Onion (Allium cepa L.) Bulb Quality and Shelf Life as Influenced by Nitrogen and Phosphorus Fertilisation

Reader Impact Factor Score
[Total: 7 Average: 4.3]

Published on International Journal of Food & Nutrition
Publication Date: June 26, 2019

Muluneh Bekele Etana, Ali Mohammed & Amsalu Nebiyu
Department of Horticulture, College of Agriculture and Veterinary Science, Ambo University
Jimma University College of Agriculture and Veterinary Medicine, Jimma University

Journal Full Text PDF: Onion (Allium cepa L.) Bulb Quality and Shelf Life as Influenced by Nitrogen and Phosphorus Fertilisation (Under Jimma Condition).

Onion (Allium cepa L.) is one of the most important vegetable crops produced in Ethiopia. Yield and productivity of the crop has been far below the regional and national standards owing to several factors; absence of location specific fertilizer recommendation being the major among others. In Ethiopia, post harvest loss of vegetables contributed up to 30% yield reduction on vegetables and fruits. Thus, a field experiment was conducted at Jimma University College of Agriculture and Veterinary Medicine Research field in dry season to study on the effects of Nitrogen (N) and Phosphorus (P) fertilizer application on quality and shelf life of irrigated onion under Jimma condition, South Western Ethiopia. The treatments were consisted of factorial combinations of four levels of Nitrogen (0, 50, 100 and 150kg N ha-1) and three levels of Phosphorus (0, 46, and 92kg P2O5 ha-1) laid out in Randomized complete Block Design with three replications. Data on bulb quality and shelf life parameters were recorded and analyzed using GenStat 12.1 version computer soft ware packages. Results of the study revealed that; N and P had shown a highly significant effect on quality parameters like TSS (oBrix), DMC (%) and bulb shape index. Similarly, keeping quality of the Onion bulbs like bulb sprouts (%), weight loss (%), weeks to 50% bulb sprouts and storage rots (%) are highly influenced by application of N and P at different levels. Excessive Nitrogen caused higher bulb rots (%); bulb sprouts (%) and weight loss (%) during the two month storage time at ambient temperature. The output of this experiment can be recommended for use by potential Onion investors or farmers in the study area. Nevertheless, more researches are needed in different locations and on different soils to come up on general recommendation

Keyword: Fertilisation, Jimma, Nitrogen, Onion, Phosphorus, Quality, Shelf life.

1. Introduction
Different cultural practices and growing environments are known to influence yield and quality of dry bulb. So far, research in the country was mainly focused on the identification of superior cultivars of onions and adopting improved management practices. Mineral nutrition is main that affects yield and quality of onion (Chung, 1989). Nitrogen and Phosphorus are often referred to as the primary macronutrients because of the probability of plants being deficient in these nutrients and because of the large quantities taken up by plants from the soil relative to other essential nutrients (Marschner, 1995). Nitrogen comprises 7% of total dry matter of plants and is a constituent of many fundamental cell components (Bungard, 1999). It is one of the most complexes in behavior, occurring in soil, air and water in organic and inorganic forms. For this reason, it poses the most difficult problem in making fertilizer recommendations (Archer, 2002). Plant demand for N can be satisfied from a combination of soil and fertilizer to ensure optimum growth.
However, Proper management techniques such as fertilizers, soil moisture and disease control, harvest time and curing enhance Onion produce (Kabir, 2007). Optimization of such practices results in significant decrease in post harvest losses and increase bulb yield in Onion. Decrease in post harvest losses will be instrumental in market stability and exploiting opportunities to export Onion and earn foreign exchange. Best quality Onion can be produced through application of well balanced fertilizers (Murashkina, 2006).
Accounting for soil N and expected release from the soil, additional N application may be necessary in order to meet the crop N requirements. The amount of N needed is usually based on soil organic matter content, crop uptake and yield levels. Nitrogen uptake levels by onion crops may vary depending on cultivar, climate, plant density, fertilization and yield levels (Hegde, 1986a; Sørensen, 1996; Suojala et al., 1998; Salo, 1999, Pire et al., 2001 and Muluneh et al,. 2018).
Depending on soil P status, cultivar and plant density, application rates of up to 200 kg ha-1 was found to maximize Onion yields (Hegde et al., 1986a; Vachhani & Patel, 1993; McPharlin & Robertson, 1999; El-Rehim, 2000; Singh et al., 2000) and reduce storage loss of bulbs. Increased P levels are also known to improve bulb size and the number of marketable bulbs in shallots (Zahara et al., 1994; Nagaraju et al., 2000). Regardless of the P status of the soil, placement of P-fertilizers in the soil near to the plant would be the most effective method of P supply to Onion plants (Brewster, 1994; Henriksen and Hansen, 2001 and Muluneh et al,. 2019).
In general, better understanding of the nutrient requirements of onion plant is needed in order to develop the management strategies, which optimize fertilizer use of the crop and thereby increase returns with premium bulb qualities to the producers. In the light of the above aspects, the present research was initiated to identify the economical level of Nitrogen and phosphorus fertilization for onion (Allium cepa L.) optimum quality and shelf life under Jimma conditions, Southwestern Ethiopia.

