A review of flower bud initiation in blueberry

Susan McCallum

The James Hutton Institute, Invergowrie, Dundee, DD2 5DA

ackg

In blueberry, knowledge of what determines vegetative vs. reproductive bud development under UK

conditions is difficult to obtain. Bud determination is under significant environmental control across

seasons leading to excessive yield instability. Unpredictable profit projections have hampered further

investment in the UK. We need to understand the environmental triggers of the phenotype prior to their

effect on yield and quality being realised, allowing mitigation strategies to be determined. Unstable

yields in high yielding environments raise serious concerns about the ability of existing crop systems to

sustain and expand. Unpredicted phenotypic variation in a range of developmental traits that have a

direct impact on yield has been evident for several seasons across a range of crops. In blueberry, yield

variation between seasons can be as much as 50%, valued around £36,000 per ha per year.

Summary

of main

• Flower bud formation occurs under short days/ long nights only.

• High temperature or water shortage during short days reduces flower bud initiation and results

in smaller flower bud formation.

• Cold temperature at night keeps growth slow and increases flower bud initiation.

• Number of flower buds per shoot is related to the number of days good weather for flower bud

initiation (longer developmental period).

• Factors including day length, temperature and plant health are all critical for flower bud initiation.

• Careful selection of cultivars depending on expected environmental conditions (early, mid-

season or late) can optimise light levels and resultant yield.

• Genetic potential of new selections can be determined through the study of key regulatory

pathways.

• Irrigation, fertigation, and pruning are areas a grower can control to maximise flower bud

initiation and subsequent yield.

ppr

This desk study was commissioned by AHDB to fill a knowledge gap in our understanding of factors

influencing bud initiation and yield in highbush blueberries (Vaccinium Corymbosum) and how

management regimes may influence these.

The study was undertaken by performing literature searches of Web of Science, Google Scholar and

the websites of grower organisations (e.g. AHDB), using search terms such as flower bud initiation, bud

development, photoperiod and plant signaling. Some plant characteristics found to be influenced by

light and temperature within flowering plants are listed in Table 1.

The study summarises information gathered from these literature searches into environmental and

management factors that have been found to influence (positively or negatively) the initiation of flower and

vegetative buds in blueberries. Although this study focusses primarily on highbush blueberry (Vaccinium

Corymbosum) some information related to Southern highbush (Vaccinium corymbosum hybrid with V.

darrowii) and Rabbiteye (Vaccinium ashei) has been reported if thought to be relevant to the study.

Finally, the study summarises additional crop management strategies that could be used to encourage

flower bud initiation and hence improve fruit yield.

Introduction

Throughout their life cycle, plants are subjected to many adverse environmental conditions including

low light levels and periods of drought or extreme temperatures which can dramatically affect plant

survival and limit productivity. To cope with such stresses, plants adjust metabolically and

physiologically. Unanticipated variation in crop development is already in evidence in a range of crop

cultivars resulting in yield instability with significant negative impacts on the rural economy,

environment, and wellbeing.

The lack of accessible information in this area severely limits the capacity for active crop management

to optimise yield or to breed for future environmental resilience. Factors influencing bud initiation and

hence yield, along with management practices to mitigate these could help inform growers how to

maximise yield and reduce crop losses.

Currently no methodologies exist to predict when a plant's development will lead to unwanted

phenotypes. In blueberry the environment has a significant role in yield determination and while we

have some understanding of yield traits, we have little knowledge in the UK of what determines the

development of vegetative vs. reproductive buds which has a great impact on yield. This is under

significant environmental control so varies greatly between seasons. Increasingly unpredictable

seasonal conditions have an impact on yield stability through the expression of inferior phenotypes

(erratic bud break, early fruit drop etc.). In blueberries, unpredictable profit projections have hampered

further investment in the UK.

