Flower quality on mature trees and its improvement

Flower quality, in terms of ability to set fruits is often estimated by measuring the Effective Pollination Period or EPP.

The flowers formed in the spurs and on short terminal shoots of apple trees are initiated very soon after fruit set in the previous year.

• Approximately 3-4 weeks after petal fall the first floral primordia are visible under high power microscopy in the spurs.
• Providing these buds develop unhindered and differentiate into flowers, they should provide flowers of the highest quality (with long EPPs) in the subsequent spring.

In contrast, flowers forming on the current season’s extension shoots, known as axillary buds, are initiated later in the season, usually once the growth of the current season’s extension shoots begins to slow down in mid-late summer.

• These axillary buds are, on most varieties, poorer quality; they flower several days later than the flowers on spurs or short terminal shoots and they have shorter EPPs.

Flower bud quality is difficult to quantify but may be best described as the relative ability of the flower to set a fruit.

• However, it is now thought that flower quality may also influence the retention of the fruit after setting and its subsequent rate of growth.
• Flower buds of differing quality are very fully described in Abbott,1984.
• Other studies have suggested that a good correlation exists between the size of the flower cupula and eventual fruit size (Stosser et al.,1996).

Many environmental and tree management factors influence apple flower quality.  The following factors are thought to be the most important influencing flower quality:

• Crop loading and flower and fruit thinning (including natural fruit drop).
• Tree health and nutrition.
• Tree pruning/training and flower bud position on the tree.
• Exposure of the sites of flower initiation to adequate light.
• Winter chilling and satisfying the dormancy requirements.
• Spring temperatures prior to bud burst.

The Effective Pollination Period or EPP

The most common measurement of ‘flower quality’ is the ‘Effective Pollination Period’ or EPP (Williams, 1970).  Trials conducted in the 1970s at Long Ashton Research Station (Stott et al.,1973) showed that short EPPs were a common cause of poor fruit set on the early fruiting variety Discovery. Although 9% of flowers set fruits if pollinated on the day of opening (anthesis) only 1% set fruits if pollinated two days after opening.

The EPP is measured by protecting flowers from external uncontrolled pollination by enclosing them in large paper or other bags at the balloon stage of development. After removing the bags (temporarily) some of the flowers are pollinated on the day they open (anthesis). Further flowers are pollinated one, two, three, four or more days later. After each controlled pollination, the bags are replaced until after fruit set.

• Flowers that set fruits only when pollinated on the day of opening, or not at all, have low EPPs and are classed as poor quality.
• In contrast, flowers which can be pollinated 4 or more days after opening and still set and retain fruits have long EPPs and are classed as higher quality.
• In UK conditions, good quality apple flowers should, under orchard conditions, have EPPs of three to five days.

The reasons for these differences in EPPs and flower quality are not fully understood. Most theories relating to apple flower quality focus on differences in the ovules of the flowers.

• Poor quality flowers are thought to have ovules which either have a short life or which develop out of synchrony with the rest of the flower.
• An example of this might be flowers where the ovule becomes receptive to the pollen tube in advance of flower opening but which has begun to degenerate by the time that pollen eventually reaches it.

Flowers that are initiated late in the previous summer, such as those formed as axillaries on one-year-old wood, develop only partially by the autumn and are usually, but not always, of poor quality, with short EPPs in the subsequent spring.

• In this instance incomplete or shortened flower development may explain the quality differences.
• This means that the window of time for effective pollination and fruit set with axillary flowers is much shorter and the risks of poor fruit set much higher than with spur or terminal flowers.

Climatic conditions have a strong influence on apple flower quality. and this is discussed more fully by Williams (1970) and Abbott (1984).

The aim of the apple grower must be to encourage the rapid production of high quality flowers on the spurs and short terminal shoots of young trees.

• With many of the newer varieties, such as Gala, Braeburn, Kanzi, Jazz and Rubens, axillary flowers can be relied upon to contribute significantly to cropping on young trees.
• These axillary flowers are more likely to set and produce good size fruit if they are produced on strong wood of approximately 10 mm diameter.

