Effects of Planting Dates on Nutritional and Phytochemical Compositions of Onion Varieties Under Rain-fed and Irrigation Facilities in Ogbomoso, Nigeria

Onion is a high-value vegetable consumed in Nigeria on a daily basis, and it forms an integral part of the diet, but the nutritional quality is low due to lack of appropriate cultural practices. The study examined the effects of planting dates on the nutritional quality of onion varieties in Ogbomoso, Nigeria. Five onion varieties (Local white, a local red, karibou, gandiol+, and safari) were subjected into four different planting dates namely, two under rain-fed (early April and August) and two under irrigation (early September and November). The 5 x 4 factorial treatment combinations were arranged in a Randomized Complete Block Design (RCBD) with three replicates. Onion bulbs and leaves were collected separately from six plants, in each treatment and used for proximate analysis. The minerals (N, P, K, Ca and Mg), nutritional, and phytochemical compositions (protein, fat, crude fiber, ash, moisture content, vitamin C and quercetin) of onion bulbs and leaves were assessed. Data collected were subjected to analysis of variance. The results showed that variety significantly (P≤0.05) influenced the minerals, nutritional and phytochemical compositions of onion leaves and bulbs. Karibou contained highest minerals, nutritional (except fat) and phytochemical compositions of onion leaves, while local white gave the least values. The vitamin C and quercetin contents of the onion bulbs were higher (20.08 and 0.59 g/mg, respectively) in karibou as compared to other varieties. Planting date exerted significant (P≤0.05) influence on the nutritional quality of onion leaves and bulbs. The highest quercetin content of 0.58 and 0.23 g/mg for onion bulb and leaves respectively, were obtained from onion planted in November, while April planting date gave the least values. In conclusion, the best nutritional values were produced by karibou variety and from onion planted in November, and hence can be recommended for Ogbomoso agro-ecological zone in the southwestern Nigeria.

Keywords: Onion Bulb; Onion Leaves; Proximate Analysis; Nutritional Quality; Quercetin Content

Onion (Allium cepa L.) is an important vegetable crop valued for its pungent or mild flavor and for being the essential ingredient of the cuisine of many regions [1]. Therefore, it is one of the most important vegetables in the world, whose utility in Nigeria is ranked second to tomatoes based on the level of consumption [2]. It is one of vegetable crops commonly consumed in Nigeria and around the world. Onion is grown for its green leaves and bulbs, which are used in every home, almost daily. It is rarely used as a sole dish or in large quantities, its main use lies in flavoring and seasoning of a wide variety of dishes. Its popularity is due to its aromatic, volatile oil, the allyl-prophlsulphide that imports a cherished flavor to food [3].

The two categories of onion varieties are: long day which is better for cool climate and short-day that is better for warm climates. They have various colors ranging from white to yellow to red with a flavor range from mildly sweet to more pungent onion [3,4]. The nutritional values of onion cultivars are 89% water, 9% carbohydrates (including 4% sugar and 2% dietary fiber), 1% protein, and negligible fat. Onions contain essential nutrients and have an energy value of 166KJ (40 calories) in a quantity 100g. Onions contribute savory flavor to dishes without contributing significant caloric content [5].

Thomas S [6], reported that onion is a rich source of nutrients, including vitamin B, and C, protein, fiber, starch and a series of essential elements. He revealed that the chemicals contained in onions are effective agents against fungal and bacterial growth; they protect against stomach, colon and skin cancers; they have anti-inflammatory, antiallergenic and antidiabetic actions; and they treat causes of cardiovascular disorders, including hypertension, hyperglycemia and hyperlipidemia while also inhibiting platelet aggregation.

