The overall goal of pollination for wild blueberry producers is berry production. Therefore, wild blueberry producers rent honey bee colonies and purchase bumblebee colonies to ensure sufficient pollination of their crops. Many practices are used to ensure that there are enough pollinators present in the crop, such as stocking density, timing, and location of hive placement. Producers can follow these practices, but there are many factors that can affect pollination success, such as adequate weather for the flight of pollinators or for plants producing nectar. It may be difficult to ensure adequate pollination of a crop, but there are many ways to assess a crop to determine pollination success.

Measuring Pollination Success of Wild Blueberries

Successful compatible pollen transfer from the anther of one flower to the stigma of another flower leads to germination and fertilization. Pollen transferred between flowers of the same clone, also known as self-crossing, and between different species can be incompatible [3]. Therefore, a flower receiving only incompatible pollen will have lower germination and fertilization rates, which results in reduced seed and berry formation. If there is successful pollination, there are many ways to tell how efficient or successful the pollination event was.

Observing visitations by insects is one way to assess pollination success, where if an insect has visited the flower, then there is a possibility of a pollination event. There are considerations to this method though because not every visit results in successful pollination. There could be insect cheaters which will feed on nectar and pollen resources, but do not exhibit the proper structure or behavior to pollinate [5]. Also, the pollen that is carried by insects may be incompatible to the observed flower [3]. Therefore, pollinator frequency, effectiveness, and efficiency are all different when considering pollination [6].

Hoverfly foraging on wild blueberry flowers (John MacDonald ©, 2024)

Some characteristics of a pollinated flower will change to prevent insect visitation of a flower that has already been successfully pollinated. This will ensure flowers that have not yet been pollinated will receive more visitors. One method is the pedal drop of flowers, where a pollinated flower will drop its pedals within a few days of being pollinated [4]. Another method which humans cannot see is color change, where ultraviolet color attracts insects and after successful pollination this color will fade [9]. Nectar and pollen production will also be reduced after successful pollination, which reduces the incentive for insects to visit the flower as well [2].

When a flower is successfully pollinated, germination commences, with the growth of a pollen tube from the surface of the stigma to the ovules, at the base of the female part of the flower. With the help of dyes and fluorescent microscopy, pollen tube formation can be observed. Methodologies for preparing flowers to observe pollen tube growth vary widely. Common methods typically include collecting flowers and removing the style, or the female reproductive part of the flower. The flower material is then placed in a solution to soften the tissues, rinsed to remove the solution, and dyed to darken the pollen tubes to make them visible. The material is then pressed flat onto a slide to view using florescence microscopy [8,10]. This method is more advanced than the others and requires supplies and equipment that may not be easily accessible. 

Fertilization occurs after germination, which can be assessed by both seed and berry formation. Seeds that have been successfully fertilized are also known as viable seeds. Fertilized and unfertilized seeds can be differentiated by size and color, where a fertilized seed is large and dark brown to red colored and an unfertilized seed is small and pale colored [1]. Successfully fertilized seeds will sprout uniformly when planted in a growing medium. Therefore, seeds can be placed in media to induce growth and then germination rates, or the ratio of sprouted to unsprouted seeds can be evaluated to show fertilization success. One consideration with this kind of test is to ensure that there is not a dormancy response in the seed that requires vernalization or scarification to allow growth [1].  

Seed growth also elicits berry formation, so berry size is another indication of pollination success. A very small berry that drops from the plant while the stigma is still attached, also known as a pinhead berry, can suggest that the flower was not successfully pollinated. The number of berries per plant can be an indication of pollination success, with large numbers indicating good pollination. Also, if a berry has more seeds, this suggests that many ovules were successfully fertilized, resulting in a larger berry [7,10]. Therefore, the size of berries and the number of seeds can also be indications of successful pollination. Another way to measuring pollination success is fruit set. This can be done by counting the number of flowers after bloom has commenced and then comparing the number of flowers to the number of fruit that successfully form after bloom [3,7,10].

Fertilized and unfertilized wild blueberry seeds (ATTTA ©, 2024)

Using yield as an indicator for pollination success should be done carefully because there are many factors that could affect yield from the point of pollination until the berry is ready to be harvested [6]. This includes, but is not limited to, disease and pest pressures, water availability, and unfavorable weather conditions. Measuring pollination success is important to ensure the pollinator used, environmental conditions, and other management practices are ideal for the most effective pollination. Please read upcoming pollination blogs on bee structure and behavior.

