Bountiful Buds for the Next Season
By Hannah Meikle, Summer Research Student, OMAFRA
As a quick glance at any apple tree might tell you, this time of year fruitlets are well on their way to becoming the apples we know so well. However, these fruitlets are not the only process apple trees are currently undergoing. While fruit growth for this year’s crop is well underway, each tree is also hard at work initiating the flower buds for next year. This process consists of three parts: flower induction, initiation, and differentiation.
Figure 1. Image of floral buds (Photo: Meikle. H, OMAFRA)
How do floral buds begin to develop?
Terminal bud set has begun, signifying an end to extension growth for this season. During this time, fully developed vegetative buds may begin the transition to floral buds. Floral buds are typically initiated on the terminal buds found on shorter shoots known as spurs. The time at which a vegetative bud meristem receives a signal to develop new flower structures is called flower induction. The exact timing of induction still remains obscure in many apple cultivars, but continues to be researched. Following floral induction, flower initiation begins; this is the distinct process of changes that the bud undergoes to become a proper floral bud. Flower initiation involves morphological changes that can be viewed under a microscope.
When does floral development occur?
Flowers are always initiated during the growing season prior to winter dormancy. A subtle doming shape at the tip of a bud signifies the beginning of transition from vegetative to floral reproductive growth (Figure 2). This may occur roughly 50 days after full bloom, but can vary between cultivars. Following this, floral meristems will continue to develop, often throughout autumn right up until winter dormancy. This is referred to as the differentiation phase. The flower development process then comes to a pause until spring.
Figure 2. Doming of apical meristem, the first visual indication of flower initiation, under microscope. (Photo: L.C. Luckwill & J. M. Silva)
When does the floral bud cycle end?
This process is typically finished by late fall. Where the transformation from vegetative to floral buds is complete and dormant, waiting for bloom the following spring. After the required chilling hours have been met between April and May, these buds will burst and bloom will begin. The release of dormancy and beginning of bud burst marks the end of the floral bud development cycle (Figure 3). These blossoms will then hopefully be pollinated during this period and result in a bountiful crop.
Figure 3. King blossom open during spring bloom period, the result of a fully developed floral bud. (Photo: Meikle. H, OMAFRA)
What factors affect floral buds?
Many questions have arisen regarding how environmental conditions could affect the initiation of floral buds, and in turn the following year’s crop. Research has shown that temperatures may not affect timing, but could affect intensity. Temperatures that cause higher vegetative vigor have the potential to reduce floral initiation. Additionally, shading at 30% or below did not affect flowering, but shading beyond 70% has been observed to inhibit this process in McIntosh apples.
Beyond these factors, one steadfast requirement has been determined by various studies for transition to occur: a critical number of vegetative nodes must be reached. This number will vary between cultivars, however the rate of node development must be fast enough that this critical number is reached before the end of the growing season. This is significantly more important to keep in mind in regions with shorter growing seasons.
Due to the potential importance of reaching this critical node number, flower bud formation has the potential to be affected by treatment applications up to 80 days before induction. As it was proposed that each cultivar may have its own individual critical node number, cultivar-specific studies have been conducted on several varieties. As it can vary by region and weather, some cultivars have had multiple values reported. Below are some of the values that have previously been determined for some cultivars:
- Golden Delicious: 16, 19
- Cox’s Orange Pippin: 20
- Baldwin: 18
- McIntosh: 22
The critical node concept has been widely accepted, however, in recent years it has been questioned as some findings point to a weaker connection between critical node numbers and flower induction. Regardless, it is still something that many keep in mind when monitoring for the timing of floral bud initiation.
Despite all the research that has already been done regarding this fascinating process, there are still many questions to be answered. Studies are ongoing to better understand just what goes into the process of floral initiation, and how it could affect things like biennial bearing. In the meantime, we can simply marvel at the work these trees are doing now to both give us delicious apples this year, and prepare the flowers for the next.
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