Ideally, cross the emasculated flower immediately, or pollinate it the following morning. If crossing is done in the greenhouse, collecting freshly opened male flowers in the morning is no problem and pollen remains viable for 12-15 hours after anthesis. Pollen to be used from several hours to one or two days later can be stored viably in a plastic bag (refrigerated).
Some genotypes are superior pollinators whereas others are better seed parents. Unless special genetic studies are to be made, use the most efficient parental donors. It is usually more convenient, and reduces risks of contamination, to remove the flowers of the male parents and use them to brush pollen on the stigmatic surface.
To expose the anther sacs, remove or slip downwards the innermost petals of mature open flowers. Use the mass of pollen on the hairy-necked style as a brush to deposit pollen grains on and immediately under the green circular disc-shaped stigma (Figure 7).
You may use one flower to pollinate 4 or 5 emasculated buds. Only the obliquely arranged disc-shaped stigma at the tip of the style is receptive (not the hairy portion beneath). Under IITA conditions, anthers usually dehisce before or around sunrise. Pollen grains are somewhat sticky and tend to form clumps that can even be seen with the naked eye or with the aid of slight magnification.
A small tag listing the cross and date is affixed to the raceme or peduncle beneath the pollinated bud. Do not allow hands, instruments and other foreign objects to touch the receptive portion of the stigma and the dehisced anther sacs.
In a well managed mesh house or greenhouse, you may leave the crossed flowers open and uncovered, as risk of contamination is minimal. However, crawling and flying insects must be excluded from the plants during and immediately following pollination. Even ants, which are often attracted to the nectaries, can cause self-pollination. To discourage thrips and other insects likely to carry pollen, apply an insecticide at regular intervals.
|Objectives, Study materials, Practicals|
|1||Principles of cowpea crossing|
|7||Suggestions for trainers|