., 2013).
One of the major problems in onion production in the area is lack of proper agronomic practice and improved variety 10 cm spacing between plants is the nationally recommendation of onion, in central rift valley condition of the country some years back.Nevertheless, farmers did not adopt the recommendation in the real situation; they practice it a bit narrower or wider spacing.In Ethiopia, the crop is more intensively consumed than any other vegetable crops produced in the country.Despite that,Gebrekorkos et al. 87 the productivity of the crop remains with the average productivity of (10.02 tonha -1 which is very low compared to world average yield of 19.7 tonha -1 (Negasi, 2014).Farmers and investors have been producing onion widely in Raya Azebo and Raya Alamata districts.Current market preference on bulbs size and variety of onion necessitate an evaluation of the response of yield and bulb size to plant populations.Results obtained from these evaluations can aid growers to decide the optimum spacing of onion.However, a research has not been conducted on improved agronomic practices like plant spacing to improve the productivity of onion in the study area.Therefore, the present study was conducted to investigate the effects of inter intra and row spacing for the productivity of onion in the study area.
Appropriate spacing enables the farmers to keep appropriate plant population in the field.Therefore, it can avoid excess or less population in a given plot of land which has negative result on yield and quality of onion.Spacing of 40 × 20 × 10 cm between furrow, row and plants, respectively has been used for onion production in Ethiopia.But producers complain 10 cm intra row spacing produces large bulb size which is not preferred by consumer for home consumption.(Habtamu et al., 2016).The present study was therefore undertaken to investigate the effects of different intra-row and inter row spacing for onion productivity in the study area.

Description of the study site
The experiment was carried out at Mehoni Agricultural Research Center testing site in 2014 and 2015 under irrigation conditions .The site is located 12°41'50" N and longitudes of 39°42'08" E. It is 678 km north of Addis Ababa.The area is situated at an altitude of 1578 m above sea level (m.a.s.l) with mean annual rainfall of 750 mm and minimum and maximum annual temperature of 18 and 25°C, respectively.The soil is clay-loam texture f with a pH value of 7.9 at the soil depth of 0-30 cm (Haileslassie et al., 2015).

Experimental materials, treatment and design
The trial contains treatment of singe planting pattern of 20 x 10, 40 x 10, 30 x 5, 30 x 7.5, 30 x 10 and 30 x 12.5 cm inter and intra spacing respectively and double planting pattern combinations of four intra plant spacing (5, 7.5, 10 and 12.5 cm) and two levels of inter spacing (40 and 50 cm) by means of total fourteen treatments were carried out in Randomized Complete Block Design (RCBD).The experiment contains three replications having a plot size of 2.4m x 2 m for each experimental unit.Bombay Red variety was used as test crop for the experiment to all treatments.The treatment (T) combination (Table 1) comprised:

Experimental procedure
UREA and DAP were used as the sources of Nitrogen and Phosphorous respectively.46kg P2O5 ha -1 was applied in band application method at transplanting time.Similarly, 23 kg ha -1 of N was applied in two splits 20 days after transplanting and the 30

Single planting pattern
Double planting pattern T1.Flat planting of 20x10 cm T7. 40 x5 cm T2.40x10 cm T8. 40 x7.5 cm T3.30x5 cm T9. 40 x10 cm T4. 30x7.5 cm T10.40 x12.5 cm T5.30x10 cm T11.50 x5 cm T6. 30x12.5 cm T12.50 x7.5 cm T13.50 x10 cm T14.50x12.5 cm days after the first nitrogen applied.Onion was transplanting when 3-4 true leaves emerged that was in 55 days from the time of sowing.The transplanting time was in the morning in order to decrease the shocking of the plant.Ridomil was applied as a protective spray before the outbreak of the fungal disease at nursery and after transplanting in the mid stage.Other cultural management practices were done according to the national recommendation for all experimental units.

Data collection and analysis
Data collection: Important morphological, phenological and yield components of onion was collected.

Morphological and phenological traits
Plant height (cm): Five plants from the net plot area were pre tagged to collect data of plant height.It was measured using ruler from the soil surface up to the tip the leaves at bulb development stage.

Days to maturity (days):
The actual number of days from seedling transplanting to the field to a day at which of the plants in a plot showing yellowing of leaves was recorded to determine the days to physiological maturity.

