Wild Blueberry Phenology in the Crop Year - Endodormancy

Thursday 27 June 2024

Successful pollination of wild blueberry crops is dependent on managed pollinators. Studying phenology stages throughout the life cycle of wild blueberry crops could help with efficiencies around the use of these pollinators. Time between emergence and tip dieback in the sprout year will determine what the stem can support in the crop year.  Evaluation of these phenology stages could support a prediction model, using sprout year data, for the numbers of pollinators required in the crop year. Wild blueberry phenology stages in the crop year include two winter dormancy periods, known as endodormancy and ecodormancy, as well as flowering, fruit set, and harvest. With a better understanding of the dormancy periods of wild blueberries, a prediction model for timing of bloom and pollinator placement may be determined earlier in the crop year to help with pollinator efficiencies.

Wild Blueberry Phenology in the Crop Year - Endodormancy

Endodormancy is the first of two winter dormancy periods that are needed for wild blueberry buds to resume normal growth in the spring, and it is driven by internal cues within the plant [5]. Wild blueberry buds become endodormant to withstand cold temperatures and dehydration and prevent buds from opening throughout the winter when growth conditions are not adequate. Endodormancy is initiated by cold temperatures late in the summer and short daylength helps to accelerate this process. Cold temperatures in the fall also help acclimatize the buds to withstand freezing temperatures throughout the winter [1,6]

Endodormant wild blueberry plants (©John MacDonald 2024)

There are many changes happening internally when the buds go endodormant, including but not limited to the bound-to-free water status and hormone levels. The water goes from a free state to a bound state with macromolecules when the buds go endodormant. Also, the two most important hormones that are related to dormancy are abscisic acid (ABA) and gibberellin (GA). There is an increase in ABA within the plant during the initiation of endodormancy, which suggests that ABA plays an important role in endodormancy initiation. There is an increase in GA and decrease in ABA during endodormancy release, suggesting that GA plays an important role in endodormancy release [7].

The release of dormancy is regulated by the accumulation of cold temperatures, which is known as the chilling requirement. This can be calculated as chilling hours, which is an accumulation of temperatures between a lower and upper threshold (e.g., between 0 and 7°C) [3]. In contrast, chilling unit models are created to consider the reduced efficacy of temperatures below or above an optimum value and hence account for the reduced or negative effects of certain temperatures. The chilling requirement of wild blueberries is estimated at 1000 hours of temperatures less than 0°C [9,12], but limited results are available to confirm this assumption.

Endodormant buds will have a delayed and reduced growth rate compared to non-dormant buds when placed in growth adequate conditions, therefore dormancy must be satisfied to have adequate flowering and fruit set [10]. Knowing the chilling requirement for endodormancy release can give insight into the timing and intensity of bloom. The state of dormancy in wild blueberry floral buds can be determined by sampling floral buds throughout the winter, while the buds are receiving more chilling hours, and evaluating them in a greenhouse or laboratory.

One way dormancy release can be evaluated is a change in water status or hormone levels using intricate laboratory techniques. Another simpler and cheaper method that will yield similar results is known as forcing buds in a greenhouse. This is done by observing the rate of buds opening after a determined amount of time [4,8,11] or observing the number of buds opening after a determined amount of time [2,7]. Dormancy release is considered to have occurred when plants exposed to more chilling hours exhibit similar rates of buds opening.

Endodormant wild blueberry plants (©John MacDonald 2024)

After endodormancy is released, we then have ecodormancy, flowering, and fruit set. Currently wild blueberry producers and beekeepers use April 1st as an approximation of when endodormancy is released and ecodormancy starts in the Maritimes. This is inaccurate as the accumulation of chilling hours or units will vary from year to year, based on endodormancy requirements. Therefore, determining when endodormancy is released in wild blueberries will give a better approximation as to when flowers will open and help with efficiencies in placing pollinators for optimal fruit set.

Check back in the following weeks for part two of Wild Blueberry Phenology in the Crop Year – Ecodormancy and Flowering.

Written by John MacDonald, ATTTA Seasonal Apiculturist


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[2] Ferlito, F. et al. (2021) ‘Assessment of chilling requirement and threshold temperature of a low chill pear (Pyrus communis L.) germplasm in the Mediterranean area’, Horticulturae, 7(3), p. 45. Available at: https://doi.org/10.3390/horticulturae7030045.

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[4] Guak, S. and Neilsen, D. (2013) ‘Chill unit models for predicting dormancy completion of floral buds in apple and sweet cherry’, Horticulture, Environment, and Biotechnology, 54(1), pp. 29–36. Available at: https://doi.org/10.1007/s13580-013-0140-9.

[5] Lang, G.A. et al. (1987) ‘Endo-, para-, and ecodormancy: physiological terminology and classification for dormancy research’, HortScience, 22(3), pp. 371–377.

[6] Li, C., Junttila, O. and Palva, E.T. (2004) ‘Environmental regulation and physiological basis of freezing tolerance in woody plants’, Acta Physiologiae Plantarum, 26(2), pp. 213–222. Available at: https://doi.org/10.1007/s11738-004-0010-2.

[7] Li, Y. et al. (2022) ‘Comparative transcriptomic analysis provides insight into the key regulatory pathways and differentially expressed genes in blueberry flower bud endo- and ecodormancy release’, Horticulturae, 8(2), p. 176. Available at: https://doi.org/10.3390/horticulturae8020176.

[8] Norvell, D.J. and Moore, J.N. (1982) ‘An evaluation of chilling models for estimating rest requirements of highbush blueberries (Vaccinium corymbosum L.)’, Journal of the American Society for Horticultural Science, 107(1), pp. 54–56. Available at: https://doi.org/10.21273/JASHS.107.1.54.

[9] Rieger, M. (2006) Introduction to fruit crops. Haworth Food & Agricultural Products Press.

[10] Schuchovski, C. and Biasi, L.A. (2021) ‘Dormancy of floral buds of rabbiteye blueberry in a mild winter climate’, Brazilian Archives of Biology and Technology, 64, p. e21190755. Available at: https://doi.org/10.1590/1678-4324-2021190755.

[11] Spiers, J.M., Marshall, D.A. and Braswell, J.H. (2004) ‘Chilling requirement studies in blueberries’, Small Fruits Review, 3(3–4), pp. 325–330. Available at: https://doi.org/10.1300/J301v03n03_09.

[12] Yarborough, D.E. (2012) ‘Establishment and management of the cultivated lowbush blueberry (Vaccinium angustifolium)’, International Journal of Fruit Science, 12(1–3), pp. 14–22. Available at: https://doi.org/10.1080/15538362.2011.619130.