onion; genus allium

8/14/2019 Onion; Genus Allium

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Onion; genusAlliumB.N: Allium cepaFamily:

USES; Onions, one of the oldest vegetables, are found in a large number of recipes and

preparations spanning almost the totality of the world's cultures. They are now availablein fresh, frozen, canned, caramelized, pickled, powdered, chopped, and dehydrated forms.

Onions can be used, usually chopped or sliced, in almost every type of food, including

cooked foods and fresh salads and as a spicy garnish. They are rarely eaten on their own,

but usually act as accompaniment to the main course. Depending on the variety, an onioncan be sharp, spicy, tangy and pungent or mild and sweet.

Historical uses:

Raw Onions

Nutritional value per 100 g (3.5 oz)

Energy 166 kJ (40 kcal)

Carbohydrates 9.34 g

Sugars 4.24 g

Dietary fiber 1.7 g

Fat 0.1 g

saturated 0.042 g

monounsaturated 0.013 g

polyunsaturated 0.017 g

Protein 1.1 g

Water 89.11 g

Vitamin A equiv. 0 g (0%)
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Thiamine (Vit. B1) 0.046 mg (4%)

Riboflavin (Vit. B2) 0.027 mg (2%)

Niacin (Vit. B3) 0.116 mg (1%)

Vitamin B6 0.12 mg (9%)

Folate (Vit. B9) 19 g (5%)

Vitamin B12 0 g (0%)

Vitamin C 7.4 mg (12%)

V itamin E 0.02 mg (0%)

Vitamin K 0.4 g (0%)

Calcium 23 mg (2%)

Iron 0.21 mg (2%)

Magnesium 0.129 mg (0%

Phosphorus 29 mg (4%)

Potassium 146 mg (3%)

Sodium 4 mg (0%)

Zinc 0.17 mg (2%)

[edit] Eye irritation

As onions are sliced or eaten, cellsare broken, allowing enzymescalled alliinases to

break down amino acidsulphoxides and generatesulphenic acids. A specific sulfenic

acid, 1-propenesulfenic acid, formed when onions are cut, is rapidly rearranged by asecond enzyme, called the lachrymatory factor synthase or LFS, giving syn-propanethial-

S-oxide, a volatile gas known as the onion lachrymatory factor or LF.[20] The LF gas

diffuses through the air and eventually reaches theeye, where it activates sensory

neurons, creating a stinging sensation. Tear glands produce tears to dilute and flush outthe irritant.[21] Chemicals that exhibit such an effect on the eyes are known as

lachrymatory agents.
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Eye irritation can, therefore, be avoided by cutting onions under running water or

submerged in a basin of water. Rinsing the onion and leaving it wet while chopping may

also be effective. Another way to reduce irritation is by chilling, or by not cutting off theroot of the onion (or by doing it last), as the root of the onion has a higher concentration

of enzymes.[22] Using a sharp blade to chop onions will limit the cell damage and the

release of enzymes that drive the irritation response. Chilling or freezing onions preventsthe enzymes from activating, limiting the amount of gas generated.Eyeirritation may be

avoided by having a fan blow the gas away from the eyes as the onion is being cut. The

amount of sulfenic acids and LF released, and the irritation effect, differs amongAllium

species. On January 31, 2008, theNew Zealand Crop and Food institute created a strain

of "no tears" onions by usinggene-silencingbiotechnology to prevent synthesis by the

onions of the lachyrmatory factor synthase enzyme.[23]

[edit] Varieties

Brown and white onionsYellow onions Flower head of a yellow onionRed onions

Production trends

Onion field during harvest, Vale, Oregon (USA).
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Top Ten Onion Producers 2005

(1000 tonnes)

India 9,793

China 5,500

Australia 4,003

United States 3,346

Turkey 2,220

Pakistan 1,764

Russia 1,758

South Korea 1,750

Japan 1,637

Spain 1,149

World Total 64,101

Source:UN Food & Agriculture Organisation(FAO)[24]

AVRDC Extension Materials
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OnionCultivation

Contents

Introduction Weed control

Cultivars Disease control

Soils and climate Insect pest control

Planting systems Harvesting

Sowing Curing

Nutrient management Storing

Water management Marketing

Introduction

Onion (Allium cepa) is a popular vegetable grown for its pungentbulbs and flavorful leaves. It is widely grown throughout the world.