During vegetative growth, one bud develops on the stem for each leaf produced. The bud is located at

the leaf axil, which is just above the point where the petiole meets the stem. These axillary buds first

develop vegetatively. Depending on the day length, temperature, and plant health, some of these

vegetative buds will convert to flower buds and this happens during late summer and throughout

autumn. Flower buds develop on the older current-season’s wood that grew during the spring first, and

then on the vigorous upright growth that develops following fruit harvest (Williamson et al., 2004). Day

length is measured by special plant pigments called phytochromes which are located within the leaves

and buds. As day length shortens in the autumn, vegetative buds are gradually converted to flower

buds with flower bud number being positively correlated with the length of exposure to short days (Strik,

2012). High temperatures and water deficit can both reduce flower bud initiation, even when days are

short enough to induce it. Moreover, flower buds initiated under high temperatures are smaller and do

not develop as well as those initiated under lower temperatures.

Blueberry is a key crop with great potential for UK production, but current supply only meets around

15% of the UK market. Demand for blueberries is at record levels with UK fresh sales valued at £450

million in 2020 representing growth of over 11% year on year. As the cost of production for the fresh

market is high, and labour is increasingly difficult to secure, there is a significant requirement to develop

cultivars which are quick to establish and produce consistent yield and quality. The changing UK

climate presents additional challenges to plant growth, particularly for woody perennial species like

blueberries. There are already indications that the trend towards warmer winters in UK and Europe is

affecting the dormancy cycle and subsequent fruit development of some berry crop species.

Due to increasing demand, prices for fresh-market blueberries have been relatively high, ranging from

£6.00 per kg early in the season to £11.00 per kg as the season comes to an end. Average five-year-old

plants would be expected to yield between 2.5-5.0 kg per plant, but this is subject to cultivar,

environment, and management conditions. Increases of 0.5kg per mature bush at the average growing

density of 4,000 bushes per ha could therefore result in the addition of up to £16,000 per ha per year

based on a rather conservative blueberry value of £8 per kg.

During each growing season, blueberry bushes form buds for the following year’s production. In spring

and early summer, new shoots which produce leaf buds are formed. Later in the summer if both light

levels and temperature are favourable, the plant produces fruit buds at the tips of these new shoots.

Heavy fruit set this year means the bush had enough energy to produce adequate fruit buds the

previous year.

In many crops, flower bud initiation (FBI) and flower bud differentiation (FBD) are clearly affected by

photoperiod and temperature with these often having interactive effects. Manipulation of photoperiod

and temperature during the FBI period has been used to alter fruiting season. In blueberry, FBI occurs

under short day length (it is the long night that is of most value) around late summer and early autumn,

beginning at the tip of the current season’s shoot (Aalders and Hall, 1964; Banados and Strik, 2006).

FBI is found to increase after eight weeks of 8-, 10-, or 12-hour photoperiod, compared with 14- or 16-

hours (Hall et al., 1963). Flower bud number has been reported to positively correlate with the length of

exposure to short days in northern highbush blueberries (Banados and Strik, 2006). The same study

was found to show FBD was incomplete in plants exposed to only four weeks of short days compared to

eight and bloom was also delayed.

In early, mid and late season northern highbush blueberries (NHB), FBI was found to only occur under

short days (SD) at a constant temperature of 22

C with the number of flower buds correlated to the

length of exposure. Plants grown under SD (8 hours light and 16 hours dark) for four or more weeks

ceased growth and entered endo-dormancy, whereas those grown under long days (LD) of 16 hours

light and 8 hours dark continued to grow but did not initiate flower buds or enter dormancy. FBI proceeds

basipetally with the number of floral buds per shoot affected by cultivar, climate, and production

practices. Shoot growth often slows under short days but this may be in response to a reduction in daily

photosynthesis as opposed to a direct photoperiod response (Strik, 2012). Flower bud development will

then continue until temperatures become too cool in late autumn. An experiment conducted over nine

years following eight different cultivars (early, mid and late ripening) reported variations in duration from

crop maturity to the end of the growing period as well as length of thermal time (heat sum of temperature

above 0

C). Early cultivars took between 99-133 days and received 1,166-1,640

C, mid cultivars 95-127

days with 977-1,664

C and late cultivars spanned 62-102 days with only 816-1,146

C (Kurlovich, 2020).

Constant 6

C for 1,000 hours chilling has been reported as being more successful for bud break than

C or 12

C in NHB. Once chilling has been reached, bud break and bloom will occur after six and 26

days with little difference seen between cultivars. Insufficient chilling will result in delayed and erratic bud

break as well as a reduction in the overall number of buds that break. Flower buds tend to have a lower

chilling requirement than vegetative buds. FBI and FBD requires at least two weeks of shoot cessation

before a flower bud can be visually identified at the apex. Equal hours of chilling in cold storage are not

equal to similar hours in the field. Currently little is known about the effect of fluctuating temperatures

and how best to simulate more “natural” chilling (Strik, 2012).