2. Materials and Methods
2.1 Description of the Experimental Site
The experiment was conducted under field condition at Jimma University College of Agriculture and Veterinary Medicine research field under irrigation condition. Jimma University College of Agriculture and Veterinary Medicine is geographically located 346 km southwest of Addis Ababa at about 70, 33’N latitude and 360, 57’ E longitude and an altitude of 1710 meter above sea level. The analysis of soil samples from the top 30 cm depth of the experimental site before the experiment revealed that the soil contains 1.46% organic carbon; 1.42% total nitrogen, 2.80ppm available phosphorus, 53.1μS/cm electrical conductivity and a pH value of 5.94.The mean maximum and minimum temperatures are 26.80C and 11.40C, respectively and the mean maximum and minimum relative humidities are 91.4% and 39.92%, respectively. The mean annual rainfall of the area is 1500mm (BPEDORS, 2000).

2.2 Experimental Design and Layout
Onion seedlings were raised in the nursery on a well prepared seedbed whose dimension was 5m × 1m. The seeds were sown in rows marked 15cm interval across the length of the seed bed and the beds were covered with dry grass mulch until emergence. Healthy, uniform and 51 days old seedlings were transplanted to the prepared field at spacing according to recommendation. All the treatment combinations were randomly assigned and there were 10 plants in each row and 60 plants per plot with three replications. During the course of the study Mancozeb was applied to prevent the damage of disease at rate of 4.0 kg ha-1 mixed in 600 liter of clean water. All other agronomic management practices were provided as per the recommendation equally for all the treatments (Getachew, 2009).
Finally, bulbs from the central four rows were harvested after 60% neck-break and used for analysis. After storage daily storage room temperature and relative humidity were recorded using digital sling Psychrometer (AZ8706 model, China). The storage time was from the month of May to July for three months under the average monthly temperatures and relative humidity of 17.23oc and 16.72oc, 75.32% and 77.65%, respectively.