Onion bulbs are rich sources of dietary flavonoid sand contribute to a large extent to the overall intake of flavonoids. Two types of flavonoids are important in alliums, the anthocyanin responsible for the color in red-skinned onion cultivars and the flavonoids, which give a yellowish due to onion flesh and are important precursors for yellow and brown skin pigments. Flavonols are the predominant pigments of onions. At least 25 different flavonols have been characterized, and quercetin derivatives are the most important ones in all onion cultivars. Moreover, onion cultivars are index according to their flavonoids content measured as quercetin. Only compounds belonging to flavonoids; the anthocyanin and the dihydroflavonoids have been reported to occur in onion bulbs. Yellow onions contain 270-1187mg of flavonols per kilogram of fresh weight (FW), whereas red onions contain 415- 1917mg of flavonols per kilogram of FW. Quercetin 4’-glucoside and quercetin 3.4’-diglucoside are in most cases reported as the main flavonols in recent literature [3].

Planting date is one of the major cultural practices in onion production, which will greatly influence its performance. Onion is grown widely during the wet and dry seasons. However, there is need for more information on the nutritional quality of onion produced during the two seasons, particularly in the Southwest Nigeria. This study assessed the effects of planting dates on the nutritional and phytochemical compositions of rain-fed and irrigated onion varieties in Ogbomoso, south western Nigeria.

The field and laboratory experiments were conducted between 2018 and 2019 cropping seasons at the Teaching and Research Farm and in the Department of Crop Production and Soil Science respectively, Ladoke Akintola University of Technology, Ogbomoso, Nigeria. Ogbomoso is on latitude 8⁰ 10¹N and longitude 4⁰10¹E in the guinea savanna zone of southwest Nigeria.

The materials used are five onion varieties namely, a local white and safari which are white in color, a local red and karibou which are red in color, and gandiol+ which is yellow in color and safari, were obtained from KAAL farmer centre, Ogbomoso, Nigeria.

The treatments involved five onion varieties (Local white, a local red, karibou, gandiol+, and safari) and four different planting dates namely, two under rain-fed (early April and August) and two under irrigation facilities (early September and November) with their various combinations. The 5 x 4 factorial treatment combinations were arranged in a Randomized Complete Block Design (RCBD) with three replicates. The experimental site was plowed and harrowed and 60 raised beds were made, with each bed containing 64 plants. These were divided into three replicates with each replicate containing 20 beds. The size of each bed was 2 m x 2 m (4m2) with the spacing of 0.5 m within and 1m between replicates, giving a total experimental plot size of 50.5m x 10m (505m2).

The nurseries for onion were established two months before each planting date. The onion seeds were sown on nursery beds containing top soil and seedlings were raised for eight weeks before transplanting to the field. Uniform and healthy seedlings were transplanted into the field in order to ensure uniformity at 8 weeks after sowing at the spacing of 30 cm x 30 cm, giving 64 plants per bed and a plant population of 111,111 stands per hectare. Supplementary water supply or irrigation was carried out after transplanting and continued during the dry period of the experiment. A recommended combined application of 65kg N/ha, 40kg P/ha and 45kg K/ha [7]. was applied by band method, at 3 weeks after transplanting. Mulching was done immediately after sowing with the use of dried Guinea and Southern Gambia grasses which were carefully placed on each bed to help retain soil moisture, reduces weed problems among the plants and improves the soil fertility. Weeding was carried out as it became necessary by manual method using hoe. Insect pests were controlled with cypermethrin insecticides at 10 ml per 10 liters of water, sprayed at two weeks interval with the use of Knapsack sprayer.

Six plants were randomly selected within the net plot, in each bed and tagged. Data were collected at 4 months after transplanting (MAT). Onion bulbs and leaves were collected separately from six plants in each treatment, dried and analyzed in the laboratory to determine the mineral contents (N, P, K, Ca and Mg), nutritional and phytochemical compositions (protein fat, crude fiber, ash, moisture content, vitamin C and quercetin) of onion as affected by treatments. Samples were analyzed according to the official method of analysis described by the Association of Official Chemist [8]. Total N was determined using macro Kjeldahl [9] the P and K contents of the plants were determined by wet digestion in nitric, sulphuric and perchloric acids. P was determined by vanadomolybdate yellow colometry method. Digested samples were diluted and used for determination of K concentration using atomic absorption spectrophometer, while the nutritional, quercetin and vitamin C contents were determined using the methods of analysis described by the Association of Official Chemist [8].

