Morphogenetic Studies of Some Genotypes of Onion (Allium cepa L.) In Jos, Nigeria

The improvement of the fresh bulb and the economic dry matter yields of onion is dependent on the morphogenetic variability and the interrelationships amongst morphological attributes which can be used to determine the quality or performance of the crop. This research was aimed to study the morphogenetic characteristics of ten genotypes of onion (Ares, Violet de Galmi, Red Creole, “Wase”, “Dan Zaria”, “Dan Garko”, “Dan Giyawa”, “Bahaushe”, “Bakana” and “Yar Aleiro”) in Jos, Plateau State. Using the seedlings raised in the nursery, the genotypes were laid out in a completely randomized design (CRD) with four replicates. Results showed that morphological attributes differed amongst the genotypes, except the number of leaves per plant, neck thickness and horizontal bulb diameter. The study concludes that morphological differences, which are responsible for differences in fresh bulb and dry matter yields, exist amongst the different genotypes of onion. These attributes could be used as selection indices in the improvement of the onion.


Introduction
Onion (Allium cepa L.) belongs to the family Alliaceae. It originated in Central Asia, but can be found all over the world today. Onion is ranked second after tomato among vegetables in terms of consumption. The total world production of onions in 2013 was 4,281,501 tons, out of which 648,247 tons was produced in Africa; 267,164 tons from West Africa and 235,000 tons in Nigeria. These tonnages were obtained from 230,180 ha, 46,469 ha, 16,221 ha and 14,000 ha with average yields of 18,600.8 kg/ha, 13,950.1 kg/ha, 16,470.3 kg/ha and 16,785.7 kg/ha, from the world, Africa, West Africa and Nigeria, respectively (FAOSTAT, 2013).
Crop varieties perform differently under different agro-climatic conditions and various varieties of the same species grown even in the same environment give different yields as the performance of a cultivar mainly depends on the interaction of genetic make-up and environment (Jilani & Ghafoor, 2003). Like many other crops, the productivity of onion depends on growth attributes such as leaf area, leaf area index and crop growth rate (Abou El-Magd, 2013). Growth analysis is a useful technique in the study of plants, because it bridges the gap between the empirical and the mechanistic approaches to modeling growth and development, which is a necessary prerequisite for the understanding of the morphology of a plant (Abou El-Magd, 2013). Ijoyah, Rakotomavo and Naikem (2008) observed that some varieties of onion performed better than the commonly grown ones in Seychelles. Morphological studies are basically used to determine the quality and performance of a crop plant. Therefore, in order to enhance the commercial production of onion to meet increasing demand, an understanding of the onion morphology may be used to establish the distinctiveness of the individual genotypes. The objective of this study was to determine the variation in the morphological characteristics of some genotypes of onion grown in Jos, Plateau State.

Raising of Seedlings in the Nursery
The seedlings of the genotypes were raised in the Green House of the Federal College of Forestry, Jos, Plateau State (Latitude 09 0 57"N, Longitude 08 0 53"E, Altitude 1,159 m above sea level) Topsoil, sharp sand and organic manure (cow dung) in the ratio of 2:1:1 were thoroughly mixed and set in fifteen (10) plastic trays which measured 70 cm in diameter, 23 cm in height and 23 cm in length. Each tray was labeled according to each genotype. Seeds of each genotype were broadcast into each tray and mulched for 7-10 days at 3-5 cm thickness of dry grass (which was removed after germination was attained at 60%). The seedlings were transplanted after eight weeks. The genotypes were laid out in a completely randomized design (CRD) with four replicates. The agronomic characteristics of the genotypes used are shown in Table 1

Preparation of Potting Mixture for Transplanting Seedlings
Topsoil was collected from a fallow field and the organic manure (cow dung) used was collected from animal farm in Jos (popularly known as Yan shanu, Nasarawa Gwong, Jos). The topsoil was mixed with the organic manure in a potting ratio of 3:1. The soil mixtures were prepared by initial irrigation to provide adequate moisture, and was then analyzed for chemical and physical properties at the Agricultural Services, Training and Marketing Centre (ASTC), Kassa, Plateau State. The result of the physico-chemical properties of the soil used for the experiment is shown in Table 2. The mixtures were placed in polyethylene pots, which measured 23 cm in diameter and 23 cm in length. Each pot was perforated at the bottom to ensure proper drainage. The pots were arranged in a completely randomized design with four replicates. Transplanting was done on August 21, 2016. Seedlings were transplanted at a depth of 1-4 cm (Olani & Fikre, 2010). Two pots represented a plot and three seedlings were planted in each pot. The pots were kept in the sun as recommended by McGee (2004) and Hanelt (1990). The experiment was rain-fed and weeding was carried out weekly.