Bulb yield and yield components
Average bulb weight (g): Five plants from the net plot area were pre tagged to collect data of average bulb weight and expressed in gram.
Bulb size (cm): Five plants from the net plot area were pre tagged to collect data of average bulb size and expressed in centimeter.
Marketable bulb yield (MBY) (t ha -1 ): Total weight of clean, disease and damage free bulbs were measured per net plot and converted to t ha -1 .
Unmarketable bulb yield (MBY) (t ha -1 ): Total weight of decay, physiological disorder such as thick necked, split and bolters were measured per net plot and converted to t ha -1 .
Total bulb yield (TBY) (t ha -1 ): Total weight of marketable and unmarketable bulbs were measured per net plot and converted to t ha -1 .
The data were collected from middle rows of a net plot area where the two outer most rows of each treatment were left as border effects.In addition, 0.05, 0.075, 0.1 and 0.12 m length in both ends for 5, 7.5,10 and 12 cm intra row spacing, respectively, of each harvestable row were also left as border effects.

Statistical analysis
All data were subjected to the analysis of variance(ANOVA)using the SAS (9.1)software computer package (SAS Institute, 2004) and significance difference among the treatment means was computed with Duncan's Multiple Range Test(DMRT) at 5% probability level (Gomez and Gomez, 1984).

Plant height
Plant height was significantly affected by planting space in both years.Tallest plant height of 37.70 cm was recorded at single planting spacing of 40 x 10 cm in 2014 where as the shortest plant height of 28.03 cm was recorded at double spacing of 40 x 5 cm.In 2015 irrigation season, the tallest plant height of 53.67 cm) was also recorded at single spacing of 40 x 10 cm while the shortest (40.33 cm) was obtained at double spacing of 40 x 5 cm (Table 2).The greatest plant height may be due to the availability of free access of environmental resources (water, nutrient and light) for the plants in the wider plants.In line with this result, Tesfalegn (2015) reported that plant height of onion plants was significantly affected by intra spacing of cultivars.The highest plant height was obtained at 10 cm and the lowest plant height at 4 cm.Similarly, Jilani et al. (2009) and Sikder et al. (2010) also reported the closer intra-row spacing resulted short plant height than wider plant spacing.

Days to 90% physiological maturity
Days to 90% physiological maturity showed no significant (P>0.05)variation to planting spacing (Table 1).This might be due to the less computation of plants for growth resources.Generally, it matured at a range of 82.33 to 88.00 starting from its transplanting time (Table 1).

Mean bulb size
There was significance difference (P<0.5)among the treatments of spacing on bulb size of onion.The highest bulb size was recorded at the spacing of of 50 x 12.5 cm followed by 50 x 10 cm in double planting manner.The smallest bulb size was also recorded at double planting of 40 x 5 cm in both years (Table 3).In agreement with the present result, Yemane et al. (2013) reported that as the intra-row spacing increased from 5 to 10 cm, the percentage of large size bulbs increased from 9.3 to 20.3.Similarilly, Jilani et al. ( 2009) reported that significant difference with the plant spacing appeared bigger at the wider spacing.

Average bulb weight
As indicated in Table 2 spacing did not significance affected the bulb weight of onion in the year 2014 but significantly influenced by plant spacing in the year 2015.Accordingly, the highest average bulb weight (87.90 g) was produced at double spacing of 50 x 10 cm, while the lowest average bulb weight (72.12 g) was observed in single planting of 30 x 7.5 cm.similarly Yemane et al. (2013) founded that the highest average bulb weight increase as the interspacing increases from 5 cm to 10 cm.This result also confirms Aliyu et al. (2008) who noticed that densely populated plants produced lower bulb weight as compared to thinly populated plants.Similarly, Jilani et al. (2009) also reported that the highest fresh bulb weight was observed at the wider intra spacing.

