Advanced Methodology For Hybrid Seed Production in Cotton    

By: Shehzad Ahmad Kang, Department of Plant Breeding and Genetics, University of Agriculture, Faisalabad, Pakistan.
 

Introduction

Pakistan is eyeing record cotton production of over 15 million bales in the 2011/12 crop year as farmers sow a wider area after domestic prices more than doubled from a year ago. Cotton and textile account for about two-thirds of the country's exports and a healthy cotton crop is vital to economic growth, which is expected to slip to around 2.75 percent in the current financial year to June, because of flood damages.

The world's fourth-largest cotton producer had hoped to produce 14 million bales in the 2010/11 crop year before the devastated summer floods last year reduced the output to about 11.70 million bales (Anonymous, 2011-12). There is more need of seeds / seed cotton (kapas) due to increasing population for the consumption of human. Breeder’s do more research work for the development of hybrid genotypes to enhance the cotton seed yield/ lint yield.

• Cultivated species of Cotton are two types viz.

Tetraploids of Cotton

• Gossypium hirsutum (2n = 52)
• Gossypium barbadense (2n = 52)

Diploids of Cotton

• Gossypium arboreum (2n = 26)
• Gossypium herbaceum (2n = 26)

Hybrids & Species

• Hybrids between the cultivated species of same ploidy level are fertile.
• Four intra specific hybrids (one in each species) and two inter specific hybrids of same diploid no. between Gossypium hirsutum x Gossypium barbadense and Gossypium arboreum x Gossypium herbaceum.
• Totally 6 different types of hybrids are theoretically possible.

Species from which hybrid developed

• Four types of hybrids are only developed for commercial cultivation,
• Intra specific hybrids in Gossypium hirsutum and Gossypium arboreum.
• Inter specific hybrids between tetraploids viz., Gossypium barbadense and Gossypium hirsutum and diploids G. arboreum x G. herbaceum.
• Inter specific hybrids between G. barbadense and G. herbaceum could not be released.

Hybrid seed production

• In cotton, there are two methods of hybrid seed production, viz.,
• Conventional method, and
• Male sterility based method.

These are briefly discussed below

Conventional Method

• Breeder or certified seed of male and female parents is used for the production of hybrid seed.
• Hybrid seed production is carried out by hand emasculation and pollination.
• This method ensures purity in seed production.
• The female and male parents are planted in the same field in separate plots in 4:1 or 3:1 ratio.
• Sowing of parental material is done in such a way that there should be nicking in the flowering time of both the parents.
• The off type plants are rogued out before initiation of crossing programme.
• The female and male parents are planted at wider spacing for easy movement during crossing.
• Higher doses of fertilizer and recommended plant protection measures are used to raise healthy crop to get continuous flush of flowers.
• Crossing work is started after one week of flower initiation.
• Flower buds which are likely to open the next day are chosen for emasculation.
• Anthers of selected buds are removed gently with the help of nail (emasculation) as suggested by Doak method.
• Emasculated buds are covered with tissue paper bag of red color to prevent natural out crossing.
• The best time for emasculation is 3:00 – 6:00 PM.
• Emasculated buds are pollinated the next day with the pollen of male parent.
• The best time for pollination is 8:00 – 11:00 AM, because stigma receptivity is maximum during this period.
• 4-5 buds are pollinated by one flower of male parent.
• After pollination, the red tissue paper bags are replaced by white tissue paper bags for identification.
• A label or thread is also tied on the pedicel for identification.
• Fertilization occurs after 12-30 hours of pollination.
• Cross buds should remain covered for 3-4 days after pollination.
• The straw tube used for cool drink is also used for covering the stigma of emasculated buds before and after pollination.

 Male sterility method

• This method is used for hybrid seed production of only those hybrids which have been developed through the use of male sterility. Use of male sterility reduces only the cost of emasculation. Pollination has to be done manually.
• Two types of male sterility systems are used in cotton, viz.,
• Genetic male sterility, and
• Cytoplasmic genetic male sterility.

Use of Genetic Male Sterility

• In cotton, Gregg male sterility source is used.
• The male sterility is transferred to the female parent through backcross technique.
• The male sterility is governed by two recessive genes (ms5 ms6).
• A heterozygous male fertile genotype which segregates only is identified.
• Cross of this male fertile genotype with sterile line will always produce male sterile and male fertile plants in 1:1 ratio.
• Fertile plants are identified after flowering are removed.
• The male sterile plants are pollinated with the pollen of male parent to get hybrid seed.
• In case of male sterile parent, 3-4 seeds should be sown per hill because 50% of the population (male fertile) is removed when flowering starts.

