The Effect of Bulb Size and Plant Density on Yield and Quality of Onion (Allium cepa var cepa L) Seed, at Ziway, Central Ethiopia

TABLE OF CONTENTS

ACKNOWLEDGMENTS

LIST OF TABLES

ACRONYMS AND ABBREVATIONS

ABSTRACT

1 Introduction

2 Literature Review
2.1 Onion Crop Production
2.1.1 Onion Cultivars
2.1.2 Climatic Requirement
2.1.3 Soil and Fertilization Requirement
2.1.4 Water Requirement
2.1.5 Planting Date
2.2 Onion Seed production
2.2.1 Bulb-to-Seed Method
2.2.2 Seed-to-Seed Method
2.3 The Effect of Bulb Size and Plant density
2.3.1 Effect of Bulb Size
2.3.2 The Effect of Plant Density
2.4 Onion Flowering
2.5 Pollination in Onion Flowers
2.6 Onion Seed Setting and Maturation
2.7 Harvesting, Threshing and Seed Cleaning
2.8 Onion Seed Quality
2.9 Biotic Limiting Stresses

3 Materials and Methods
3.1 Research Site Description
3.2 Experimental Treatments and Design
3.3 Management practices
3.4 Data Collection
3.5 Data Analysis

4 Results and Discussion
4.1 Effect of Bulb Size and plant density on Bulb Sprouting and Plant Growth
4.1.1 Bulb Sprouting and Plant Stand
4.1.2 Plant Height
4.1.3 Umbel Size
4.2 Effects of Bulb Size and Plant Density on Seed Yield Components
4.2.1 Number of Flower Stalk per Plant
4.2.2 Number of Seeded Florets per Umbel
4.2.3 Seeds per Floret
4.3 Effects of Bulb Size and Plant Density on Seed Yield
4.3.1 Number of Seeds per Plant
4.3.2 Number of Seeds per 100m2
4.3.3 Seed Yield (qt per ha)
4.4 The effect of Bulb Size and Plant Density on seed quality
4.4.1 Thousand Seed Weight
4.4.2 Germination percentage
4.5 Correlation Analysis

5 summary and Conclusions

6 References

7 Appendix

ACKNOWLEDGMENTS

I am very much grateful to my advisors Dr. Lemma Desalegn and Dr. Demelash Kefale for their careful supervision, excellent guidance, and assistance from the very beginning of the proposal initiation up to the final write up. I would like to pass my sincere thanks to SGS/HU for offering financial assistance to conduct my thesis research work.

I am very indebted to express my wholehearted gratefulness to my wife W/ro Yemisrach Gebre (S/r) for her love, patience, encouragement and wholehearted support in every challenge that I faced through out the whole course of my study.

I wish to express my hearty appreciation to my brothers Ato Haileberhan Zena and Ato Tariku Zena, my friends Ato Tesfaye Labiso and Miss Yemisrach Mekonnen and my Brother-in-law Ato Firew Gebremariam for their invaluable helps, morally and financially during all phases of my study. Without their assistance this thesis work could not be finished.

The cooperation rendered by Engida Shiferaw, the technical staff of Melkassa Research Center during the fieldwork is highly acknowledged. I am greatly obliged to the staff members of Horticultural Research Division for their assistance and encouragement while doing my research work.

My particular gratitude and appreciation goes to my friends and class mates especially Abiyot Aragaw, Abdulwahab Ali, Kemeria Barsenaga, Khedija Hussen, Berhan Abdulkadier and many more for their love, understanding, and encouragement who had directly or indirectly contributed for the successful completion of this thesis work.

