Hybrid seed production its methods & benefits
In agriculture and gardening, hybrid seed is seed produced by cross-pollinated plants. Hybrid seed production is predominant in modern agriculture and home gardening. It is one of the main contributors to the dramatic rise in agricultural output during the last half of the 20th century. The alternatives to hybridization are open pollination and clonal propagation. All of the hybrid seeds planted by the farmer will produce similar plants, while the seeds of the next generation from those hybrids will not consistently have the desired characteristics. Controlled hybrids provide very uniform characteristics because they are produced by crossing two inbred strains. Elite inbred strains are used that express well-documented and consistent phenotypes (such as high crop yield) that are relatively good for inbred plants.
Hybrids are chosen to improve the characteristics of the resulting plants, such as better yield, greater uniformity, improved color, disease resistance. An important factor is the heterosis or combining ability of the parent plants. Crossing any particular pair of inbred strains may or may not result in superior offspring. The parent strains used are therefore carefully chosen so as to achieve the uniformity that comes from the uniformity of the parents, and the superior performance that comes from heterosis.
Hybrid is produced by crossing between two genetically dissimilar parents. Pollen from male parent (Pollen parent) will pollinate, fertilize and set seeds in female (seed parent) to produce F1 hybrid seeds. For production of a hybrid CROSSING between two parents is important, the crossing process will results in heterosis. In self pollinated cross it is difficult to cross but in cross pollinated crops it is easier.
In nature to create genetic variability and for its wider adaptation in different environmental conditions, flowering plants has adopted many mechanisms for cross pollination. Cross-pollination results in genetic heterogeneity and show wider adaptations.
Flowering plants have evolved a number of devises to encourage cross-pollination. Those mechanisms are;
1. Dicliny:Flowers are unisexual. In monoecious plants male and female flowers are borne on the same plant eg., cucurbits, maize, castor and coconut. In dioecious plants male flowers are borne on different plants eg., papaya, cannabis, mulberry.
2. Dichogamy:Time of anther dehiscence and stigma receptivity are different forcing them for cross-pollination. The time gap between the two may vary from one day to many days. In protoandry anthers dehisce earlier than the stigma receptivity eg; maize, sunflower. In protogyny stigma become recetive earlier than the anther dehisce eg., Pearl millet mirabilis.
3. Self-incompatibility:self fertilization in avoided by recognizing the self pollen by the stigma. Eg., Brassica, Petunia, Lilium .
4. Herkogamy:there is spatial separation of the anthers and stigma. Their relative position is such that self fertilization
cannot occur. The stigma projects beyond the anthers and therefore pollen cannot land on stigma. Eg., Lucerne stigma is covered with a waxy film. The stigma does not become receptive until this waxy membrane is broken by visit of honeybees resulting in cross-pollination.
5. Male sterility:Absence or atropy or mis or malformed of male sex organ (functional pollen) in normal bisexual flower. Male sterility is of three types: genetic male sterility, cytoplasm sterility and cytoplasmic- genetic male sterility.
6. A combination of two or more of the above mechanisms may occur in some species. This improves the efficiency of the system in promoting cross-pollination Method of Hybrid Hybrid –rice can be produced in the following ways. 1. Three –line system. The hybrid seed production involves multiplication of cytoplasmic – genetic male sterile line (A line) , maintainer line ( B line) and a restore line ( R line) and production of F1 hybrid seed ( AXR). 2. Two-line system. The hybrid seed production involves the use of photo-period sensitive genetic male sterile (PSMS). Any normal line can serve as a restorer. 3. By using chemical emasculators. Chemicals that can sterilize the stamen, with little or no effect on the normal functioning of the pistil, can be used to emasculate female parents for hybrid rice production. The advantages are obvious, no special development of male sterile or restore lines is required and extensive varietal resources are available. Chemical emasculators such as male gametocide 1 (ME1) and male gametocide 2 ( MG2) were developed in China and have been successfully used in hybrid rice production.
In chemical emasculation, physiological male sterility is artificially created by spraying the rice plant with chemicals to induce stamen sterility without harming the pistil. In hybrid seed production , two – varieties are planted in alternate strips, and one is chemically sterilized and pollinated by the other. Benefits of Hybrid Breeding Benefits of Hybrid Breeding Benefits of Hybrid Breeding Benefits of Hybrid Breeding Benefits of Hybrid Breeding Benefits of Hybrid Breeding Hybrids represent around 60 % of the oilseed rape grown in the United Kingdom today. The area has increased over the last few years as new, improved varieties become available, and farmers realise the real benefits hybrids have to offer. New hybrid breeding techniques allow greater genetic diversity to be introduced to varieties leading to enhanced levels of success in producing desirable agronomic characteristics.
Yield Advantage In Germany, hybrids account for more than 84% of the oilseed rape area, 77% in France and about 99% in North America. These modern hybrids are helping average yields push upwards to 5t/ha. Farmer surveys have shown that on average, a hybrid can produce around 350-500kg/ha more than a conventional variety. Oil Content With bonuses being paid at 1.5% of the contract price for every 1% extra oil over 40% the importance of oil content to the overall profitability of a crop can not be ignored. Oil content is a very stable characteristic and with the increased reliability of seed yield given by a DSV hybrid this added benefit should not be underestimated. Hybrid Vigour It is well known that in general hybrids are more vigorous than conventional varieties. This means they are more able to compensate in the field under difficult growing conditions. Autumn vigour helps with establishment as well, closing crop gaps to reduce the number of landing areas for pigeons and recovery from weather damage after the winter. Spring vigour means a fast growth rate therefore the plants are less prone to pest damage. They also recover better from weather damage after the winter. This vigour also helps the plants out compete weeds if herbicide efficacy is poor, a common problem under adverse conditions. Wider Sowing Window Hybrids appear to have better establishment across a wide range of drilling dates and conditions compared with conventional varieties. Most hybrids offer the possibility of a later sowing date, which can be very important in a difficult year and in helping to spread the work load after harvest.
Reduced Seed Rates Hybrids require lower seed rates to get the best performance. They perform best with a density of around 40 established plants/m². This allows the right canopy structure to establish and the lower population helps eliminate any risk of lodging. At the beginning of plant establishment a field of hybrid rape generally doesn’t look as dense as a conventional field which is often sown at double the rate. Reduced Plant Stress Hybrid oilseed rape plants tend to suffer less from environmental stresses than conventional varieties. Hybrids are particularly useful in less fertile, more challenging situations, as the technology has been used to develop characteristics that help the plant overcome stress such as a lack of water or extreme cold weather, as well as improving yields. They are also more responsive to inputs than conventional varieties. Drought Resistance and Root Structure Hybrids have well developed roots which prove to be beneficial in common summer droughts. This extensive root system also improves nutrient uptake. Hybrids generally offer better utilisation of water and nutrients. Stem Characteristics Breeders are trying to combine characteristics that reduce the height of hybrids, increase standing power as well as improving genetics. DSV are in the process of breeding hybrids which are shorter and stiffer, for instance, Primus at 155cm is one of the shortest non semi dwarf varieties available. The height of hybrids can easily be controlled with the use of a plant growth regulator which is normally part of every fungicide spray programme. Therefore, in treated trials only very little difference in height is seen between conventional and hybrid varieties.