Environmental factors

Plants are sessile organisms which are unable to migrate to new locations should they find their

current environmental conditions unfavorable. They are exposed to an everchanging environment

throughout any given growing season, to which they must adjust accordingly if they are to thrive. These

responses are tightly regulated by a number of complex signaling pathways and those are initiated into

action depending on the stimuli received. During its lifetime, a plant must make numerous decisions,

based on the input they receive from the environment, in order to survive. Weather events such as

extremes in temperature, water availability or sub-optimal soil pH leads to the plants suffering from

abiotic stress which can result in poor plant growth (Lamers et al., 2020).

Photoperiod

Photoperiod is an environmental factor that changes in a predictable manner year on year for a

particular combination of latitude and day. Plants have evolved to sense these changes and adapt their

physiology and metabolism to the oncoming season. Shortened photoperiods in the autumn precede

cold winters. This is especially important in deciduous woody perennial plants native to high latitudes,

which alternate their annual growth cycle between a period of active growth in the spring and summer

and one of dormancy in the autumn and winter. Highbush blueberry is native to latitudes of 40 to 45

where the natural photoperiod ranges from 16 to 8 hours of light. The UK has a latitude between 51 to

N and also experiences between 8 to 16 hours of daily light levels. The influence of short-day

photoperiod on flower bud induction in blueberry shows that lowbush (Vaccinium angustifolium Ait.),

highbush (Vaccinium corymbosum L.) and rabbiteye (Vaccinium ashei Read) all develop flower buds

under an 8 hour photoperiod but not under 16 hours (Strik, 2015).

Vegetative growth was highest under 16-hour photoperiods and least under 8-hour. Maximum flower

bud initiation occurred under 10-hour photoperiods and this was attributed to the greater shoot growth

produced under 10-hour photoperiod which provided more shoots on which to produce flower buds

(Darnell, 1991).

Light

Without light plants cannot survive. Plants use light as a trigger for all stages of growth and development

such as seed germination, juvenility and flowering. Physiological acclimation responses to the

environment are also triggered by light and include stress tolerance, pigment and secondary metabolite

synthesis (Table 1) (Pocock, 2015). Decreasing levels of light intensity reduces FBI and flower bud

number as well as rate of fruit maturation (Aalders et al., 1969). In rabbiteye blueberries 83% of all

flower buds are formed in the top 0.6m of the canopy with relatively few buds formed in the lower

canopy where light levels tend to be less than 9 to 18% of full sun (Yanez et al., 2009). Principal

component analysis and multiple linear regression can be used to determine links between variable

temperature and luminosity (sunshine) and effect on growth and production. Development of fruit

consists of two phases: cell division and cell expansion. Both phases are related to the number of hours

the fruit was exposed to different temperature levels. Phytochrome is a light-sensitive, blue-green

pigment. It occurs in plant tissues in minute quantities - about one part in 10 million. The pigment acts as

an enzyme in other words, it activates certain processes without itself being used up. During daylight,

phytochrome is converted to an active form, and in darkness it is converted to an inactive form. If the

active form of phytochrome gets to a specific level for that plant, the enzyme can start changes in the

plant (Gettens and Klein, 1974).

Table 1. A selection of processes involved in temperature and light sensing in flowering plants.

Process

Level

Description

References

Photoreceptors

Cellular

Detect and respond to

dynamic changes of spectral

composition, light direction and

duration.

Galvao and

Fankhauser, 2015

Stomatal

opening

Biochemical

Opening and closing of

stomata for the control of gas

exchange and aid in

photosynthesis depending on

available light.

Zoulias et al.,

2018

Photoperiod

response

Molecular

Alignment of development for

favourable times of the year.

Bastow and

Dean, 2002

Hormone

signalling

Cellular

Regulation of plant growth and

development relies on energy

from light.

Liu et al., 2017

Seed

germination

Cellular and

molecular

Light activates and regulates

cellular and molecular

functions involved in cell

proliferation and growth.