LIST OF TABLES

Table 1, Plant density and respective plant population per hectare

Table 2. The influence of plant density and bulb size on the plant establishment and growth parameters at Ziway, 2006

Table 3. The effect of plant density and bulb size on seed yield components of onion at Zway in 2006

Table 4. The effect of plant density and bulb size treatments on quantity of seed produced per plan and seed yield in kg per plot of onion at Zway in 2006

Table 5. The effect of plant density and bulb size treatments on the seed weight, laboratory germination percentage and seed vigor of onion at Zway in 2006

Table 6. Simple Pearson correlation analysis for bulb sprouting and plant performance; yield components; yield parameters and seed quality parameters

Dedicated to my beloved Wife

W/ro Yemisrach Gebre (S/r)

ACRONYMS AND ABBREVATIONS

illustration not visible in this excerpt

The Effect of Bulb Size and Plant Density on Yield and Quality of Onion (Allium cepa var cepa L.) Seed, at Zway, Central Ethiopia

By

Name of Student: Zerihun Zena (B.Sc)

Name of Advisors: 1Lemma Dessalegn (Ph.D) and 2Demelash Kefale (Ph.D)

¹ Center director, EIAR, Melkassa Research Center, Po Box 436, Nazereth, Ethiopia

² Asociate Vice Dean, Hawassa University College of Agriculture, Po Box o5, Awassa.

ABSTRACT

Plant density and bulb size trial was conducted at Ziway in order to identify the optimum agronomic management practices, which contribute to maximized seed production in onion crop both in terms of quantity and quality. The experiment was laid down in split plot design with three replications, keeping plant density as main plot factor and bulb size as sub plot factor. The effects were studied by collecting data on bulb sprouting and plant performance, yield components, seed yield and seed quality parameters. Analyses of variance, mean comparison using least significant difference, correlation analysis were made using SAS computer software. Plant density significantly influenced bulb sprouting, umbel size, seed yield per 100m2and germination percentage. The largest umbel size (4.9cm) was obtained from low plant density; where as the smallest (4.4cm) from high plant density. Maximum seed yield per 100m2(1,163,446 seeds) was collected from high density while low density yielded only 416,240 seeds per 100m2. However the quality of seed in terms of germination was highest for seeds obtained from low density. The influence of bulb size was significant on plant height, number of flower stalks per plant, number seeded florets per umbel, seed per plant, seed per 100m2, seed yield (qt/ha) thousand seed weight. Plants grown from large bulbs produced plants with best height (72.3cm), highest number of flower stalks (8.5), seeded florets per umbel (368.7), seed per plant (1,539), seeds per 100m2(994,434) and seed yield in qt/ha (10.42). Seeds with best thousand seed weight (4.2g) were obtained from medium bulbs and small bulbs performed least in all aspects. Correlation analysis was made to study the relationship among plant growth, yield component, seed yield and quality parameters and their respective influence on yield and yield quality. Positive correlation existed between plant height and umbel size with seeded florets per umbel; flower stalk per plant, plant height, umbel size, seeded florets per umbel and number of seeds per floret with seed yield per plant, which was highly correlated with seed yield (qt/ha). However, further studies ought to be conducted in order to understand clearly, why bulb size and plant density interaction effect was insignificant

Key words: Plant density, bulb size, bulb sprouting, plant performance, seed yield and seed quality

1 Introduction

Onion (Allium cepa L.) belongs to the family Amaryllidaceae (amaryllis) or Liliaceae and is one of the most important monocotyledonous, cross-pollinated and cool season vegetable crops. Onions originated in Iran and Pakistan and were established staple foods of Egypt and India around 1500 B.C. By the middle ages, they were in Europe and brought to North America by Spanish settlers. Since ancient time, it has been of great economic importance for its medicinal and dietetic values (Brewster, 1999). Onion is an important vegetable crop worldwide and has been used in various forms as food. It is widely used as a condiment all over Latin America, Africa and Asia. In Pakistan almost all spicy dishes contain onion as one of the important ingredient used for culinary purposes. People consider it as an indispensable part of human diet and is commonly used both by rich and poor (Tindal, 1983).

The unique flavor and odor of onions have made it an excellent food source that provides zest to many dishes. Moreover, the numerous ways in which onions are prepared from boiling, frying, stewing, baking, pickling, to eating raw make them a versatile food source. The recent popularity of a health conscious world for salad bars has further increased their agricultural importance (FAO, 1998). Nutritionally, onions are low in calories (about 40 calories for an average size onion) and high in ascorbic acid (Jones and Man, 1963). Onions can also be used in different forms. To a lesser extent the processing industry uses it in the form of dehydrated onion flakes and powder, one of the remarkable features of onion is its excellent transportability.