The bulb is composed of concentric, fleshy, enlarged leaf bases orscales. The outer leaf bases lose moisture and become scaly andthe inner leaves generally thicken as bulbs develop.

The green leaves above the bulb are hollow and arise sequentiallyfrom the meristem at the innermost point at the base of the bulb. Thestem is very small and insignificant during vegetative growth. Aftervernalization at temperatures below 10C, the stem elongatesrapidly, eventually producing compound umbels. Bolting has beenreported to be related to the length of day. However, long days donot induce reproductive growth but tend to accelerate developmentof the seedstalk once it has been initiated by vernalization.Temperature has a major role in inducing bolting.

The onion root system is fibrous, spreading just beneath the soilsurface to a distance of 30 to 46 cm. There are few laterals, and totalroot growth is sparse and not especially aggressive. Therefore, inmonoculture, onions tolerate crowding, particularly in loose, friablesoils such as peat and muck. Competition from aggressive rootsystems (as from weed growth) severely limits onion growth.

Cultivars differ substantially with respect to the threshold daylengthrequired for bulbing. Other factors such as temperature may interactwith daylength to modify the bulbing response. In all cultivars,bulbing is accelerated with increasing temperature.
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Temperature extremes not only affect the rate of bulbing, but alsoaffect the bulb shape. Thick and elongated necks are common inplants exposed to 6 or lower.

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Cultivars

At one time, all onion cultivars were open-pollinated, and many ofthese cultivars are still offered by seed companies. The discovery ofmale sterility in onion led to a rapid change to F1 hybrids, possiblydue to simplicity and low cost of seed production. Male sterility is agenic-cytoplasmic factor, and male fertility can be restored in plantscarrying the sterility factor by introducing a single dominant allele.Any line carrying the sterile trait must be cross-pollinated, and seedsharvested from male sterile plants isolated with a normal pollen-

bearing parent will be hybrid seed. Hybrids have higher yield, largerand more uniform bulb sizes than open-pollinated cultivars.

The bulb onion cultivars are grouped into short, intermediate, andlong-day types. Short-day onions (12 to 13 hour threshold) aregenerally mild, soft fleshed, and suitable for storage. Long-dayonions (over 14.5-hour threshold), if grown in the lower latitudes, willnot form bulbs, and only green onions would be produced. Incontrast, short-day types grown in the higher latitudes will bulb veryquickly and will be little more than sets in size.

Short-day onions include the Bermudas and Grano-Granex types;long-day cultivars include yellow, white and red globes. Theintermediate-day cultivars (13.5 to 14.5-hour threshold), are relativelysoft-fleshed and used primarily for fresh trade. They are grown inareas of mild temperatures lying between 32 and 38 latitudes.
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Types of onion cultivars (open-pollinated cultivars are underlined)

Daylength Color Pungency Representative cultivars

Short Brown Sweet Awahia

Red Sweet Red Granex

Red Pungent Red Creole

White Sweet White Granex, Crystal Wax

Yellow Sweet Grano, Granex

Yellow Pungent Yellow Creole

Intermed. Brown Moderate Cochise Brown

Red Moderate Stockton Early Red

White Moderate Fresno White

Yellow Moderate Rialto

Long Brown Pungent Australian Brown

Red Pungent Carmen, Southport Red Globe

White Pungent White Lisbon, Ivory

Yellow Sweet Fiesta, Sweet Spanish

Yellow Pungent Autumn Spice,Downing Y. Globe

True pearl onions are classified asAllium ampeloprasum becausethey form just one storage leaf. In practice, short-day onion cultivars,such as Grano, Crystal Waxand others, when grown in northernlatitudes, will develop pearl-size bulbs and be marketed as such.

Most are used in pickling or in frozen mixtures of peas, broccoli andother vegetables.

Green onions, scallions, multiplier, and bunching onions are all usedin the immature stage. Green onions generally are bulbing-type,white cultivars harvested at the miniature bulb stage. Scallions arewhite cultivars ofA. cepa that do not form bulbs. Multiplier onions arecultivars ofA. cepa of Group Aggregatum with white flesh and yellowor brown scales. These are distinguishable from the shallot by thelatter's red scales and more delicate flavor. The shallot can be usedboth in the immature stage and as a dry bulb. A cross of shallot with

pink-root resistantA. fistulosum gave rise to the tetraploid cultivarBeltsville Bunching.A. fistulosum includes the bunching onions, alsocalled Japanese bunching onions or Welsh onions.