Vandebrink et al.,

2014

Photosynthesis

Molecular,

biochemical

and cellular

Complex dynamic process

transforming radiant energy

into chemical bond energy

within carbohydrate molecules.

Brown and

Schwartz, 2008

Protein

Interactions

Molecular

Interactions between protein –

protein or protein – DNA can

be affected. Lower affinity

between proteins and weaker

DNA binding by transcription

factors.

Struk et al., 2019

Protein Stability

Molecular

Protein function can be

disturbed, or proteins

degraded by temperature

changes.

Vogt et al., 1997

Shade

Blueberries are an under storey crop which are found growing naturally in deciduous forests across

northern areas of the United States. The use of shade netting can be used to protect plants from some

environmental stress (temperature, radiation and relative humidity) without reducing potential yield.

Retamales et al. (2008) looked at different coloured shade netting in a blueberry plantation in Chile over

two seasons and reported an increase in yield using white, grey, or red nets (Retamales et al., 2008).

The positive effect of shading under extreme radiation is attributed to avoiding supra-optimal light levels,

plant heating and photosynthesis inhibition (Dale, 1992; Retamales et al., 2008). It was found that the

use of black netting however, reduced the level of PAR radiation by around 50% and saw decreases in

yield. It also saw an increase in shoot and leaf growth as more carbohydrates were partitioned towards

vegetative growth. Light is a major factor affecting leaf traits and photosynthesis, regulating plant growth

and survival and determining the geographical distribution of a plant. Photosynthetic photon flux density

(PPFD) is often below the saturation level of photosynthesis because of canopy shade and weed

shading, which often curtail growth and production in plants (Xu, 2019).

When plants are exposed to low PPFD, most plants show shade avoidance syndromes (decrease or

increase of leaf blade area and elongation of the petiole). The shade avoidance syndromes are typical

examples of leaf shape plasticity. Leaf responses to shade vary widely among plant species. In general,

leaf adaption to shade during leaf development considerably differs in structure, photosynthetic pigment

content, electron carriers and photosynthetic rates as compared to sun leaves. Shapes and sizes of

leaves are important factors influencing the absorption of light energy. To absorb sufficient light energy,

leaves must be as wide as possible. At the same time, to facilitate gas exchange (CO

, O

and H

0),

leaves must be as flat and thin as possible. However, if leaves are too wide and too thin, they will quickly

become desiccated. Thus, leaf area and thickness are mainly restricted by the availability of water.

Shade leaves usually invest assimilates in increasing leaf size to capture the maximum amount of

sunlight available for photosynthesis in a place where light levels are low (Jin Kim et al., 2011). The use

of shade nets has been shown to alter both the quality and quantity of light which a plant receives

(Shahak et al., 2004). Plants can detect these subtle changes as well as the orientation of the light and

this helps them to optimise their growth and development in any given environment as they determine

the distribution of carbohydrates, water and nitrogen to survive (Rajapakse et al., 1999).

Day length

Research has shown that the conversion of vegetative buds to flower buds is controlled by day length

and temperature. Day length is measured by special photoreceptors found within the leaves and buds.

As days shorten during the Autumn, vegetative buds gradually convert to flower buds. Experiments have

shown that high temperatures reduce flower bud initiation, even when days are short enough to induce

them. Additionally, flower buds initiated under high temperature are smaller and do not develop as well

as those initiated under lower temperature (Williamson et al., 2004).

In Autumn, blueberry bushes become dormant and prepare for winter. The blueberry bush uses the cue

of shortened day length in the late summer to prepare for winter. The next is lower temperatures near

freezing. Freezing temperatures are the final cue, and the bushes go dormant waiting for winter.

Providing the cold weather does not arrive too abruptly the bushes have little to worry about. The

cultivars can easily handle temperatures down to 0

C while below freezing temperatures stimulate the

plant to acclimate and increase its cold hardiness. Significantly colder temperatures of -15

C and below

can result in winter injury to buds and shoots.

Temperature

Perennial woody fruit species cultivated in temperate zones synchronize their annual growth patterns

with seasonal environmental changes. During unfavourable winter conditions, temperate fruit species

use bud dormancy as a defensive mechanism. Bud dormancy is classified in two stages (Figure 1):

Endodormancy: Buds are latent due to internal factors, and even under favourable environmental

conditions they are unable to grow. Dormant buds will not grow until certain biochemical changes occur.