Onions can be grown under a wide rage of climatic conditions but are more successful under mild season without extremes of heat or cold and excessive rainfall (Saleem and Ghaffoor, 2003).

Ethiopia’s diversified agro-climatic conditions which is suitable for the production of a broad range vegetables and flowers, allows successful production of onion crop (FAO, 1998). Though shallots are the traditional crop in Ethiopia, onions are becoming more widely grown in recent years. Currently, onion is important cash crop for the farmers in Ethiopia; and hence the crop is produced in different parts of the country for local consumption and for export market, including the export of the flowers by air to European market. The best growing altitude for onions under Ethiopian condition is between 700 and 1800 m above sea level (Shimelis, 2004).

According to CSA (2005) annual per capita consumption of different vegetables in Ethiopia is 5.8 kg and out of which onion consumption is 1.7 kg per capita. The total land area under onion production is 17,980 hectares and 951,709 households owning the land under onion production with total production of 1.4 million tones of dry bulbs per annum. Oromia region has the largest area coverage and the largest number of households owning it.

In the last few years researches have been conducted to come up with improved crop varieties and management practices. Cultivars like Adama red, Melkam, Bombay red are growing profoundly. Adama red and Melkam cultivars are most known and accepted for their profound seed yielding potential (Lemma and Shimelis, 2003).

The demand for increased seed production arises from several factors like demand to meet the needs of ever increasing onion consumption; the development of irrigation schemes with good supply of water encouraging farmers to diversify their production system; the shift of some farmers to intensive commercial onion seed production and the increased demand for vegetable with increasing urbanization (FAO, 1998).

The major problems of onion seed production are (i) the unavailability of skilled manpower in the seed production techniques; (ii) the unavailability of quality and pure seeds of known and accepted cultivars in the market; (iii) the lack of convenient cultural and agronomic practices that easily and widely practiced by commercial seed producers and local farmers; (iv) poor post harvest handling and storage techniques; and (v) poor marketing infrastructure and (v) the less population of pollinating insects (Abdella, 1984).

Earlier investigations conducted at Melkassa (Lemma and Shimelis, 2003) and Werer (Geofery-sam-agrey et al., 1986) to determine optimum bulb sizes, plant spacing, sowing/planting date and standard cultural practices for bulb and seed production under Ethiopian condition. those findings contributed much for the improved onion production until recent years. In order to fill the prevailing research gap this research, combining two important agronomic factors termed as plant spacing, bulb size and their interaction, was conducted under Zway condition.

With growing onion production demand in the rift valley in order to increase the volume of quality seeds produced locally, reducing the cost of production for commercial seed and bulb producers and reducing dependence on seed import from abroad is becoming necessity. Therefore it would be important to identify agronomic factors that influence onion seed yield and quality. This research project was initiated aiming at studying the effects of basic agronomic practices influencing the general plant performance and seed yield with the following specific objectives:

- To evaluate the effect of plant density on the quality and quantity of onion seed produced and;
- To evaluate the influence of mother bulb size used as planting material on onion seed production.

2 Literature Review

2.1 Onion Crop Production

Common onion constitutes the large bulk of economically important varieties. They all form a single bulb and are mostly grown from seed. When the plant is grown for production of green or bulb onions, it is treated as an annual, which rarely gets more than about 31cm tall. The seeds are planted in the field or started in protected areas then transplanted, when 10 to 12 cms tall, into the field. A few weeks later, when the top growth or the bulb has reached the proper size or condition, the entire plant is harvested and the desirable parts marketed. No seeds are produced and no pollination is involved.

In the springtime, the bulbs initiate normal growth, and then produce from 1 to 20 flower stalks, about 90cms tall. This is referred to as "bolting," an undesirable trait in green or bulb onion production but essential in seed production (www.oregonstate.edu). Bolting is strongly influenced by day length and temperature, and cultivars are bred to bolt at certain times in different areas. For this reason, northern and southern types are not interchangeable. Many cultivars have been developed for different regions and purposes northern and southern, purple and white bulbs, strong and mild-flavored (Robinowitch and Brewster, 1990).

