Familiarity with crop classifications according to the methods of pollination is important in plant breeding. This is so because of their effect on the applicability of certain breeding methods and techniques. For example, pureline selection applies to highly self-pollinating crops. In crop plants with high percentages of natural cross-pollination, controlled selfing is necessary to avoid contamination by foreign pollen.
From the plant breeding standpoint, seed-producing crop plants may be grouped into the following methods of pollination that they usually perform (Poehlman 1977):
- normally self-pollinated,
- normally cross pollinated, and
- both self- and cross-pollinated.
Normally self-pollinated crops are those which pollinate mostly through self-pollination or selfing while normally cross-pollinated crops, as the term also implies, are those in which their most dominant mode of pollen transfer is by cross-pollination. In both crop classifications, however, the descriptive words “self-pollinated” and “cross-pollinated” do not necessarily mean that there is an exclusive occurrence of the mode of pollination in any crop.
The percentage occurrence of natural crossing and natural selfing within any of the above-mentioned natural methods of pollination is expected to vary. The following factors were found to have pronounced effects:
1. crop variety or strain,
2. seasonal conditions,
3. wind speed and direction, and
4. population of insects and other biotic pollinators.
The amount of natural selfing and crossing can be quantified by conducting a Progeny Test. The procedure is basically as follows, as provided by Poehlman (1977) for field crops that are normally self-pollinated:
Select two varieties that are pure for a contrasting set of characters, one recessive and the other dominant (for example Mendel,s wrinkled seeds and round seeds of garden pea, respectively). These characters should be easily recognizable. Then grow the varieties simultaneously in such a way that individual plants exhibiting the recessive character are completely surrounded by plants exhibiting the dominant character. Harvest the seeds from the recessive plants and grow the progenies. Count separately the number of plants that exhibit the recessive and dominant characters and calculate the percentages. Those which remain recessive arise from self-pollination while those which exhibit the dominant character represent the plants that arise from natural cross-pollination.
In crops that are classified as normally self-pollinated, natural cross-pollination may occur from 0 to 5 percent. The same percentages apply to the occurrence of natural selfing in crops that are normally cross-pollinated.
But crops that are classified as both self-and cross-pollinated such as cotton, sorghum, and sudangrass, even if largely self-pollinated, perform natural cross-pollination in varying degrees. For example, the normal range in natural cross-pollination in cotton is from 5 to 25%. However, there were reports of up to 50% in some locations where insects were abundant (Poehlman1977). Broadbean, a legume, likewise belongs to this crop grouping according to Allard (1960).