Examples of cherry cultivar compatibility. The blue pistils (stigma, style, and ovary) are Bing maternal tissue. Yellow circles are the pollen grains of the variety listed at the top.
A team of Washington and Oregon cherry researchers are working to better understand fruit set, a crop fundamental that can vary dramatically and unpredictably from year to year and variety to variety.
In the same year, in a block with different varieties and rootstocks, the percent of fruit set has ranged from a high of 90 percent in Bing on Gisela 5 rootstock to a low of less than 22 percent for Rainier on G.6, Dr. Matthew Whiting, Washington State University Extension horticulturist, reported during the Northwest Cherry Institute meeting in Yakima, Washington. "The orchard looked the same during bloom. And just because Bing had a 90 percent fruit set that year doesn’t mean it won’t be 20 percent the next year."
Fruit set is a problem in cherries when there is too much crop or too little. Though overset is less of an issue, Whiting noted that high crop loads tend to result in small fruit, increase the disease pressure, and can make the orchard difficult to pick. "And you lose the variety’s genetic potential for large fruit size."
An underset crop with a low proportion of flowers that were fertilized happens more frequently in cherries, he said. The low crop load results in a low yield, though growers tend to get bigger fruit. But there is excessive vigor, and frustratingly, a loss of income to the grower.
Cherries are either self-compatible or self-incompatible. Varieties that are self-compatible are also known as self-fertile. Whiting pointed out that self-fertile varieties are not necessarily more productive than self-sterile varieties.
Maternal tissue (the pistil) has two S alleles whereas the pollen grains have only one of the two alleles present for the cultivar. In self-sterile varieties, the S allele of the pollen grain must be different from one of the two alleles in the maternal tissue for successful fertilization. "You only need one allele for successful fertilization because only one pollen tube must reach the ovule," he said.
For example, maternal tissue of the Bing flower has the S3 and S4 allele. When a pollen grain from Bing (which will be either S3 or S4) begins to grow through the style, for reasons unknown to scientists, growth is arrested in the pollen tube and neither allele makes it to the ovary. "That’s why a pollinizer is needed for Bing and other self-incompatible varieties."
He explained that Rainier pollen on Bing is an example of what is sometimes referred to as a partial compatibility because Rainier pollen is either S1 or S4. The S1 allele will grow through the style to the ovule, while the S4 terminates growth. Black Tartarian is "fully" compatible, with S1 and S2 alleles, and both pollen types can make it to the ovule. Selah, with the S3 and S4 prime allele, has partial compatibility because the S4 prime will make it to the ovule. All self-fertile cherry varieties have the S4 prime allele and serve as universal donors, he noted.
Factors critical for successful fruit fertilization are that the pollen is viable, compatible, abundant, and there is bloom overlap.
"You need a high density of compatible pollinizers," Whiting said, adding that placement of pollinizer trees is important.
"You need to know your cultivar and climate. You’ve got to know when your orchard blooms."
Bees also play an important role in facilitating the transfer of pollen to the stigmatic surface of the flower. The bees must be abundant and actively foraging, Whiting said, and the pollen must be compatible with the variety being grown.
To achieve a more consistent fruit set, he suggested that growers use a high density of beehives that are well distributed throughout the orchard. "Timely introduction of hives is key—you don’t want to put them in too early or too late." He also recommended that the hives be well protected from wind and be placed in sunlight. Flowering weeds should be controlled to minimize any distractions of the bees.
The stigma must be receptive to the pollen for fertilization to occur. However, the stigma is only receptive for a certain period of time, he said, adding that researchers are trying to learn more about how long the stigma is receptive. In a sense, the bees are working against the clock. "That’s why you don’t want to short yourself on the number of bees. It’s not worth the worry."
Rapid pollen tube growth through the style is needed to get the pollen to the ovary. Once the pollen gets to the ovary, it must be viable for fertilization to occur. Scientists have learned that one of the limiting factors to fertilization is that the ovules are not viable indefinitely, he explained.
Poor fruit set causes
There are a host of reasons for poor fruit set:
• Incompatible pollen
• Lack of bloom overlap
• Poor pollinator activity (could be due to placement of hives)
• Insufficient pollinators
• Environmental conditions (freezing temperatures during bloom)
• Macro versus micro factors
Whiting said that research is being done on the causes of low fruit set, investigating both paternal and maternal factors. "Do we blame the dad or the mom?" he asked.
He noted that there is tremendous variability in the viability of pollen, from a range of 97 percent viable pollen germination of Black Tartarian to a 3 percent germination rate from the Sam variety.
In an experiment where Sam (with a very low percent of germination) was used to pollinate Attika, a known low-fruit-set cultivar, the result was 40 percent fruit set.
"That’s telling us that pollen germination rates don’t tell us much," Whiting said, adding that pollen viability has more to do with a receptive stigma, growth in the style, and viable ovules.
Initial research indicates that maternal factors may be more limiting than paternal factors when it comes to fruit set.
Overcoming poor set
He encourages growers to aim for a high fruit set, knowing that fruit set will be variable. Plant high density, compatible pollinizers, and use windbreaks if the orchard is in a windy site.
Researchers are focusing efforts to understand the genetic and environmental controls of fruit set and hope to have more answers in the future.
Additionally, Whiting and other scientists are studying hormones and growth regulators to see if they will improve or reduce fruit set. He is encouraged by early results of work involving synthetic cytokinin (CPPU), chlorophenoxyacetic acid (4-CPA), various chemistries of gibberellin, and Apogee (prohexadione calcium).