2.1.1 Onion Cultivars

Because onions have been cultivated for so long and because their bulbs and inflorescence development must be closely adapted to the temperature and photoperiods that prevail where they are grown, there exists large range of cultivars and land races developed over the centuries to fit the diverse climates and food preferences of the world (Brewster, 1983).

Cultivars are developed based on the distinguishing features of the foliage (leaf length and leaf erectness); shape of the bulbs (globe, a flattened globe, flat top and cylindrical); the uniformity of the bulb shapes; skin color of the bulbs (white, yellow) nature of inflorescence including its fertility, number of flower in the umbel, the petal and anther color in order to attract pollinating insects, the presence or absence of bulbils (small bulbs) in the inflorescence and whether an inflorescence is indeed produced or if reproduction is normally vegetative. The above features are highly heritable and easily seen with a naked eye although many are subject to environmental variability (Brewster, 1999).

The day length requirement for bulbing can be judged from the location where the cultivar is grown and the time of year it bulbs. Features of flowering, which are characterized in comparative trials include the length of flower stalk, the time of flowering and the cold requirement for bolting. Features of great practical significance and which require comparative screening against standard varieties include susceptibility to pests and diseases and ability to tolerate soil or climatic stresses (Ronald et al., 1999).

Onion cultivars are frequently classified into short, intermediate and long day length types. This refers to the minimum day length needed to stimulate bulb development. Short day onion cultivars can initiate bulbs when the day length exceed 11 to 12 hours, intermediate day types when day length exceed 13 to 14 hours and long day types require above 16 hours (Brewster, 1983).

Experiments show that this criterion to be subject to the working experiences of the professionals who undertake the work, however the division of cultivar in to these broad response groups gives the general indication of their suitability for bulb production in different regions. Because the wide spread intercrossing and breeding of new cultivars that is continuously occurring, there is little point in drawing up a formal classification schemes for grouping varieties. Each country or region tends to have its traditional varieties so any discussion of specific cultivars is inevitably highly selective and limited (Currah and Proctor, 1990)

2.1.2 Climatic Requirement

When seed-to-seed method is employed for seed production the plant grows as biennial crop. Two growing seasons are required to complete the cycle of growth; first season for bulbing and the other for flowering and seed setting. For successful vegetative growth seed sowing has to be made when the temperature is about 13 0C which is optimum for about 70 percent germination and emergence of seedlings. The growth rate of the crop during plant establishment and early juvenile growth is slow compared to other crops due to slow leaf expansion associated with poor light interception. However, when bulb-to-seed method is employed the crop completes its cycle in one season (Ronald et al., 1999).

As it is noted by Comin (1946) standard seed production technology is an important factor to achieve the maximum yield in most of the vegetables. Thrips, bad weather (high rain fall and humidity) during flowering, infection by downy mildew and lack of sunshine in the ripening period reduce seed yield.

Most cultivars are sensitive to temperature. Flower rarely forms at high temperature since a cool period is required after bulb formation for the initiation of flower stalk and inflorescence. It is preferable, however, for the vegetative stage of growth to be completed during relatively cool season with temperature range of 18 0c to 250c. The most important limiting factors determining onion seed yield for onions growing in the drier areas is without irrigation are the amount of rainfall and its distribution throughout the growing season (Brown et al., 1977).

Under Ethiopian condition, highest seed yield was obtained from bulbs planted fro September to December followed by June to august plantings for the two under production cultivars (Adama red and Red Creole) (Lemma and Shimelis, 2003). Warm and drier conditions and ample sunshine is desirable for seed maturity, ripening, harvesting and seed processing operations. Excessive rainfall and cooler condition during flowering leads to disease and poor seed setting and ripening and makes harvesting difficult (Tindal, 1983; Lemma and Shimelis, 2003).

The selected bulbs suitable for seed production, which were produced from planting seedlings obtained from seeds sown are stored under cool temperature of 8 0C to 13 0C for the purpose of vernalization (flower stalk initiation). The process might take one month or more depending on the variety under use. The bulbs planted will produce one or more flower stalks per bulb and this is referred as bolting. Variety, plant size and temperature influence when bolting to occur. Poor vernalization results in poor inflorescence development with respective low seed yield. (Ronald et al., 1999).

