You are here.
Effective pollination period (EPP)
The effective pollination period (EPP) is the difference between the period of time for pollen tube growth and that of ovule longevity. The longer the effective pollination period, the greater the likelihood of adequate fertilization and seed development. Pollination must occur within 2 to 4 days after the flowers open; otherwise, the embryo sac will degenerate before fertilization can occur. Studies have shown that this period can vary depending upon cultivar.
The growth of the pollen tube and eventual fertilization of the embryo is largely dependent upon temperature and its relationship to the effective pollination period. The EPP was introduced in the mid-1960s as a way of establishing the time frame between when a flower is pollinated and when the embryo becomes unreceptive. After pollination it takes a certain time for the pollen tube to reach the embryo sac. Once a flower opens, the embryo has only a limited time when it is receptive. If the pollen tube does not reach the embryo before it degenerates, then the flower will not set. The length of the EPP will vary by cultivar, flower position within the cluster, and by certain cultural practices. In general, the EPP can be as short as 3 days and as long as 12 days; Delicious has one of the shortest EPPs.
In 1970, Williams and Wilson developed a temperature response index to allow the estimation of the time required for a pollen tube to grow to the embryo (see below). The index is based upon the daily mean temperature over a period of days. When the index reaches or exceeds 100 percent, the pollen tube should have reached the embryo and fertilized the egg.
As an example, suppose the average mean temperature over the past five days had been 50°, 54°, 50°, 52°, and 59°. Pollen tube growth would be expected to be 14 + 20 + 14 + 17 +50 = 115 percent, meaning pollen tube growth would have taken slightly less than 5 days.
Table 1-6 gives a partial listing of the pollen nature of several apple varieties. Your extension pomologist can provide information about the pollination requirements of varieties not listed.
Effective Pollination Period Index
Five conditions are necessary for satisfactory cross-pollination:
- Pollinizer and main variety bloom periods must overlap.
- The pollinizer variety must have viable diploid pollen.
- The pollinizer variety must be located near the producing tree.
- Bees and other insects must be present in the orchard and be active at bloom.
- Weed blossoms, such as dandelions, mustard, and wild radish, should not be present in quantity since they attract bees away from fruit tree blossoms.
Certain varieties have a biennial bearing tendency. During the "off" year of the pollinizer, the adjacent variety, although an annual bearer, will tend to become biennial because of the lack of cross-pollination. This is not as serious when one of the two is relatively self-fruitful; however, the problem must be considered along with all the other factors that affect pollination.
A pollinizer is the source of pollen necessary to set fruit. A pollinizer might be another tree with compatible pollen or a bouquet of flowering branches placed in the orchard; whereas a pollinator transfers pollen from tree to tree. The most important pollinators are bees.
Trees that provide sufficient compatible pollen for the main cultivar(s) in the block are necessary for pollination. A desirable arrangement is a pollinizer located not more than 100 feet from the variety to be pollinated. In larger blocks, plant two rows of pollinizer (starting on the windward side of the block), four rows of main variety, two rows of another pollinizer, four rows of the main variety, then two rows of the first pollinizer, etc., and repeat the arrangement across the block. We recommend that no fewer than three pollen-compatible varieties be planted in an orchard. Thus, for example, if you are planting a Delicious block, select two additional suitable pollinizer varieties to plant.
Where additional pollen is needed, graft a pollinizer branch into each tree. Select main variety and pollinizer trees with overlapping annual bloom times. Large bouquets of pollinizer branches placed in drums of water near main variety trees may be used when no other pollinizers are available. The nearer the pollinizer to the producing tree, the better distribution by the bees of pollen to all blossoms.
All fruit species require pollination to set fruit. Some species are self-fruitful and do not require more than one cultivar per block. Peaches, nectarines, tart cherries, most apricots, and some European plums are self-fruitful, and a solid block of one cultivar may be planted. Apples, pears, and sweet cherries require mixed plantings of different cultivars for adequate cross-pollination.
Improving conditions for pollination can lead to increased production owing to larger fruit size and/or a greater number of fruit per tree. Research in Massachusetts has shown that fruit size and calcium content are directly related to the number of seeds per fruit, with the number of seeds being dependent on good pollination. The more pollinizer trees in a planting, the better the potential for cross-pollination. However, using pollinizers means the loss of some efficiency in orchard operations. Having two or more cultivars in an orchard may pose problems in spray-to-harvest limitations and cultural practices, and it may confuse pickers, resulting in bins of mixed cultivars. There also may be an inefficient use of land owing to differences in growth habits. These disadvantages, however, are far outweighed by the greater yields associated with pollinizer use.