Use of Cytoplasmic Genetic Male Sterility

• In cotton, G. harknessii cytoplasm is used as a source of cytoplasmic genetic male sterility.
• The male sterility is transferred to the female parent and restorer gene to the male parent by backcross technique.
• The male sterile and restorer lines are planted in the same field but in separate plots in 4:1 or 3:1 ratio.
• The crop is grown at wider spacing under irrigated conditions to get continuous flush of flowers for seed production.
• Crossing is started after one week of flower initiation.
• The male sterile parent (female) is pollinated with the pollen of restorer (male) parent.
• After pollination, flowers are covered with tissue paper bags to avoid natural out crossing with other plants.

Materials required

• Tissue paper bags (10 x 15 cm) red and white colored or straw tubes. Magnifying glass,
• Tray,
• Thread,
• Note book and pencil etc.,

Crossing techniques

• In cotton, hybrid seed production is carried out by artificial crossing. The crossing refers to hand pollination. The crossing technique consists of three main steps, viz.,
• selection of bud,
• emasculation, and
• Pollination.

Selection of Bud

• The selection of flower bud for emasculation is an important step in hybrid seed production. The crossing work is initiated after one week of flower initiation. The flower buds of proper stage (buds which are likely to open the next day) are selected for emasculation. Such buds have generally cream color and are well developed.

Emasculation 

• The process of removal of anthers from the selected flower bud is referred to as emasculation.
• Anthers of selected buds are gently removed with the help of nail of the thumb as suggested by Doak (1934). The emasculated buds are covered with tissue paper bag of red color to prevent natural out-crossing. The best time for emasculation is 3-6 PM. Some people use straw tube to cover the ovary of emasculated bud. Emasculation is not required when hybrid seed is produced using male sterility.

Pollination 

• Emasculated buds are pollinated the next morning with the pollen of male parent. The best time for pollination is 8:00 – 11:00 AM, because the stigma receptivity is maximum during this period. Generally, 4-5 buds are pollinated by one flower of male parent. After pollination, the red tissue paper bags are replaced by white tissue paper bags. For identification, a label or thread is also tied on the pedicel of crossed bud for identification of crossed bolls.

Picking 

• Pick completely opened crossed boll (kapas) as and when ready in baskets and sort out.
• Any bolls without thread be kept aside and only genuine crossed bolls are kept separately for use.
• Remove hard locks, stained kapas (seed cotton) etc. keep good crossed boll kapas for processing.
• Dry well-cleaned kapas in shade after each picking and store in a good place picking-wise lots.

Ginning

• Slow ginning is to be practiced to recover good quality seed and without cutting the seed. After ginning, the seeds may be kept well spread, air dried, without heaping.

Standards for Certified Hybrid Seed 

• There are four basic requirements for production of certified hybrid seed. These are:
• Notified hybrid,
• Genetic purity,
• Physical purity,
• Proper germination. A hybrid released either by State Variety Release Committee or Central Variety Release Committee is selected for production of certified hybrid seed.
• It is also essential that the hybrid should be notified one.
• Genetic purity refers to absence of seeds of other variety. Genetic purity is determined by grow out test.

Advantages and Disadvantages

Advantages

• Superior yield performance coupled with superior fiber properties.
• Wider adaptability
• Good price for the produce
• Employment generation through labor intensive hybrid seed production.
• Short duration hybrids can fit into multiple cropping system under irrigated areas.
• By inclusion of cotton in double and multiple cropping, cotton will find place in crop rotation in new areas, thus contributing to increased area under irrigated cotton without impairing the area of other crops.

Disadvantages

• high cost of seed,
• high cost of cultivation,
• difficulty in seed production and
• Neps and motes especially in interspecific hybrids.

High Cost of Seed

• In cotton, the hybrid seed is usually produced by hand emasculation and pollination which is very costly.
• This high cost of seed can not be offered by small and marginal farmers.
• Even if the male sterile line is used, the pollination has to be done by hand. (Extent of natural out-crossing in Pakistan is about 6%)

High Cost of Cultivation

• The cultivation of hybrids is input intensive. Hybrids require more inputs in terms of fertilizers and pesticides than varieties. The high cost of cultivation can not be offered by small and marginal farmers. The high cost of seed and cultivation act as barriers in the expansion of area under hybrid cotton. Hence, there is need to develop cotton hybrids of low input technology.

Difficulty in Seed Production

• The diploid hybrids could not become popular among the farmers due to difficulty in seed production. The seed setting in diploid crosses is very low (about 25%) which posses problems in hybrid seed production.

Upland cotton hybrids in Pakistan

• Alsemi, White Gold

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Corresponding author’s email: shehzadpbg@gmail.com

Courtesy: PAKISSAN Team