Bulbing is not required before bolting, but the plants must have leaf bases with diameters greater than 10 mm before flowering can be induced. Seed stalk initiation requires a period of chilling known as vernalization. Induction of flowering occurs when plants or bulbs are subjected to temperatures of 8° to 13°C for about one month or longer, depending upon the variety. Insufficient vernalization results in poor inflorescence development and low seed yields. The seed stalks elongate as temperatures (Jones and Mann, 1963).

Onion seed production requires two growing seasons (for seed-to-seed method), both seasons can occur in a single ten-month period if the seeds are planted in late summer. This allows enough time for the plants to achieve sufficient size in the fall, vernalize during the winter and produce seeds the following summer. Flowering, pollination, and seed development follow. Mature seeds will naturally fall from the inflorescence (shatter) if not harvested in its due season (www.agohio-state.edu)

Onion seed production requires low-humidity ambient conditions during the spring and summer. Warm temperatures and low relative humidity enhance disease management, pollination, and seed maturation. All foliage diseases are more prevalent under humid conditions, and bees are poor pollinators during rainy weather. Pre- or post harvest seed drying is also achieved most easily in low humidity climates. Climates that are cool in the winter and warm to hot with low rainfall and low humidity in the spring and summer are best suited for onion seed production (Ronald et al., 1999).

2.1.3 Soil and Fertilization Requirement

Marschener (2002) stated that onion crop has shallow and fibrous root system with most roots concentrated in the top 30cms. Since onion is shallow rooted crop and is generally grown in loose soils, it is quite responsive to fertilization. The optimal fertility program has to provide nutrients to the upper 15 to 40 cm of the soil over the entire growing season. Typically, no more than one-third of the nitrogen (N) should be available at planting, one-third at early season (3- to 4-leaf stage), and one-third at midseason or when seed stalks are visible.

Onion crop can be successfully produced on most fertile soils. The onion crop requires an alluvial or sandy loam soil with high level of organic matter for optimum vegetative growth. Nitrogen and potash are required through out the growing period for optimum growth. Phosphate is required for the leaf development. Nitrogen requirement for the crop is one of the most difficult issues to present efficiently. The onion crop is called gross feeders since they require high fertile soil to maintain maximum yield (Brewster, 1999). Typically yield increases with N application. A PH of 6 to 7 is considered to be favorable but the crop may also grow on relatively acid pit soil; however, yields are severely reduced by soil salinity and they are among the most sensitive crops in this regard (Tindal, 1983).

In Ohio state, USA, Application of 120 kg N per ha significantly increased the number of flower stalks, umbel size, seed yield and seed germination. Potash alone had no effect but in combination with N gave positive results. Applying 120 kg N coupled with 40 kg K per ha gave the best result in terms of the number of flower stalks, umbel size and seed yield. This combination produced the highest seed yield of 499 and 555 kg per ha and germination of 82 and 87% during first and second year, respectively (www.ag.ohio-state.edu).

As stated by Sutherland and Waverly (1995b), Onions are sensitive to ammonia, so formulations that contain high levels of ammonia should be avoided. However, fall foliar applications (soon after planting) of liquid ammonium nitrate have proven beneficial to onion growth, and may have the side benefit of controlling young weed plants. Total supplemental nitrogen needs may vary from 110 to 450 kg/ha, depending on soil and cropping history and irrigation efficiency. High rates of phosphorus (225 kg/ha) may be necessary if beginning soil levels are low or deficient; moderate rates are sufficient in other soils. Onions are not responsive to potassium in most California soils. 11 to 22 t/ha of composted manure are sometimes used to meet planting and early season N requirements and other nutrient needs (Ronald, 1999). According to Sutherland and Waverly (1995b) soil tests and tissue analyses are required for all nutrients, and preliminary quick test on tissues for N is needed. Soil analyses are the best indicators for phosphorus (P), potassium (K), and micronutrient needs, while tissue analyses combined with soil and cropping history are the best indicators for nitrogen (N) needs. For phosphorus, less than 8 to 10 ppm P with sodium bicarbonate extraction is a deficient level; for potassium, less than 80 to 100 ppm K with ammonium acetate extraction is a deficient level. Micronutrients, if needed, are most effectively applied at planting time, banded 5 to 10 cm below the seed or, if bulbs are planted, at the depth of the bulb bases but to the side of the planting row. For zinc, less than 0.5 ppm Zn by DTPA or dithizone extraction is a deficient level; zinc is commonly applied to onion seed crops, both at planting and as foliar application. Other micronutrient applications may be needed depending upon the specific micronutrient’s availability in the soil.

