THE SUCCESS and sustenance of hybrid seed technology depends on the efficient and economic hybrid seed production, so as to provide good quality seed at reasonable price to the farmers. Various requirements for successful seed production such as ideal locations/seasons, seed rate, row ratio, supplementary pollination techniques etc., have been worked out and a seed production package has been developed.
Hybrid seed began with maize in the 1920s, and became extended to vegetables and flowers; and more recently, rice, cotton and some other crops. Hybrid seeds are produced from naturally out-breeding crops, from which inbred lines are produced by repeated self-pollination. The established inbred lines are being crossed to produce first generations (F1) hybrid seeds.
The hybrid seeds are prized because they produce uniform plants benefiting from the effect called heterosis (hybrid vigor). Heterosis can result in a large increase in yield over the inbred lines or comparable lines that are out-crossing. The precise basis of heterosis is still unclear, but epistasis and over-dominance are thought to be involved. Epistasis is the interaction between different genes, and over-dominance is a condition where the heterozygotes (genes represented by two different versions) are superior to either homozygotes (gene represented by the same versions).
The F1 hybrid seed is heterozygous in many genes.Hybrid seed is planted to produce a crop that is harvested for use. Saving seed from the crop and planting it is undesirable because the two different versions of the genes in the F1 hybrid segregate out in the offspring, producing an extremely variable progeny. In other words, the superior qualities of the F1 hybrid will have all disappeared.
Thus, only the seed companies produce hybrid seeds, and farmers must buy those seeds from the company every year.Hybrid seed production involves the crossing of a female line to a male line. Either line can be the female or male parent, but normally the best seed yielder is selected as the female parent. Both parents should be pure, preferably being self pollinated for more than six generations (this is called inbreeding).
The inbred parents are selected for their desirable traits e.g., high yields, disease resistance, fruit quality, earliness, etc. Ratio of male to female plants is also important in hybrid seed production. It is necessary to have plenty of pollen available for making hybrid crosses. Normally, a ratio of one male for every four female plants is recommended.Self-pollination cannot be allowed in hybrid seed production. The female flower must be pollinated by the pollen from the male line. To prevent self-pollination, remove the stamens from the flower buds of the female line before they shed their pollen. This process is called emasculation.
The female flowers are emasculated usually with a forceps at 2 days before anthesis. Too early emasculation can damage the bud and too late stage increases the chances of selfing. Pollination is done first thing in the morning. The emasculated flowers are pollinated on the day of flowering. It is important to introduce enough pollen onto the stigma to ensure high seed set because normally pollination is done only once in a flower and one pollen can give only one seed. Fully matured fruits with full color are harvested and any selfed fruit found should be discarded. The fruits can be immediate seed extraction or kept in a cool place for 3- 4 days for post-harvest maturation before extraction.
The development of hybrid seed had left seed production to seed companies for the practical reason that it is the most economical way to maintain appropriate inbred lines. But it prevents farmers from saving and replanting seeds, making it necessary to purchase seeds every season.Advantages of Hybrid Seed ProductionHybrid varieties have many advantages compared to open-pollinated varieties. Hybrids usually produce higher yields; they generally mature earlier and more uniformly; and many hybrids have better fruit quality and disease resistance.
With all of these advantages, many farmers prefer to sow hybrid seeds in spite of the higher seed cost.Constraints of Hybrid Seed ProductionHybrid seed production is not so easy because it requires much labor but fortunately, this is not a problem in developing countries like Pakistan, where affordable labor is available. It requires the mastery of special skills and a close attention to detail. And ideal growing conditions are needed to produce high quality seed. Seed production during the rainy season generally leads to low yields and poor seed quality.
Consequently, many organic growers want to grow open-pollinated as well as hybrid cultivars. To be considered a hybrid, the seed must be the offspring of two parents that differ in one or more heritable characteristics. For example, a green bean cultivar that has excellent flavor but is susceptible to a certain pest and a different cultivar that is tolerant of the same pest could be crossed, and the new hybrid might have both excellent flavor and be resistant to the pest, provided the parents are homozygous. Seeds collected from a grown hybrid cultivar will not be true to type when replanted and thus cannot be saved to plant next year’s crop.
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