Growers should strive to produce flowers of high quality with Effective Pollination Periods of three days or more. Flower quality is improved by:

• Minimising the spring and summer use of nitrogenous fertilisers, which encourage strong vigorous shoot growth and extended growth into the late summer and autumn.
• Limiting strong shoot growth by branch bending and brutting techniques especially in Cox and Bramley.
• Improving light penetration into the tree canopy.
• Optimising crop loading by appropriate flower or fruitlet thinning techniques.
• Not delaying fruit harvesting too late in the season.

The importance of planting well-feathered trees

Research conducted largely at HRI-East Malling in the 1980s showed the importance of planting high quality trees in increasing flowering in the first few years following planting.

• Tree quality is generally measured in terms of tree height and the numbers of feathers (lateral branches) present on the tree at the time of planting. Trees with six to eight strong feathers with wide branch angles are ideal.
• Another tree quality factor, sometimes ignored, is the health status of the tree. Flowers formed on well-managed healthy trees are invariably of higher quality than those on diseased or poorly nourished trees.
• Some virus infections will have a negative influence on flower quality and fruit set. Wherever possible, trees of high health status (especially virus freedom) should be chosen.

 

 

• Trees planted as ‘whips’, with a single leader and few or no feathers, take a minimum of one year longer to begin flowering significantly following planting and are not recommended.
• In contrast, maiden trees with 6 to 8 good feathers (lateral branches) at the time of planting begin to flower in the second year (leaf) following planting and by the third year can be producing worthwhile crops.
• The first flowers produced on poor quality trees are formed only in the axillaries of the leaves on one-year-old extension shoots and, as such, are usually of poor quality.
• On well-feathered trees, the production of high quality flowers on spurs or short terminal shoots on two-year-old or older wood is achieved sooner following planting.
• The onset of flowering is advanced and the abundance and quality of flowers improved if well-feathered trees are planted in good fertile soils.
• It is also important that these relatively large trees receive good management to minimise the transplanting check to growth. Excessive transplanting check to growth will cause tree stunting and may lead to the production of branches with bare wood.
• The use of ‘knip’ trees (see below) will encourage early fruiting and significant yields can be achieved in the second leaf after planting.

Nursery and other techniques aimed at improving young tree quality

Some apple varieties, such as Spartan and Bramley’s Seedling, are inherently poor in branching and often produce no feathers in the nursery when grown as ‘maiden trees’ but with the use of special techniques to promote branching can produce a well-feathered 2 yr old nursery tree.  Feathering on trees of Bramley, Spartan and other varieties may be improved by:

• Growing trees on for a second year in the nursery.
• Producing the trees using the ‘knip boem’ or ‘snip tree’ technique.
• Producing trees using interstems.
• Sequential removal of the lamina (blades) of the leaves at the shoot tip of the scion central leader.
• Spraying trees with ‘branching agents’.

Growing trees on for a second year in the nursery

As might be expected, growing ‘maiden’ trees on in the nursery for a second season greatly improves the number and quality of feathers produced. The height of these feathers above ground level can also be controlled such that fewer of the low and unwanted feathers are produced.

• Two-year-old trees are, inevitably, more expensive and growers can expect to pay perhaps 50 pence more for a two-year-old tree.
• Usually, nurseries undercut the root systems of the trees at the end of the maiden year, even when the trees are left for a further year in the nursery.
• This aids lifting at the end of the subsequent season and encourages more fibrous root systems.
• Two-year-old trees are larger at the time of transplanting to the orchard and often receive a slight check to growth following transplanting.
• This can be advantageous in reducing shoot growth and increasing the number and quality of the flowers produced.
• However, excessive checks to growth and severe tree stress must be avoided.

Producing the trees using the ‘knip boem’ or ‘snip tree’ technique

Trees with much improved feathering are produced using the ‘knip boem’ or ‘snip tree’ technique which has been common practice amongst the best Dutch nurseries for many years. The benefit of this technique is that a tree of approximate two-year quality is produced within the normal time span needed for producing a maiden tree.  Knip boem trees are generally less expensive than two-year old trees produced from maidens grown on for a second nursery season. The methods are as follows:

• Rootstocks are either grafted or budded and grown in the nursery at normal spacings.
• At the end of the first season of growth, a single-stemmed ‘whip’ is produced. This is headed back to 70 cm height by removing all but the top bud, strong growth is encouraged, with abundant feathers  between 70 and 100cm from the ground.