Total bulb yield and marketable bulb yield
Effect of the intra and inter spacing showed significance (P< 0.05) difference on the bulb yield of onion in both years (Table 3).Accordingly, the highest total bulb yield of 21.28 ton ha -1 and marketable bulb yield of 20.60 ton ha -1 was recorded at the spacing of 40 x 5 cm followed by single planting 20 x 10 cm (20.99 ton ha -1 ) for total bulb yield and 40 x 7.5 cm (19.69 ton ha -1 ) for the marketable bulb yield in 2014 in double planting manner (Table 4).
Bulb onion produced with a spacing of 40 x 5 cm was increased by 45.25% as compared to bulb yield produced with spacing of 30 x 12.5 cm (14.65 ton ha -1 ).Similarly, the highest total bulb yield and marketable bulb yield 32.92 and 31.30ton ha -1 was recorded at the spacing of 40 x 5 cm in 2015 with double planting manner.Minimum total bulb yield (18.68) was obtained from double planting of 50 x 12.5 cm in 2015.The highest total bulb yield gave 76.23% yield advantage over the lowest ones.The lowest total bulb yield production would be due to extreme wider spacing at which the required population ha -1 could not be accommodated; and this result in low production of total bulb yield because of low population density.Moreover, the pooled mean result indicated that double planting inters and intra spacing of 40 and 5 cm, respectively gave high yield of onion in both the total and marketable bulb yield.The current result was found consistent with Jan et al. ( 2003) reported the highest total bulb yield (549.90 kg ha -1 ) was obtained at a closer spacing, whereas the lowest total bulb yield was recorded from a wider spacing.Similarly, Yemane et al. (2013) reported that as intra-row spacing increased from 5 to 10 cm, total bulb yield in tons ha -1 decreased.Significantly, the highest total bulb yields of 36.14 and 33.82 tons ha -1 were recorded at 5 and 7.5 cm intra-row spacing, respectively.An intra-row spacing of 10 cm showed the lowest total bulb yield (28.51 tons ha -1 ).This finding was in agreement with similar study of Habtamu et al. (2016) concludes that yield increases significantly as population density increases.

Unmarketable yield
Significance difference was observed on unmarketable yield (P<0.05) by the inter and intra row spacing in both 2014 and 2015 irrigation season.The highest unmarketable yield (2.25 ton ha -1 ) was recorded at 50 x 5 cm in double planting manner in 2014 cropping season.Whereas the lowest unmarketable yield (0.87 ton ha -1 ) was recorded at 30 x 12.5 cm in single planting manner.In 2015 cropping season the maximum unmarketable bulb yield (2.72 ton ha -1 ) was observed at the flat bed of 20 x 10 cm.Moreover, the pooled mean result indicated that the highest unmarketable yield was observed at flat planting of 20 x 10 cm inter and intra row spacing (Table 3).This is for the reason that flat bed planting resulted in decayed bulbs and large sized bulbs which was unwanted in the local market.
This finding was contrast with the result of other workers (Yemane et al., 2013;Habtamu et al., 2016) who concluded that plant density has an impact on unmarketable bulb size.

Conclusions
Optimum plant spacing has a promising impact in increasing the productivity and production of Onion.The findings of the present experiment showed that plant height, bulb size, total bulb yield and above ground dry biomass yield were significantly affected by inter and intra spacing of Onion.The tallest plant height was obtained at spacing of single planting 40 x 10 cm.The maximum total bulb yield and above ground dry biomass yield were obtained at the double planting of 40 x 5 cm in both years as compared to the other treatments.It is, therefore, concluded that spacing of double planting 40 x 5 cm can be recommended for the growers in the study area as the total bulb and marketable bulb was high compare to the other treatments to increase Onion productivity.Moreover further investigations need on different varieties, soil types, quality aspects together with other agronomic management practices to identify best technology on productivity and production of onion.

Table 2 .
Mean number of days required for physiological maturity and plant height of onion under Irrigation.
Means followed by the same letter (s)in the same column are not significantly different at 5% probability level according to Duncan's Multiple Range Test; NS= non-significant, *=significance at 5%.

Table 3 .
Mean bulb size and bulb weight for onion as influenced by plant spacing under irrigation.

Table 4 .
Mean total bulb yield, marketable bulb yield and un marketable bulb yield for Onion as affected by plant spacing under Irrigation.Means with the same letter (s) in the same column are not significantly different; *= Significant at 5% probability level; **= significant at 1% probability level; NS= non-significant.Sin planting =single planting pattern, dou planting = double planting manner.