Accumulation of chilling promote the changes that break dormancy of buds.

Ecodormancy: After chilling is completed the plants are dormant only because environmental factors

(cold or cool weather, day length) are preventing growth. Buds can be induced to break by exposure to a

specific ‘amount’ of heat. Evidence suggests that chilling and heat needs are not entirely fixed, but can

partially compensate for each other, but the exact nature of this compensation is poorly understood.

Figure 1. The relative contribution of the various types of dormancy during a hypothetical dormant

period (Lang et al., 1987).

Climate change is resulting in declining chilling (causing poor and erratic bud break) seen in

some environments and is predicted to continue to decline, which has resulted in winter chill

being recognised as a potential limiting factor for fruit production in deciduous woody plants.

Adequate winter chill is a pre-requisite for successful commercial production of many

temperature fruit crops (Atkinson et al., 2013). Chilling requirement (CR) generally calculated as

cumulative hours at temperatures of 7.2

C or below varies between 150 and 1500 hours across

blueberry species. Blueberry floral bud initiation often starts before endodormancy. Sufficient

chill accumulation during endodormancy is required to break dormancy along with a warming of

temperature. Hormone and flowering pathways are also involved in dormancy release.

Crop management strategies

An appropriate balance of vegetative and reproductive buds is required to maximise yields of high-

quality fruit. If there are too few flower buds, then the yield potential will not be maximised, too many,

relative to the vegetative buds, then the berries may exceed what the bush can support. This can result

in poor leafing, small berries, delayed harvest, poor fruit quality and plant stress or even plant death.

Blueberry yields (Kg per plant) are dominated by two principal factors: the number of fruits per plant and

their fresh weight. The number of fruits available can be estimated from the number of flower buds left

after pruning and by the bud density per branch (Bowen and Eaton, 1983).

The yield of blueberry plants depends on a series of both internal (genetic) and external factors (growing

practices, stimulants, climate). The flower buds in blueberries contain clusters of flowers. There are five

to 12 flowers per cluster depending on cultivar. The flowers emerge as a tightly packed cluster. As the

flowers develop, the corolla (petal tube) is visible as pink tissue at the tip of the flowers (pink bud). As

the flowers develop, the corolla expands and the pink fades to white. Bloom begins when the tip of the

corolla opens (first bloom). The flower clusters at the shoot tip bloom first, with bloom then progressing

down the shoot. The flowers at the base of each cluster open first. The first flowers pollinated have the

potential to be the largest fruit. Fruit from the later blooms are smaller. The early period of fruit growth is

very important in determining final fruit size. For several weeks, the fruit grows quickly by cell division.

Then, cell division in the fruit stops, and the fruit switches to growth by cell enlargement. Bigger fruits

have more cells, so the final size of the fruit is determined in the month immediately after bloom

(Longstroth, 2020).

As the fruit begins to grow, the fruit and shoots compete for the available resources and shoot growth

slows down. Maintaining good soil moisture during the growing season allows the roots to maintain an

adequate flow of water to the leaves, sustaining growth. By the time the fruit starts its final swell before

harvest, most of the shoot and leaf growth for the season has stopped. After harvest, the plant prepares

for next year’s growth by storing reserves, but this has been found to be limited in some

blueberry cultivars, at least in the UK (Petridis et al., 2018). In the autumn, flower buds are formed for

next year’s crop. The terminal shoot bud becomes fat and plump as it changes from a leaf bud to a

flower bud and under favourable conditions, other leaf buds below the terminal bud will also change to

fruit buds (Longstroth, 2020).

Nutrients

Nutrient management in blueberry production is crucial for successful and productive plantings. When

various chemical elements are inadequate, the general health, yield and quality of the crop may be

reduced so long as other factors are not limiting. Severe nutrient deficiency can lead to discoloured

leaves, wilted flowers, reduced yields, decreased cane initiation and growth, and eventually plant death.