2.3 Statistical Analysis
The data were analyzed using GenStat versions 12.1 (2009) with the REML variance component analysis. Mean differences were tested following least significant difference (LSD) at (P<0.05). 3. Results and Discussions 3.1 Quality Parameters of Onion Bulb 3.1.1 Total Soluble Sugars Content (TSS) Regarding the total soluble sugars content (TSS), the interaction of N-P had shown a highly significant (Table 1) difference. The highest TSS value (11.67 oBrix) was recorded in the combined application of N-P at the rate of 150:92 kg ha-1; while the minimum TSS value (8.08 oBrix) was recorded in control treatments. Regardless of the levels, maximum combined application of N-P (150:92 kg ha-1) increased the TSS by about 30% as compared to control (8.08 oBrix). Table 1: TSS of onion bulb as influenced by combined effects of N and P Nitrogen (kg N /ha) Phosphorus levels (kg P2O5/ha) Means in a column followed by the same letter(s) are not significantly different at 5% 3.1.2 Bulb Shape Index Considerable variation was observed in the result of bulb shape index. The shape of onion bulb can vary from flat to globe to torpedo which is in different markets having different requirements. The onion bulb shape was assessed by the bulb shape index; this was determined by the ratio of bulb length to diameter. The result of this study revealed that application of N and P at different levels had shown a highly significant (P<0.001) difference on the bulb shape index, while their interaction did not (Table 2). This result also showed that the null and lower application of N and P fertilizers increased the percentage of shape rejects as compared to the plot received higher levels of N and P (150 kg N ha-1 and 92 kg P ha-1), respectively. Similarly, Geremew et al., (2009) reported as bulb shape of onion is affected by mineral nutrients. Table 2: Bulb shape index as influenced by applications of N and P Means in a column followed by the same letter(s) are not significantly different at 5% Regardless of levels, higher application of N at (150 kg ha-1) and P at (92 kg ha-1) increased the bulb shape index by about 3% and 3% over control (Table 2). The reason why N and P fertilization increased the bulb shape index of onion may be because of their vital role in plant growth and development. Kimani et al. (1993) reported as bulb shape difference is among onion cultivars and affected by growing environment and also further explained that globe shaped (shape index=1) are preferred by the consumers. 3.1.3 Dry Matter Contents Regarding the dry matter contents, main application of N and P had shown a highly significant effect, while their interaction did not (Table 3). The increasing levels of N and P encouraged bulbs with a significantly higher dry matter contents as compared to the unfertilized plot. The maximum dry matter content of onion bulb (10.54%) and (10.30%) recorded with higher application of N and P at rate of 150 and 92, respectively. The minimum dry matter contents (9.26%) and (9.54%) detected in control respectively. This finding is in consistent with the result of Mojsevich (2008) who reported that with the increase of doses of the main fertilizer N and P 70 and 45kg ha-1 to N and P 110 and 75kg ha-1 caused the increase of dry matter content in bulbs from 14.6% to 15.5% respectively. Table 3: Dry matter contents as influenced by applications of N and P. Means in a column followed by the same letter(s) are not significantly different at 5% 3.2 Shelf Life Parameters of Onion Bulb 3.2.1 Bulb Storage Rots Percentage N application had a highly significant (p<0.05) effect on the bulb rotting percentage during the storage time; while P application did not (Table 4). The highest percent of bulb rot percentage (3.69%) recorded in the plots received 150 kg N ha-1 and the least bulb rot percentage is recorded with unfertilized plots. Jones and Mann (1963) similarly reported that onion bulbs produced without nitrogen application resulted in lowest rotting (22%), while highest rotting (36-54%) was recorded in bulbs produced under higher dose of nitrogen. Similarly in India, Singh and Dhankar (1991) and Patel and Patel (1990) reported that increasing the rate of applied nitrogen (N) from 50 to 150 kg ha-1 led to significant increases in storage rots of onion during 4 to 5 months under ambient conditions. Table 4: Storage rots of bulb (%) as influenced by applications of N and P. NS = not significant; Means in a column followed by the same letter(s) are not significantly different at 5%. Numbers in parenthesis are square root transformations. 3.2.2 Physiological Weight Loss Percentage N had a highly significant (P<0.05) effect on the weight loss percentage of stored onion bulb during the two month storage time (Table 5). Large weight loss percentage (39.53%) was seen at plot that received maximum N at 150 kg ha-1. This maximum weight loss may be associated with the resumption of higher incidence of sprouting and rotting presumably through increase in the rate of respiration. Regardless of the level, maximum N application at 150 kg ha-1 showed high weight loss percentage (19%) as compared to the control. Dankhar and Singh (1991) also reported similar result that weight loss of bulbs increased with the increase in the nitrogen level. Table 5: PWL (%) and bulb sprouts (%) of onion as influenced by applications of N & P NS = not significant; Means in a column followed by the same letter(s) are not significantly different at 5%. Numbers in parenthesis are square root transformations. 3.2.3 Bulb Sprouts Percentage and Weeks to 50% Bulb Sprouts Sprouting is physiological change that occurs on bulbs of onion in storage. N application had shown a significant (P<0.001) difference on percentage bulb sprouts (Table 5). The highest incidence of sprouting was seen in the plot received maximum N at rates of 150 kg ha-1; while the least record observed from unfertilized plots at the end of two months storage. There are similar reports by Bhalekar et al. (1987) who observed that sprouting was increased with increasing nitrogen levels from 0 to 150 kg N ha-1. Dankhar and Singh (1991) also reported that high dose of nitrogen produced thick-necked bulbs that increased sprouting in storage due to greater access of oxygen and moisture to the central growing point. 4. Summary and Conclusions Onion is a vegetable bulb crop that produced for it is high bulb yield and extended post harvest life and storage. In order to maintain and improve such bulb quality and post harvest keeping ability (shelf life) of the crop, special cultural practices should be taken in to consideration for maximum exploitation of such a feature. Both physical and chemical parameters have to be preserved during the post harvest life of this crop. For persisted quality nature of Onion crop in shelf life both pre harvest and post harvest management practices should be given due attention. Storage of bulbs after harvest is crucial to ensure availability during off season. Many cultivars do not keep long in ambient storage because they tend to sprout and rot shortly after harvest. In addition, due to poor storage facilities, Onion breakdown within 2 to 4 months after harvest is a common phenomenon. Yield, quality and shelf life of onion bulb plants were affected with application of N and P at different levels. Higher mean bulb weight (49.78g, and 47.52g) at higher levels of N (150kg/ha) and P (46kg/ha) application; lower unmarketable bulb yield (0.47ton/ha and 0.62ton/ha at higher levels of N (150kg/ha) and P (92kg/ha) application; higher harvest index (0.80%, 0.77% and 0.78%) at higher levels of N (150kg/ha), and P (46kg/ha) application; higher dry matter content (10.54%, and 10.30%) at higher levels of N (150kg/ha), and P (92kg/ha) application; higher storage rot percentage (3.69%) at higher levels of N (150kg/ha); higher bulb weight loss (39.53%) at higher levels of N (150kg/ha); higher bulb sprouts percentage (8.12%) at higher levels of N (150kg/ha). In general, from marketable yield, post-harvest quality and storability point of view, N and P fertilization was very sound; especially for our country farmers where their production is once in a year. If these methods are integrated and well applied, year round production of this crop may not be required. In addition, problem of market glut could be stabilized with balanced costs from stored bulbs dispatch. Therefore, the result of this study has shown that N and P fertilization have a sound and promising impact for post-harvest quality that could be applied for onion production. 5. Acknowledgment We would heartedly like to thank and praise the Lord Almighty God in giving us strength and wellbeing to successfully complete the study. We sincerely thank Jimma University for all necessary support. We also want to thanks all the data collectors who help us in data collection and for all their concern and moral support. Finally, all the reference materials used in this article are dully acknowledged 6. Conflict of Interest Regarding the publication of this manuscript, there is no any conflict of interest.