Breeding of Vegetable, Spice and Tuber crops (2+1)

Onion

• Onion Allium cepa L. is an important vegetable crop grown throughout the world. It is a cool season vegetable. It is grown for its bulbs.
  
• In India, the most important onion growing states are Maharashtra, Tamil Nadu, Andhra Pradesh, Bihar and Punjab. Onion is one of the crops which India exports.
• A global review of area and production of major vegetables shows that onion ranks second in area and third in production in the world.
  
• About 25.40 million tonnes of onion is produced in the world from 1.79 million hectares of land. India ranks first in area and second in production (0.29 million hectares and 2.45 million tonnes).
  

Origin

• The Russian scientist Vavilov reported Central Asia as the primary centre of origin and near east as the secondary centre of origin. Onion originated from the region comprising north-west India, Afghanistan, the Soviet Republic of Tazik and Uzbek and Western Tiensens. Western Asia and the area around Mediterranean sea are its secondary centres of origin. It is now cultivated throughout the world.
• Ploidy level= 2n=2x=16

Floral biology

• The flower structure is called an umbel which is an aggregate of many small inflorescences (cymes) of 5 to 10 flowers, each of which opens in a definite order causing flowering to be irregular and to last for two or more weeks.
  
• Each individual flower contains 6 stamens, 3 carpels united into one pistil and 6 perianth segments.
  
• The pistil contains 3 locules each of which has 2 ovules.
  
• The flower also contains nectarines which secrete nectar to attract insects for cross-pollination.
• The flowers are protandrous and anthers shed pollen over a period of 3-4 days prior to the time when full length of style is attained. Anthesis occurs in early morning (6-7 AM).
• Anther dehiscence is between 7.00AM and 5.00 PM and on next day also with peak between 9.30AM and 5.00 PM. Pollen fertility is maximum on the day of anthesis.
  
• Thus, stigma becomes receptive 3-4 days after shedding of pollen grains and protandry leading to favour cross pollination.

Breeding goals

• Long bulb storage life
• Resistance to diseases (purple blotch, basal rot, stemphyllium blight, bacterial storage rot)
• Resistance to insect pests (thrips)
• Resistance to abiotic stresses (moisture stress, high temperature, salinity, alkalinity)
• Bulb quality (size, shape, colour, pungency, firmness, dormancy, amount of soluble solids)

Cytogenetics

• The Allium cepa species are diploid with basic chromosome number of x=8 (2n = 16). Occasional tetraploids have also been reported.
  
• There are morphological and cytological similarities between the species of section cepa, but still strong crossing barriers exist between them.
  
• This prevents gene flow between the two even where sympatric distribution of two species occurs.
  
• Introgression of genetic material from wild to cultivated species is also difficult. Low success has been recorded in several interspecific hybridizations.