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The following points highlight the five main steps of hybridization technique. The steps are: 1. Selection and Preparation of Parents 2. Emasculation 3. Bagging 4. Pollination 5. Selection.
Hybridization Technique # Step 1. Selection and Preparation of Parents:
The plant breeder must have a clear idea about the plants which he wants to use as parents — a subject which may be called crop botany. The time of flowering, the stage of flower development at which the anthers burst (anthesis) and the stigmas become receptive and also the time period for which the pollens remain viable and the stigmas remain receptive must be known beforehand.
Crop plants are mostly annual plants and in such plants the pollens remain viable for several days after anthesis, and so pollen viability is not a critical factor. But in annuals stigmas remain receptive for a short period, usually for several hours and very often for not more than a day. In many plants the stigma becomes receptive at a particular time of the day; as in rice, it becomes receptive in the morning, at around 8 a.m.
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The receptivity period of the stigma is a vital factor, because if pollination is not done within this period, fertilization normally does not occur. Similarly, if pollination is done with immature pollens or with pollens which have lost their viability, fertilization normally does not take place.
The plant breeder should take all necessary precautions so that no unwanted pollination occurs. This, in turn, depends upon the nature of the crop — whether it is a naturally self-pollinated or cross-pollinated crop, as also the percentage of cross-pollination.
To prevent any unwanted pollination the flowers are kept covered by bags long before they open. Necessity of isolation increases with increase in the percentage of natural cross-pollination.
The plant breeder first selects the two varieties which he wants to use as parents mid also determines which variety will be used as male parent and which one as the female parent. The two parents are grown in adjacent plots.
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The breeder must have prior knowledge about the cultivation of the crop, the natural growing season of the crop, the time of sowing of the seeds as also, the preparation of the land, i.e., ploughing, manuring and watering of the plot.
The anthers of the male parent should be just ripe when the stigma of the female parent become receptive. For this, sometimes it becomes necessary to space out the sowing of the seeds of the two parents.
In plants where the flowers are rather small and remain too much crowded on the inflorescence, it is necessary to remove some of the flowers early, so that the breeding work can be done easily. This is specially true in case of cereals like rice and wheat where just 10-12 flowers are left on the inflorescence and the rest clipped-off.
Hybridization Technique # Step 2. Emasculation:
At a certain stage of flower development the anthers become ripe and dehisce to liberate the pollens. This is known as anthesis. These anthers must be removed before anthesis from the flowers of the female parent to prevent self-pollination. This is called emasculation. Fig. 4.1 shows the plant breeder’s kit which is specially needed for emasculation.
The exact technique of emasculation varies from plant to plant depending upon the structure of the flower. In plants with relatively large flowers, such as tomato or brinjal, the corolla is forcibly opened just prior to anthesis, the anthers are plucked with a forceps and thrown-off. The forceps must be sterilized by dipping in rectified spirit.
Care should be taken not to injure the floral parts, particularly the carpels. The anthers should not be broken while plucking them off. Emasculation becomes progressively difficult as the flowers become smaller and smaller and is most difficult in the cereals where the glumes are very small and often brittle as in rice. In such plants, special techniques of emasculation are applied.
Hybridization Technique # Step 3. Bagging:
The emasculated flowers should be kept properly covered so that no untoward cross-pollination takes place. This is done by enclosing the emasculated flowers in muslin, oil paper or polythene bags (Fig. 4.2).
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Depending upon the nature of the plant, the flowers, the inflorescence, or the entire plant is bagged or caged. The bags are kept loosely tied at the bases of the plants so that gaseous exchange remains unhampered. Ordinarily the bags are kept till seed setting is complete.
Hybridization Technique # Step 4. Pollination:
When the stigma of the emasculated flower becomes mature (receptive) it is artificially pollinated by pollens taken from the male parent. During pollination whole, freshly dehisced anthers are plucked from the flowers of the male parent by means of forceps (then sterilized with rectified spirit) and dusted on the stigma. The bags are temporarily removed at the time of pollination and replaced, as before, after pollination.
The crossed flowers should always be kept properly tagged or labelled. Fine, rectangular and thin sheets of aluminium make good tags. Other types of tags are also available.
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The tag should be brief but complete. It should bear the names of the parents. The name of the female parent should be written first. It must contain a number referring to the field notebook. All necessary particulars about the cross should be recorded in the field notebook under that number.
Hybridization Technique # Step 5. Selection:
The seeds are allowed to mature within the bags. They are then harvested and stored with proper labelling. From the next growing season onwards the hybrid seeds are grown, allowed self-pollination and selection is carried on to find out the desired phenotypes and make them homozygous.
Section can be done following the Bulk Method or the Pedigree Method. Selection is followed by field trial and then only it can be recommended for large scale cultivation.