Calcium and Boron have been used as supplemental fertilisers in blueberry production and have the

potential to positively influence fruit set and yield. The absorption of nutrients into the fruit tissue relies

on the passage of nutrients through the cuticle and stomata (Toselli et al., 2009). For effective uptake of

nutrients to fruit tissue with foliar applications, adequate surface contact is required. A soil application of

Nitrogen (4.5-6.5kg per Acre) may help to stimulate leaf growth. Frequent light fertilizer applications

throughout the summer are recommended to promote new growth of fruit-bearing wood and for longer

leaf retention into autumn. A warning however, that excessive late summer or autumn fertilization could

delay cessation of terminal growth and reduce or delay flower bud formation (Williamson et al., 2004).

Calcium is an essential macronutrient in blueberry production. The roles include structural (calcium

bound to pectin cell walls), defence and communication (between cells and tissues). Calcium is

particularly important for the function of calcium ion transporters and their subsequent role in

environmental and biotic responses. A major concern with commercial production of some cultivars i.e.

Draper, is abscission of unripe fruit just prior to berry colouring. Reports of crop losses due to

premature fruit drop in ‘Draper’ have been reported as high as 65%. While the main cause of premature

fruit drop remains unclear, research has suggested a calcium (Ca) deficiency in fruit is involved and that

foliar applications of Ca can reduce subsequent drop. One study showed that fruit drop was reduced

following the application of Ca while the exposure to 50% shade was found to exacerbate fruit drop

(Arrington and DeVetter, 2017). Strik and Vance (2015) previously demonstrated a predisposition to

poor Ca partitioning in ‘Draper’ and some cultivars with ‘Draper’ parentage have shown a similar pre-

disposition. Calcium is immobile in the plant and tends to be found in elevated concentrations in tissue

adjacent to the xylem (Shear and Faust, 1970). Speculations surrounding the Ca deficiencies in ‘Draper’

pointed towards the result of many competing shoot tips during fruit development. This is likely to result

in Ca movement in the xylem to transpiring leaves, thus supporting observations of increased fruit drop

in vigorous plantings with excessive growth. Seasons with reduced solar radiation and daytime air

temperature can see increased fruit drop as transpiration rates are reduced under such conditions.

Carbohydrate status

Blueberry leaves are the major organs that produce photosynthates and perceive changes in day length.

They are therefore vital for flower bud initiation and development. In Rabbiteye plants, net CO

assimilation was found to increase dramatically in leaves of ‘Beckyblue’ and ‘Climax’ developed under

short days. This increase in net CO

was also accompanied by an increase in stomatal conductance

(Darnell, 1991). For woody, perennial crops such as blueberry, naturally shortened photoperiods

precede the onset of dormancy, and carbohydrate reserves required for dormancy and subsequent

spring bud break begin to increase. Therefore the increase in CO

assimilation found in blueberry under

short days may be in response to the onset of dormancy and the initiation of carbohydrate reserve

accumulation. It has been reported however, that blueberries grown in the UK tend to have limited

carbohydrate reserves and instead work on a “current account” of newly assimilated carbon (Petridis et

al., 2018).

Irrigation

Water stress can occur quickly in blueberries and takes only a few days without irrigation to result in

reduced growth and fruit production due to photosynthetic limitation. Over watering is equally damaging

in blueberries and can result in root function impairment, and an increase in soil erosion and

nutrient leaching (Bryla, 2011). During periods of warm weather, the net photosynthesis in blueberries

declines significantly and the higher leaf temperatures results in an increase in water use. If these high

temperatures coincide with fruiting, as they often do in the UK, this can result in both water and

carbohydrates being diverted away from the fruit to the leaves (Bryla, 2011). Plant water deficit during

FBI period can also reduce flower bud and fruit number for the following season (Perrier et al., 2000). It

is important to maintain good soil moisture in August and September. While the demand for water

declines with cooler and shorter days, there is still a significant demand for water. Daily water losses can

often exceed 0.2 inches per day during the hot summer days in a well irrigated field. This can fall to less

than 0.1 inch per day during the cooler days. This means the bushes still need approximately half-inch

of water or more every week. Continuing irrigation throughout Autumn is a good way to guarantee you

have a good number of flowers for the following spring’s bloom. Autumn is also when the plant develops

the cold hardiness necessary to withstand winter cold. Plants that are suffering from drought-stress have

less reserves and cannot withstand as much cold as healthier plants. Having good soil moisture leading

into winter also ensures the plant will be better able to withstand winter cold by moderating soil

temperatures (Bryla, 2011).