Field Observations and Data Collection
Field observations and data collection were carried out fortnightly after transplanting.

Plant Height
Plant height was measured using the metre rule from the soil surface to the top of the longest leaf.

Number of Leaves Per Plant
The total number of outer leaves in each plant was counted and recorded.

Neck Diameter
Neck diameter (thickness) was measured at the narrowest point using the Vernier caliper.

Fresh Plant Weight
Fresh plant weight (including leaves, bulbs and roots) was measured using a digital weighing balance (Laboratory Analytical Balance 6354 model).

Horizontal and Vertical Bulb Diameter
Horizontal and vertical bulb diameters (that is the height of the bulb and the average width at the widest point in the middle portion of the mature bulb) were measured using the Vernier caliper.

Root Length
The length of the longest root was measured, using a measuring tape.

Bulb Shape Type and Shape Index
Bulb shape types were identified visually by comparing the shape of the bulbs with the description by previous researchers and the use of shape index. Shape index was computed as the ratio of horizontal diameter to the vertical diameter of the bulb.

Fresh and Dry Weight Of Bulbs
Fresh bulbs were weighed using a digital weighing balance (Laboratory Analytical Balance 6354 model) after uprooting and washing the whole plants. The bulbs were dried for 72 hours at 80±2°C and thereafter weighed using the digital weighing balance (Razia, Rafiq, Jayanti, Vandria, Sheikh & Koul, 2014). Four plants were sampled from each plot.

Results and Discussion
The significant differences observed in most of the attributes measured indicated the existence of variability among the genotypes used in this study. Alan, Mutschler, Brants, Cobb and Earl (2003) also reported genetic variability amongst genotypes of onion. Table 3 shows plant height of some genotypes of onion grown in Jos in 2016. The genotype "Bakana" had the highest plant height of 49.00 cm, which differed significantly (P<0.05) from the genotype "Dan Zaria" which was observed to be the shortest (18.88 cm). Plant height varied with genotype in this study. Mohanty, Hossain and Prusti (2002) observed the interaction of genotype and environment on the expression of plant height, which they noted was quantitatively inherited. Ibrahim (2010) as well as Trivedi and Dhumal (2010) also observed differences in plant height amongst onion genotypes.

Neck Thickness
The neck thickness ranged from 1.80 cm in the variety "Dan Zaria" to 4.38 cm in the genotypes "Bahaushe" and "Yar Aleiro" but the difference was not significant (Table 2). Whereas Ishwori, Gautam and Govinda (2016) observed no difference in neck thickness, Hirave, Wagh, Alekar and Kharde (2015) reported significant differences in neck thickness amongst the genotypes they studied as those with narrow neck thickness stored longer than those with wide neck thickness. Neck thickness is believed to influence the storability of onion.

Number of Leaves per Plant
The highest mean number of leaves per plant (10.5) was observed in the genotype "Bahaushe", while the lowest was observed in Violet de Galmi (4.75 cm). Differences in the mean number of leaves per plant among the genotypes were not significant (Table 2). Unlike plant height, the number of leaves per plant did not vary with genotype, which contradicts the reports of Boukary, Haougui, Barage, Adam, Roumba and Saadou (2012) and Dwivedi, Kushwah and Sen-Gupta (2012). It has been observed that the number of leaves in onion is also influenced by the environment (Ijoyah et al., 2008). Table 4 shows the effect of genotype on fresh plant weight of some genotypes of onion grown in Jos in 2016. The highest fresh plant weight (115.0 g) was observed in the genotype Violet de Galmi which differed significantly from the genotype "Dan Zaria" (62.5 g). The genotypes "Wase", "Dan Zaria" and "Dan Giyawa" did not differ significantly in fresh plant weight, with fresh plant weights of 75.8 g, 62.5 g and 69.5 g, respectively. Fresh plant weight differed with genotype in this study, being highest in the genotype Violet de Galmi and lowest in the genotype "Dan Zaria". The result indicated that this attribute is genotypically controlled. The attribute is believed to define the general performance of each genotype before bulb, root and leaf separation. It is used as a major index of fresh bulb yield in the onion.