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Soils and climate

Onions can be grown successfully on any fertile, well-drained, non-crusting soil. The optimum pH range, regardless of soil type, is 6.0 to6.8, although alkaline soils are also suitable. Onions do not thrive in

soils below pH 6.0 because of trace element deficiencies, oroccasionally, aluminum or manganese toxicity.

Onion is cool-season biennial, and is tolerant of frost. Optimumtemperatures for plant development are between 13 and 24C,although the range for seedling growth is narrow, 20-25C. Hightemperatures favor bulbing and curing.

Prior to planting, soils should be plowed and disked sufficiently toeliminate debris and soil clods. In most commercial areas, beds 0.9to 1.0 m wide are formed, and two to six rows are seeded or planted

on the bed. If two rows, they may be two-line (twin) rows with plantsstaggered to achieve proper spacing and high population density.

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Planting systems

Three systems of planting may be employed:

1. Direct seedling is preferred and gives excellent results wherethe season is sufficiently long to provide early prebulbinggrowth.

2. Transplants normally have three to five well-formed leaves attransplant time. Transplant leaves are pruned during growthprior to field setting, facilitating handling and increasing planthardiness.

3. Sets are used in some areas to ensure large bulb size anduniform maturity. Sets are small dry bulbs, approximately 12mm in diameter, produced the previous season by seedingthickly or growing under conditions favoring rapid bulbing.

Any of the above systems may be used for early green onionproduction.

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Sowing

The ultimate yield of onion is determined by the number of leavesthat are formed prior to bulbing. Since bulbing in each cultivar istriggered by a specific daylength, early planting is the most effectivemethod of improving bulb size and is a primary factor contributing
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toward yield. If, however, early planting coincides with cool airtemperature or cool wet soils, the stand and ultimately the maturity ofthe crop will be erratic. Some cultivars of the Bermuda type also maybolt if substantial growth precedes exposure to cool temperature.

Seeds are sown 6 to 18 mm deep in heavy mineral soils, deeper inlight mineral soils and mucks. Excessively thick seedlings of bulbonions may delay maturity, however necks tend to be thinner than insparse seedlings, and bulbs are somewhat more globular in shape.

Using coated seed (shown) and precision seeding, the seeding ratecan be adjusted easily for projected bulb size. For normal storageonions, seeds are spaced 7.5 cm apart. When small boiling, picklingor pearl onions are desired, spacing would be reduced to 2.5 cm inthe row. Large bulb size is promoted by spacings of 10 cm or more.

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Nutrient management

Onion responds very well to organic manure. Organic manure at 25to 40 t/ha is recommended to obtain high bulb yield.

Fertilizer is applied either as a broadcast, or more commonly, as aband 5 to 10 cm directly below the seed set or transplant. Onionplants utilize substantial amounts of nutrients. Based on a yield of 18t/ha of bulbs, the plants remove an average of 66, 11, and 70 kg ofN, P, and K respectively. Soils differ widely in fertilizer needs,depending on production history, soil type, and analysis.

Mineral soils on average contain 90 to 112 kg/ha of N and 56 to 168kg/ha of P2O5 and K. N, P, and K application of about 160, 90, and40 kg/ha, respectively, is recommended for mineral soils. One or twoside dressings of nitrogen are applied during a season. These side

dressings may be applied through the irrigation system. Fall seededonions require only P2O5 before seeding and require N when activegrowth starts in the spring and twice thereafter. Insufficient N willinduce early maturity and reduce bulb size; high N may increase bulbsize and cause large nicks and soft bulbs with poor storage quality.

If heavy fertilization rates are indicated by soil tests, the materialshould be incorporated throughout the plow layer, or if banded,placed 15 cm to the side of the row.

Minor element deficiencies, particularly zinc and copper, may be

encountered. Suggested corrective rates are 11 kg/ha of zinc or 17to 28 kg/ha of copper, applied every two to three years. Relativelyhigh levels of sulfur are utilized by onions, but corrective applicationsvary widely, according to soils, leaching losses, and presence ofsulfur contaminants in the atmosphere. If applied, sulfur will acidifythe soil, and therefore, liming rates should be adjusted accordingly.

Similar fertilizer ratios are recommended for green onions; however,due to short growing period, the application rates would be reduced.