Pruning

One of the goals of winter pruning is to bring fruit and leaf buds into balance on the bush. Heavier

pruning reduces fruit but increases leaf canopy for the following year. A bush without leaves cannot

produce the energy it needs to ripen the berries. So even though there may be many berries, they will

likely be undersized and under sweet if they ripen at all. Further, the bush will not have sufficient energy

to put into a) next year’s buds or b) root system which could lead to plant stress (Figure 2).

Fruit removal could also be carried out manually to reduce fruit load in overburdened bushes. This

should be done by early July especially in bushes less than three years old or the excess stress could

limit future plant growth. Fruit removal will also increase the quality of remaining fruit as well as reducing

stress and increase potential yields for the following year.

Figure 2. Dormant blueberry pruning of “Bluecrop” bushes, before and after (Pavlis, 2005).

Plant growth regulators

Plant flowering and size are affected by a range of phytohormones such as abscisic acid (ABA),

auxin, cytokinin, ethylene, brassinosteroid, jasmonic acid, nitric oxide, gibberellic acid (GA),

peptide hormone and salicylic acid. Pathway genes for the major phytohormones have been

associated with the regulation of transcript levels (upregulated or downregulated) in plants found

to have altered plant growth (dwarfing habit) and delayed flowering (Gao et al., 2016).

Stimulants are often used across the USA to encourage vegetative budding and accelerate the

development of flower buds, an example of which is hydrogen cyanamide

sprays (H

). The concentration required and its application rate and timing, would depend on

the amount of the cold weather to which the plant was exposed to during the winter (Williamson

et al., 2001). Hydrogen cyanamide however, is not approved for use in the UK. Gibberellic acid

can also be used to control the number of flower buds in early-fruiting cultivars, improving the

yield in subsequent harvests which currently has UK approval (Black and Ehlenfeldt, 2007).

Plant size is determined by genetic potential as well as environmental (biotic or abiotic)

conditions. The development of new genomic and biotechnological tools will facilitate the

knowledge within blueberry genetics and genomics and this will lead to an increase in breeding

efficiency. Traditional breeding is both time consuming and expensive but as genetic resources

develop it may be possible to select for genes within flowering pathways that can influence the

regime of blueberry flowering and increase fruit production (Walworth et al., 2016). Targets

could include flowering pathway genes like flowering locus C (FLC) and MADS affecting

flowering-2 like (MAF2), phytohormone related genes such as ABA related 2 (ABF2) and

ethylene responsive and brassinosteroid related (RAV1) or photoperiod pathways like constans-

like 5 (col5).

The floral transition is an essential process in the life cycle of flowering plants because their

reproductive success depends on it. Flowering plants must therefore respond to many environmental

signals and stimuli to adapt and cope effectively. While growers are limited by their own environment, in

terms of light and temperature, there are certainly measures that can be put in place to optimise bud

initiation and fruit yield in blueberries. Maximising the light levels by using reflected mulches or

appropriate tunnel polythene alongside selecting cultivars proven to be adaptive to local climate is a

good place to start but further work is required to confirm how well they would work under UK

conditions.

Further work

1) Demonstrate that good management practices such as nutrition, pruning and cultivar

selection can influence bud initiation in blueberries and look at optimising light levels

in tunnels using reflective mulches.

2) Assess the effect of different polythene on light levels and temperatures within

tunnels and the impact on bud initiation, yield stability and fruit quality.

3) Compare the performance of a wide range of commercial cultivars and how they can

be grown for optimum yield under UK conditions.

4) Study the genetic potential for flowering and yield by identifying key genes across the

regulatory pathways and comparing their expression between cultivars and under

differing environmental conditions.

eferen

Aalders, L.E. and Hall, I.V. (1964). A comparison of flower bud development in the lowbush

blueberry V. angustifolium Ait. Under greenhouse and field conditions. Proc. Amer. Soc. Hort.

Sci. 85: 281-284

Aalders, L.E., Hall, I.V. and Forsyth, F.R. (1969). Effects of partial defoliation and light intensity

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