Vertical Bulb Diameter
The highest vertical bulb diameter was observed in the genotype Violet de Galmi (6.6 cm), but this did not differ significantly from genotypes Ares (4.65 cm) and "Wase" (5.74 cm). The lowest vertical bulb diameter (3.35 cm) was observed in the genotype "Bakana" (Table 4). Vertical bulb diameter differed significantly with genotype in this study, contrary to the result obtained by Ishwori et al. (2016), who observed that onion bulbs with high vertical bulb diameter stored longer than those with low vertical bulb diameter.

Horizontal Bulb Diameter
The horizontal bulb diameter varied from 3.81 cm in the genotype "Wase" to 6.60 cm in the genotype Violet de Galmi (Table 4). The differences in the horizontal bulb diameter were not significant. Onion genotypes with high horizontal bulb diameter are known to have large bulbs and small neck diameter, attributes which are believed to make them not to store well (Ishwori et al., 2016). Bulb diameter contributes to bulb size and shape index, both of which differed with genotype in this study.

Fresh and Dry Bulb Weights
The highest fresh bulb weight of 76.7 g was observed in the genotype Red Creole, which differed significantly from genotypes "Wase" (46.3 g), "Bahaushe", "Bakana" and "Yar Aleiro" with a fresh bulb weight of 51.3 g ( Table  4).The highest dry bulb weight of 11.60 g was observed in the genotype "Dan Garko" followed by genotypes "Dan Giyawa" (10.8 g), Red Creole (8.2 g), "Bahaushe" (6.7 g) and Violet de Galmi (6.0 g). The lowest dry bulb weight of 4.1 g was observed in the genotype "Wase". Like the fresh bulb weight, the dry bulb weight also differed with genotype. The genotypes with higher dry bulb weight also had higher total fresh bulb yield, implying that dry bulb weight is a major determinant of fresh bulb yield in the onion. Dry matter content has been reported to contribute largely to the firmness and tunic thickness of "Taherpuri", a local onion variety in India. Dry matter content is also believed to influence long storage period in the onion in India (Best, 2000;Mahanthesh, Ravi, Harshav, Vishnuvardhana & Janardhan, 2008) as well as in Nigeria (Kabura, Musa & Odo, 2008;Ibrahim, 2010). Genotypes with high dry matter have longer shelf-life and are recommended for industrial processing.

Shape Type
The genotypes Violet de Galmi, Red Creole, "Wase", "Bahaushe" and "Yar Aleiro" all had thick flat shape while the genotypes Ares and "Dan Garko" had globe and flax shapes, respectively. Both genotypes "Dan Zaria" and "Bakana" had granex shape. The genotype "Dan Giyawa" had both Spanish and granex shape ( Table 5). The shape of onion bulbs is genotypically influenced, and is an important marketing attribute. Different onion shapes were observed in this study. In South Africa and for the export market, onion bulbs with a round shape are preferred for export market and by consumers (Eksteen, Van Den Klashorst & Van, 1997;Bosch-Serra & Currah, 2002).

Shape Index
The shape index in the genotype "Wase" (1.56) was statistically similar to those of genotypes Ares (1.0), Violet de Galmi (1.20), Red Creole (1.07), "Dan Zaria" (0.91) and "Dan Giyawa" (0.89). The lowest shape index of 0.74 was observed in the genotype "Dan Garko" (Table 5). An onion bulb with a shape index of 1.1 ± 0.02 is classified as a globe bulb, while a bulb with a 1.8 ± 0.03 index is regarded as a flat bulb. Those with a 1.3 ± 0.05 index are regarded as a flattened shape and those with a 1.1 ± 0.02 index are said to have a complete spherical bulb shape (Hasegawa, Bressan, Zhu & Bohnert, 2001). Round bulbs have a shape index of 1.00 (Mallikarjun, 2006).

Conclusion
Results of this study showed that morphological attributes differed amongst the onion genotypes, except for the number of leaves/plant, neck thickness and horizontal bulb diameter. The study concludes that morphological differences, which are responsible for differences in fresh bulb and dry matter yields exist amongst the different genotypes of onion cultivated in Nigeria. These attributes could be used as selection indices in the improvement of the onion.