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Water management

Onions require uniform moisture throughout the growing season.Fields that suffer growth retardation may produce excessivenumbers of doubles or splits, reducing the number of Grade 1 bulbs.Furrow irrigation is generally used. Light sandy soils are irrigatedwith overhead systems or by subsurface seep irrigation where the


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soil profile allows. Onions at the bulbing stage utilizing substantialamounts of water, although excessive moisture must be avoidedduring the growing season.

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Weed control

Onions are not good competitors with weeds. Cultivation, if used,must be shallow to avoid root damage, and growers usually favorchemical control. Pre-emergent broadcast applications of DCPA orone of several organic compounds have been used with somesuccess.

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Disease control

Both field and storage diseases reduce profitability. Field diseasesinclude purple blotch, Stemphylium blight, anthracnose, downymildew, Botrytis leaf blight, pink root, smut, smudge, and severalbasal rots. Storage diseases include some of the common field rots,black mold, botrytis neck rot, and bacterial soft rot.

Purple blotch (Alternariaporri) attacks onion, garlic, shallot andother Allium crops. Initially, small white sunken spots develop on theleaves. These enlarge, become zonate and under moist conditions,turn purple. These are also prominent on the inflorescence stalks.Infection can cause a semi-watery rot on necks of bulbs that turnyellow-red in color. Infected bulb tissues eventually become papery.This pathogen is widespread. The optimum temperature for diseasedevelopment is 21 to 30C. Therefore, it is most serious in hot,humid climates.

The fungus is seed-borne, but the relevance of this phase in initiatingdisease outbreaks in hot climates is not well documented. Infectedonion debris has been implicated as an infection source.

Host-plant resistance is yet to be exploited. Some cultivars appear tobe less susceptible, e.g. Red Creole. Taliana Redin Hungary isreported to be resistant.

Cultural control methods include long rotations with unrelated cropsand good drainage. Lowering the density of transplanted crops willreduce infection, as will the application of high rates of calciumsuperphosphate and potassium fertilizer. Nitrogen fertilizer at lowand high rates will incrase the prevalence of disease. Routine(weekly interval) field sprays with dithiocarbamate fungicides,


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particularly mancozeb and chlorothalonil have been reported to beeffective.

Stemphylium leaf blight (Stemphylium vesicarium) has beenreported from Europe, Africa, North and South America, and Asia.Foliage losses of 80 to 90 percent have been reported. Diseasesymptoms are very similar to purple blotch. Lesions are light yellowto brown, watersoaked and progress from the tip to the base ofleaves. The conidia have up to six transverse septa, besides severalvertical septa. Wet and warm conditions favor the disease spread.Control measures are similar to purple blotch.

Anthracnose (Colletotrichumgloeosporioides) favors hot (24 to29C) and wet conditions. The disease overwinters in sets and soil,and spores are spread by wind, splashing water, and tools. Theleaves become twisted due to infection.

Downy mildew (Peronosporadestructor) also attacks young plants,appearing as white specks, usually confined to the oldest leaves ofyoung plants,. A white mold develops rapidly in cool damp weatherand progresses down the sheath, and plants eventually fall over anddry up. The fungus overwinters in bulbs and sets and on plant debris.Spores are carried long distances by air currents. For control, youngplants can be treated with mancozeb at weekly intervals until bulbingbegins.

Botrytis leaf blight, commonly termed blast, is caused by severalBotrytis species. The disease first appears as white specks onleaves, expanding to cause a dieback from the leaf tips. Tops maybe killed completely within a week, and entire fields may be affected.Frequently, blight follows previous damage from insects, disease,mechanical damage, or air pollution. Control is achieved throughmancozeb sprays at approximately 7-day intervals.

Several bulb rots may occur either in the field or in storage. Basalrot, caused by Fusarium species, results in a breakdown of innerscales. Outwardly, the bulb may appear normal. It eventuallybecomes soft, however, and will develop a watery rot under moistconditions or a dry shriveled bulb in a dry environment. The diseaseis most severe in warm areas with poor soil drainage. Botrytis neck

rot (shown) is an extension of the leaf blight disease and canbecome serious in storage.