Written by John MacDonald, ATTTA Seasonal Apiculturist

Connecting with ATTTA Specialists

If you’d like to connect with ATTTA specialists or learn more about our program, you can:

visit our website at https://www.perennia.ca/portfolio-items/honey-bees/

Email attta@perennia.ca

References:

[1] Castro, C., Olarte, Y., Rache, L., and Pacheco, J. 2012. ‘Development of a germination protocol for blueberry seeds (Vaccinium meridionale Swartz)’, Agronomia Colombiana, 30(2):196-203.

[2] Cromie, J., Ternest, J.J., Komatz, A.P., Adunola, P.M., Azevedo, C., Mallinger, R.E., and Munoz, P.R. 2024. ‘Genotypic variation in blueberry flower morphology and nectar reward content affects pollinator attraction in a diverse breeding population’, BMC Plant Biology, 24:814.

[3] Dogterom, M.H., Winston, M.L., and Mukai, A. 2000. ‘Effect of pollen load size and source (self, outcross) on seed and fruit production in highbush blueberry cv. 'Bluecrop' (Vaccinium corymbosum; Ericaceae)’, American Journal of Botany, 87(11): 1584-1591.

[4] Drummond, F.A. 2020. ‘Wild blueberry fruit drop: a consequence of seed set?’, Agronomy, 10:939.

[5] King, C., Ballantyne, G. and Willmer, P.G. 2013. ‘Why flower visitation is a poor proxy for pollination: measuring single-visit pollen deposition, with implications for pollination networks and conservation’, Methods in Ecology and Evolution 2013, 4: 811–818.

[6] Ne’eman, G., Jurgens, A., Newstrom-Lloyd, L., Potts, S.G., and Dafni, A. 2009. ‘A framework for comparing pollinator performance: effectiveness and efficiency’, Biological Reviews, 2009:1-15.

[7] Noone, R.E., Doucet, S.E., and Jones, P.L. 2022. ‘Pollination ecology of lowbush blueberry (Vaccinium angustifolium Aiton) in an island ecosystem’, Canadian Journal of Plant Science, 102(3).

[8] Noormets, M. and Olson, R. 2005. ‘Observations on the gynoecial pathway for pollen tube growth in sweet lowbush blueberry (Vaccinium angustifolium Ait.)’, Journal of Applied Botany and Food Quality, 80:6-13.  

[9] Schulte, A.J., Mail, M., Hahn, L.A., and Barthlott, W. 2019. ‘Ultraviolet patterns of flowers revealed in polymer replica – caused by surface architecture’, Beilstein Journal of Nanotechnology, 10(1):459-466.

[10] Stavert, J.R., Bailey, C., Kirkland, L., and Rader, R. 2020. ‘Pollen tube growth from multiple pollinator visits more accurately quantifes pollinator performance and plant reproduction’, Scientific Reports, 10:16958.

Wild blueberries go through a series of developmental stages before producing a harvestable berry. These stages extend over a two-year period, or production cycle, consisting of a sprout and crop year. During these two years, wild blueberry floral buds undergo three dormancy periods before bud burst and bloom, the flowers then need to be pollinated and germinated, and the seeds need to be fertilized. Read this week’s blog to get an overview of these processes and what is needed to get harvestable wild blueberries.

Wild Blueberries - From Dormant Buds to Ripe Berries

This journey starts with the first dormancy period known as paradormancy. During paradormancy the apical meristem, the growth region at the tip of the stem, has dominance over the lower or lateral meristems, where the floral buds will form [4]. During the fall of the first year, or sprout year, short daylength and cold temperatures drive tip-dieback, or apical abortion. Once apical dominance is broken, then the floral buds can form. The buds then enter a second dormancy period known as endodormancy, which helps the buds survive harsh winter conditions [4]. The buds will acclimate throughout endodormancy as temperatures get colder [1]. This stage can be measured by an accumulation of cold temperatures, known as the chilling requirement. The chilling requirement of wild blueberries is approximately 1000 hours of temperatures less than 0°C [7]. The floral buds will then enter the third dormancy period known as ecodormancy, which prevents the buds from opening too early in the spring [4]. The floral buds deacclimate throughout ecodormancy as the temperatures rise [1]. This stage can be measured by an accumulation of daily mean temperatures, known as growing degree days (GDD). It takes approximately 400 GDDs for the start of bloom, with peak bloom happening around 550 GDDs [6].