Data collected were analyzed using Standard Analysis System [10] for one way analysis of variance (ANOVA). Difference among treatments means was computed using Duncan Multiple Range test at 0.05 significant levels.

The mineral composition of onion bulbs showed that variety had significant (P≤0.05) effect on the P, K, Ca and Mg contents of onion bulbs (Table 1). Karibou had the highest (5.28 g/mg) nitrogen content which is statistically similar to the values obtained for gandiol+ (5.15 kg/mg) and safari (4.99 g/mg) while local white gave the least (4.68 g/mg) value. M Karibou had the highest P, K, Ca and Mg content of 35.08, 161.49, 161.40 and 10.11g/mg respectively, followed by Gandiol+ while local white recorded the least values. The phosphorus and potassium contents of the bulb of onion planted in November (34.39 and 161.97 g/mg, respectively) were significantly higher (p≤0.05) than the others, but no statistical difference was observed in the values obtained between December and August planting dates. Also, the planting date had significant (p≤0.05) effect on the calcium and magnesium contents of onion bulbs. The highest N, Ca and Mg contents of 5.55, 158.79 and 9.61 g/mg respectively were obtained from bulb of onion planted in November followed by December while April planting date gave the least values. Significant (P≤0.05) interaction was recorded between variety and planting date on the mineral compositions of onion bulbs.

The mineral composition of onion leaves is presented in Table 2. The nitrogen, phosphorus, calcium and magnesium contents followed the same trend and significantly (P≤0.05) higher for karibou (0.23, 21.72, 130.97, 14.10 and 7.11 g/mg, respectively) as compared with other onion varieties. Planting date exhibited significant (P≤0.05) influence on the mineral compositions of onion leaves. The leaves of onion planted in November recorded the highest N, P, K, Ca and Mg contents (0.23, 21.7, 131.9, 14.25 and 7.33 g/mg, respectively) than in December, August and April planting dates. The interaction between variety and planting date exerted significant influence on the P, K, Ca and mg contents of onion leaves.

The onion bulb nutritional and phytochemical compositions revealed significant (P≤0.05) differences in the crude fiber and fat contents of the onion bulb among the varieties (Table 3). Although, the highest crude fiber and fat contents (1.52 and 0.31 g/mg, respectively) were recorded for karibou, but no statistical difference was observed between the values obtained for Karibou and Safari. The ash and moisture contents of karibou were significantly (P≤0.05) higher (1.75 and 86.81 g/mg, respectively) than the other varieties. Planting date exerted significantly (p≤0.05) effect on the nutritional qualities of onion bulbs. Onion planted in November had the highest crude fiber, fat and ash contents (1.60, 0.31 and 1.77 g/mg, respectively) followed by those planted in December while April planting date recorded the least values. Moisture content of onion bulb was significantly influenced by the planting date. The highest moisture content was recorded at April (87.14 g/mg) while December planting date gave the least (83.28 g/mg) value.

Although, the Vitamin C content of onion bulbs is not significantly differs among the varieties, the highest value of 20.08 g/mg was obtained for karibou while local white gave the least (9.58 g/mg) value. The bulbs of onion planted in August recorded the highest (17.54 g/mg) Vitamin C content while April planting date gave the least (9.16 g/mg) value. Karibou provided significant highest Quercetin content (0.59 g/mg) which is statistically similar to that of Gandiol+ (0.54 g/mg) while local white recorded the least (0.39 g/mg) value (Figure 1).

Quercetin content (0.58 g/mg) obtained from bulbs of onion planted in November was significantly higher (p≤0.05) than those recorded in December, August and April planting dates (Figure 2). The variety and planting date interaction effect had a significant influence on the Ash and Moisture contents of onion bulbs.

The nutritional and phytochemical compositions of onion leaves revealed that karibou had a significantly higher Crude Fiber content that is statistically similar to that of local red and white onion, while no statistical difference in the Crude Fiber content was observed between Gandiol+ and Safari (Table 4).