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Insect pest control

Thrips(Thripstabaci) are minute insects that cut or "rasp" theepidermis of leaves or stems and suck the plant sap resulting in
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white blotches on leaves. Severe infestations result in leaf blastingand collapse. Bulbs become distorted and undersized. Infestationsare more severe in dry seasons than in moist, and entire fields maybe destroyed. The insect has many host plants. Adults and nymphsoverwinter on plants or plant debris, or in weeds bordering the field.

Most of the insects are female, which can reproduce without a male.Eggs are thrust into the leaves and will hatch in 5 to 10 days.

Diazinon sprays at 7-10 day intervals are recommended to controlthrips. Up to six applications may be necessary and good coverageis essential.

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Harvesting

Onions are ready for harvest when the leaves collapse. For storage,

onion tops should have broken over before harvest and the necksshould collapse and dry. Storage bulb maturity can be acceleratedby withholding irrigation water or by pruning the root system. Bulbsfor storage may be harvested when 50 percent of more of the topshave broken over, but the bulbs must be cured and dried thoroughlybefore being placed in storage. Bulbs intended for immediate usecan be undercut when 15 to 25 percent of the tops are down.

To harvest, first a knife or lifter is drawn under a bed or row, cuttingroots and loosening the soil. Then the bulbs may be dug or allowedto cure further before digging. Under dry conditions, bulbs may beleft to cure in the field, either in place or in windrows. To avoid

damage from direct sunlight, however, onions normally are placed infield containers and moved to a dry shady location for subsequentcuring.

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Curing

The purpose of a curing period is to allow natural dormancy todevelop and to dry the onion sufficiently. A properly cured onion willhave a dry shrunken neck and dry outer scales. The respiration rateof a cured bulb is lower than that of an uncured bulb. Fully mature

bulbs are harvested and cured by exposure to temperatures up to35 in low (less than 50 percent) relative humidity. Air movementmust be provided at the rate of 1 cubic foot/minute/cubic foot ofonions (60 cubic meters/hour/cubic meters of onions). Immatureonions require twice the rate of air exchange.

Following curing, the temperature of stored onions is loweredgradually to 0C, or slightly higher, with the relative humidity at 60 to


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70 percent. Air exchange in the storage facility is important toprevent any condensation on the bulbs. Also, when the bulbs areremoved from the storage, they should be conditioned for severaldays at 20 and 50 percent relative humidity.

The tops and roots are removed during harvest. When this is notpossible, they should be removed after curing, before storage orsale.

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Storing

Freshly harvested onions are dormant and will not sprout for avariable period of time (depending on cultivar). Storage will prolongthis dormancy. Sprouting will increase in storage temperatures

above 4.4C, decreasing again as temperatures exceed 25C. Toreduce the frequency of sprouting after the rest period, onions maybe field treated with maleic hydrazide (MH-30) at 2.2 to 3.4 kg/hawhen the tops are still green but beginning to senesce.

Cultivars intended for long-term storage should be firm with a thickdry neck; free from greening, root growth, sunburn, or freezedamage; and well covered with dry scales. Bulbs with fleshy, softnecks are susceptible to persistent rot, especially if storage humidityexceeds 70 percent.

Flavor in onion is associated with pungency (propyl disulfides andother disulfides) and with sugars (glucose, fructose, and sucrose).Both sugar content and pungency are related to percentage drymatter. Short-day and long-day types differ in their flavors. Pungencyand dry matter content are important quality attributes in onions forprocessing.

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Marketing

Onions normally are shipped in 22.7-kg mesh bags. The bulbs are

graded by size, with jumbo and pearl sizes frequently used byprocessors. Those intended for international trade are packed in 25-kg bags.

Green onions are pulled before bulbing, when the basal diameterexceeds 6 mm, and the roots are trimmed near the base. Theyshould be washed free of soil. Discolored stalks are discarded. Thegreen onions are bunched and packed in ice to preserve crisp


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texture and quality. Vacuum cooling is possible but requiresprewrapping in ventilated polyethylene bags to retard wilting. Storagelife of green onions is limited to approximately one week at 0C and90 to 95 percent relative humidity.

Shallots are harvested by hand when the bases are at least 6 mm indiameter. The outer leaf is stripped off and the roots are trimmedbefore washing and bunching. If grown for dry bulbs, they arehandled in a similar manner as onion bulbs.

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Taken from Onion Cultivation and Seed Production, written by S. Shanmugasundaram. Edited by T.Kalb. 2001. This document is also available as a Training Guide in PDF format.
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onion; genus allium

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