Endodormant wild blueberry floral buds (John MacDonald ©, 2024)

Once all three dormancy periods are satisfied, the floral buds will start to burst and these can be measured by the five stages of bud burst, known as T1 to T5 [3].  The T1 stage is known as the bud swell stage when the buds start to expand, and a green tip can be seen. The T2 and T3 stages are known as the early bud burst and bud burst stages, respectfully, when the buds continue to swell and the bud scales are separating. The T4 stage is known as the tight cluster stage when flower pedals become visible and elongated. Finally, the T5 stage is known as early flower which is just before bloom when the flowers are still closed. The next stage is floral bloom when the flowers have fully opened.

The stigma, where the pollen lands on the receiving flower, on individual flowers is only receptive to pollen for up to nine days, where pollination within the first four days will result in the highest fruit set [2]. The bloom period for an entire field is typically three to four weeks [2]. Wild blueberry fields typically contain two species of wild blueberries, Vaccinium angustifolium and Vaccinium myrtilloides, and pollination is incompatible between these species. Wild blueberries are also self-incompatible, so wild blueberry flowers require pollen transfer from a different clone of the same species to get successfully pollinated [2].

Pollination can be one of the most limiting factors in wild blueberry fruit production. Pollination is the transfer from pollen from the anther of one flower to the stigma of another flower on a different clone of the same species. Wild blueberries are entomophilic with respect to pollination, meaning that they need an animal pollinator [2]. The conditions during bloom need to be ideal for the animal pollinators to ensure successful pollination. Wild pollinator populations vary between fields, so managed pollinators are used to get even pollination. Therefore, placement and removal of managed pollinators is crucial in providing efficiencies around pollination.

Bumblebee pollinating wild blueberry flowers (John MacDonald ©, 2024)

Once the pollen is successfully transferred from the anther of one plant to the stigma of another clone of the same species the next stage, known as pollen germination, can start. If conditions are favorable for the flowers, germination typically starts two to three hours after successful pollen transfer [5]. During germination, a pollen tube forms into the ovary at the base of the style, or female flower parts, and extends to an individual ovule. The pollen sperm travels down the pollen tube until they reach the ovule. Pollen germination can take three to four days, if conditions are favorable [5]. The next stage is known as fertilization, when the pollen sperm reaches the eggs and nuclei they are fertilized, and the seeds start forming. Once the seeds are formed, the fruit starts developing around the seed which provides protection and helps with seed dispersal. There are different maturity stages of blueberries from pin head, to green, to red, and then to ripe blue harvestable fruit.

For more information on how to tell if a flower was successfully pollinated, please check back in for the next blog on wild blueberries.

Written by John MacDonald, ATTTA Seasonal Apiculturist

Connecting with ATTTA Specialists

If you’d like to connect with ATTTA specialists or learn more about our program, you can:

visit our website at https://www.perennia.ca/portfolio-items/honey-bees/

Email attta@perennia.ca

References:

[1] Deslauriers, A., Garcia, L., Charrier, G., Butto, V., Pichette, A., and Pare, M. 2021. Cold acclimation and deacclimation in wild blueberry: Direct and indirect influence of environmental factors and non-structural carbohydrates Agricultural and Forest Meteorology, 301–302: 108349. (ACCLIMATION)

[2] Drummond, F. 2019. Reproductive biology of wild blueberry (Vaccinium angustifolium Ait.). Agriculture, 9(4): 69.

[3] Hildebrand, P.D., and Braun, P.G. 1991. Factors affecting infection of lowbush blueberry by ascospores of Monilinia vaccinii-corymbosi. Canadian Journal of Plant Pathology 13(3): 232–240.

[4] Lang, G.A., Early, J.D., Martin, G.C., and Darnell, R. 1987. Endo-, para-, and ecodormancy: physiological terminology and classification for dormancy research, Horticulture Science, 22(3): 371–377.

[5] Noormets, M., and Olson, A.R. 2005. Observations on the gynoecial pathway for pollen tube growth in sweet lowbush blueberry (Vaccinium angustifolium Ait.), Journal of Applied Botany and Food Quality, 80: 6-13.

[6] White, S.N., Boyd, N.S. and Van Acker, R.C. 2012. Growing degree-day models for predicting lowbush blueberry (Vaccinium angustifolium Ait.) ramet emergence, tip dieback, and flowering in Nova Scotia, Canada, Horticulture Science, 47(8): 1014–1021.

[7] Yarborough, D.E. 2012. Establishment and management of the cultivated lowbush blueberry (Vaccinium angustifolium), International Journal of Fruit Science, 12(1–3): 14–22.