An In-Depth Look At Bloom
Most fruit growers are familiar with the general concepts of pollination. Pollinating insects seek sugar-rich nectar, or protein-rich pollen for food. Apple trees lure these insects using visual cues (petals), chemical attractants (fragrance), nectar, and pollen.
While the pollinators visit flowers to collect food resources, apple blossoms use pollinator visitation as an opportunity to capture pollen and procreate.
After apple pollen (Figure 1) is deposited on the stigmatic surface of an apple pistil (female portion of the flower, see Figure 2), the romantic notion that accompanies the plant-pollinator interaction suddenly disappears and is replaced with a series of complex biochemical interactions between pollen and pistil. Pollen tube growth of apple has been studied for more than 100 years, and we still have more to learn. So how does it all work?
Pollen Germination And Growth
In the process of gathering food resources, pollinating insects come in contact with the stigmatic surface (Figure 2) and deposit pollen. The stigmatic surface of an apple blossom is comprised of globular cells that begin to break down as soon as the blossom opens. As these cells break down they emit a sticky secretion, primarily composed of carbohydrates and proteins, which aids in pollen adhesion and germination. Pollen grains adhere to the stigmatic surface, hydrate, and germinate. After penetrating the stigma, pollen tubes begin growth toward the ovaries in the intercellular spaces of the style (Figure 3). Pollen tubes grow exclusively at the tip, and under optimal conditions, can travel the length of the style in 2 or 3 days.
During germination and early growth in the style, pollen tubes primarily use stored energy from the pollen grain. After these reserves are exhausted, the pollen tube relies on nutrient resources provided by the style. Pollen tube growth is directed by resources and signals emitted by style and ovary tissues.
Recent evidence suggests that resources are limited in the lower style when compared to the upper style, prompting competition between pollen tubes. These resources/signals guide pollen tubes through the style, eventually leading to the ovary. If successful, pollen tubes reach and fertilize the ovule(s).
Individual apple blossoms don’t open at the same time in a blossom cluster: king blossoms open first and lateral blossoms open later. This extends the opportunity for pollinator visitation, and increases the chances of pollination and fruit set at each spur. Flower position within blossom clusters may also influence the duration of stigmatic receptivity, and the ability for pollen grains to adhere to the stigmatic surface.
Recent evidence suggests that king blossoms are receptive for a shorter period of time when compared to lateral blossoms, however, greater numbers of pollen grains adhered to and germinated on stigmas of king blossoms. Maximum receptivity for king blossoms was observed at or shortly after opening, while receptivity for lateral blossoms was slightly delayed.
If this process wasn’t already complicated enough, consider this: female flower tissues can begin to degrade as soon as the blossom opens. Temperature is the primary factor that influences the rate of flower degradation. High temperatures increase the rate of pollen tube growth, but hasten the degradation of stigmatic and ovule receptivity. Conversely, cool temperatures reduce the rate of pollen tube growth, but the duration of stigmatic and ovule receptivity is increased. Extremely hot or cold temperatures during bloom can be detrimental to fruit set.
The number of days that a blossom can potentially produce a fruit is referred to as the Effective Pollination Period (EPP). EPP is determined by the rate of ovule and stigma degeneration, due to temperature. Insect activity is also influenced by temperature, so warm temperatures increase pollinator activity while cool temperatures reduce activity.