The crude fiber content was significantly influenced by the planting date, with leaves of onion planted in December recorded the highest (1.69 g/mg) while the least (1.46 g/mg) was obtained for April planting date. Variety exhibited significant effects on the Fat, Ash, Moisture, Vitamin C and Quercetin contents in the onion leaves. The highest Fat, Moisture and Vitamin C content (0.80, 20.37 and 9.43 g/mg) were obtained from karibou while local white gave the least values. Local white variety had higher Ash content than the others. The highest Quercetin content of 0.22 g/mg was obtained from local red closely followed by Karibou (0.20 g/mg), while local white gave the least value (Figure 3).

The nutritional and phytochemical compositions of onion leaves were significantly affected by the planting date. The leaves of onion planted in December gave the highest Crude Fiber followed by November (1.61 g/mg), while the least (1.46 g/mg) value recorded at April planting date. The November planting date recorded the highest Fat, Ash, Vitamin C and Quercetin (Figure 4) contents (0.89, 0.63, 9.68 and 0.23 g/mg) while local white gave the least value. The Moisture content was higher (20.13 g/mg) in the onion leaves at April planting date as compared to the other planting dates.

The combined effect of variety and planting date had significant influence on the Crude Fiber, Fat and Vitamin C contents of onion leaves.

The presence of nutrient elements in both the leaves and bulbs showed that onion is a rich source of nutrients. This corroborates the findings of Brewster (2016), who reported that onions are brimming with vitamin C, B1, B6, potassium, phosphorous, fiber, protein and starch. The nutritional compositions obtained in this study are very close to those reported by USDA (2017), that recorded about 89% water, 2% dietary fiber and negligible (0.1) fats. The low mineral nutrient contents obtained in this study are similar to those obtained by Atanda (2015) who reported low amount of essential nutrients in onions.

The statistically significant differences that exist between onion varieties in nutritional and phytochemical contents, particularly quercetin agree with the earlier work of [11]. They reported differences in the polyphenols content of onion varieties. Also, the differences in the nutritional and phytochemical contents among the onion varieties confirmed the findings of Ansari (2007) and Young et al. (2004) [12,13], who reported that red onion, the mildest, sweet onion, contains a higher amount of antioxidants compounds and higher flavonoids than white and yellow onion. They reported that quercetin which is a polyphenol compound, is one of the beneficial compounds in red onion. Moreover, the higher quercetin, vitamin C and nutrient contents recorded for karibou which is a red onion, in comparison with other white and yellow varieties, supported the work of Olsson (2010) [14] that reported the presence of quercetin in sweet and red onion cultivars at harvest. However, Local white and safari which are white onions, contained more fiber than red onion, as previous reported by Ansari (2007) [12].

Brewster (1994) [15], reported wide range of diversity or variation in quality attributes and skin color among onion cultivars. He stated that yellow onions have the highest total flavonoid content, an amount 11 times higher than in white onions. While, red onions have considerable content of anthocyanin pigments, with at least 25 different compounds identified representing 10% of total flavonoid content [11]. The nutritional quality obtained in this study is in agreement with the values reported for onion bulbs by Holland et al. (1991) and Atanda (2015) [16,17]. The significant differences in the nutritional and phytochemical contents of onion bulbs and leaves might be due to genetic differences among the cultivars examined.

The lower nutritional values obtained for rain-fed onion in April and August are in agreement with the earlier findings of Ansari (2007) [12] who attributed poor performance during the raining season to the excessive rainfall and pest and diseases.

In conclusion, karibou variety and November planting date exhibited significant higher nutritional and phytochemical qualities, and therefore can be recommended for the high nutritional values of onion in Ogbomoso, southwestern agro-ecological zone Nigeria.

I appreciate TETFUND and Ladoke Akintola University of Technology, Ogbomoso, Nigeria for giving me the TETFUND institution-based research intervention fund for this study.