2.1.4 Water Requirement

Uniform application of water is needed to achieve maximum and quality seed yield. Furrow irrigation is most common practice in onion seed production in Ethiopia. Studies indicated that on sandy loam soil 5 cm deep water applied 3 to 6 days interval gave highest seed yield (Lemma and Shimelis, 2003). Miller et al . ( 1971) and McGillivray (1948) also showed that highest seed production occurred at Davis, California, when plants received more than sufficient irrigation. Likewise, Hawthorn (1951) obtained consistently higher seed yields with higher soil moisture. Jones and Mann (1963) stated that since most of onion plants grow in regions of deficient rainfall, irrigation is usually necessary for maximum yields.

2.1.5 Planting Date

Planting must be early enough for the plants to attain sufficient size to be vernalized before growth slows down with the cooler weather. Under Ethiopian condition, bulbs are generally planted in mid September in mid September to November (in the cooler season) which is conducive for flower stalk development and subsequent seed set (Lemma and Shimelis, 2003).

According to Thompson and Kelly (1957), length of day affects flowering and bulb formation in a certain varieties of onion. The best time for planting depends on locality, cultivar and methods of production employed. Early planting results in higher yields since it allows more leaves to develop before flower or bulb initiation. Before planting, it should be ensured that sufficient isolation distance is maintained around the seed field. Since onions are pollinated by insects, fields for seed production should be separated from same cultivar by a minimum of 600m to 800m (Lemma and Shimelis, 2003).

2.2 Onion Seed production

2.2.1 Bulb-to-Seed Method

The bulb-to-seed method of seed production is when the onion bulbs are grown, harvested, stored and replanted for seed production purpose. This method permits the grower to easily discard off-types, diseased or otherwise undesirable bulbs prior to planting the bulbs for seed production purpose. It is practiced primarily for maintenance of onion seed stock of the same cultivars. More flower stalks are produced on plants grown from bulbs than those grown from seeds (Lemma and Shimelis, 2003). Mother bulbs are vernalized by cool temperature for one to two months during storage for flower initiation. The vernalized bulbs are planted at a depth of 10 to15 cm. The rate of progress to seed ripening increases with temperature and about 600 degree days above 100C are needed from anthesis to ripening of seed (Brewster, 1999).

2.2.2 Seed-to-Seed Method

The seed-to-seed method is when seed is planted in the summer, carried over through the winter as bulbs in the field, followed by flowering in the spring. This method results in higher seed yields than the bulb-to-seed method because there are more plants per hectare (acre) and less time is invested in bulb establishment. While overall costs are less than the bulb-to-seed method, careful attention must be given to pest control. Roguing programs to eliminate off-type plants are also required (Jones and Mann, 1963).

In Bangladesh, bulb to seed method is an established practice for seed production. A comparative study was conducted to produce seeds in bulb to seed method and seed to seed method during 1987/88 by Amzud and Jamiul (1993). Accordingly, some 616.7kg/ha of seeds were produced from bulb to seed method as compared to 34.9kg/ha seeds from the seed to seed method of production. Benefit cost ratio confirmed superiority of bulb to seed method of onion seed production.

Research experience at Melkasa research center showed that bulb to seed method of seed production has greater advantage over the seed to seed method of seed production. Several flower stalks of 2 to 17 were formed per bulb with 250 to 1000 florets per stalk. It produced large and well-developed umbels that are favorable for high and quality seed production. It takes 5 to 6 months for stalk development and seed development. But from seed to seed method of production employed it was obtained only 2 to 3 flower stalks per plant with very low seed yield (Lemma and Shimelis, 2003).

[...]