Producing trees using interstems

Trees raised using other scion varieties or rootstocks as interstocks or interstems are generally produced using either the knip boem technique (see above) or as two-year-old trees.

• For the reasons outlined above, such trees are usually better in terms of their feather number and position than maiden trees.
• Often trees purchased from a continental nursery will have an interstem of another scion variety.
• Golden Delicious is a commonly used interstem.
• Such trees should give no problems and will usually come into cropping earlier than similar maiden trees without interstems.

Sequential removal of the lamina of the leaves at the tip of the scion central leader

Research has shown that the removal of the top half of the leaf blades of the unfolding leaves at the tip of the scion central leader can help the induction of feathering.

• These leaves are thought to produce plant hormones, such as auxins, which suppress the growth and development of the lateral buds into feathers.
• By removing them, the buds immediately below the tip are freed from this inhibition and can develop into feathers.
• The process may be carried out three to five times during the growing season, when leader growth is stong.
• It is vital that the apical meristem of the central leader is not removed during this operation.
• The objective is not to pinch the leader, only to remove the unfolding leaf blades immediately below it.

Spraying with chemical branching agents

Research trials in many parts of the world have shown that feathering of apple trees may be improved using sprays of various plant growth regulating chemicals.  Three types of chemical branching agents have been shown to have beneficial effects on young apple trees.

• Auxin transport inhibitors: Chemicals known as auxin transport inhibitors slow down the movement of auxins from the shoot tip. These auxins inhibit the growth of feather-forming lateral buds on the main stem. Reduction in the auxin flow allows these buds to break and grow into feathers.
• Gibberellins and cytokinins: Research conducted principally in the USA has shown that sprays of Promalin can greatly improve feather production on apple trees in their maiden year.  Promalin is a mixture of the gibberellins A₄ and A₇(GA₄₊₇) and the cytokinin,  benzyl adenine (BA). Sprays at high volume (at the USA label recommended     concentrations) are applied when the scion central leader is growing actively. Unfortunately, Promalin is not approved for use in the UK.
• Surfactants: Research conducted in Canada and the USA has shown that, occasionally, certain types of surfactant can, when sprayed onto actively growing apple trees in the nursery, induce feathering. None of the surfactants shown to have this beneficial activity are available for use in the UK.

The influence of rootstock or interstock (interstem)

Precocity of flowering is greatly influenced by choice of rootstock or interstock. Irrespective of the vigour of tree desired, the apple grower should always choose the rootstock, or rootstock/interstock combination, which is likely to induce precocious cropping.

• Trees grafted onto most of the dwarfing rootstocks currently available begin flowering more quickly following planting than trees raised on most of the more invigorating rootstocks.
• However, the semi-vigorous rootstocks MM.106, and M.116 (AR 86-1-25) and the vigorous rootstock M.25 are exceptions to this rule.
• All three of these rootstocks induce much better precocity in scions than most other rootstocks in the same vigour categories.

 Dwarfing apple rootstocks that induce good floral precocity

Super dwarfing	Dwarfing	Semi-dwarfing	Semi-vigorous	Vigorous
M.27	M.8	M.26	MM.106	M.25
M.20	M.9*	Mark	M.116
J-TE-G	Ottawa 3	P.14
P.22	P.2	Bud 9
P.16	J-TE-E

• Increasing the height above ground level of budding apple scions onto dwarfing rootstocks increases the dwarfing effect on the scion and may also increase precocity of flowering.
• Using a dwarfing rootstock as an interstock, between the scion and a more invigorating rootstock, dwarfs trees more than if trees were raised directly on the invigorating rootstock.
• These interstocks also improve precocity of flowering.
• Increasing the length of interstock used, up to 25 or 30cm, also increases the effects on scion dwarfing and precocity.

Dutch research has shown that use of other scion varieties as interstems (between the rootstock and the main scion variety) can also improve precocity of flowering.

• Interstems, of the varieties Golden Delicious or Zoet Aagt, have both induced such benefits in scions.

The importance of tree age and transplanting ‘check’

Trees planted as two-year-olds generally come into flowering quicker than trees planted as one-year-old maidens. Part of this effect is explained by the increased branching (feathering) that is commonly found on two-year-old trees.

• However, a recent trial planted at East Malling Research showed that, even when the length and number of feathers on one-year-old and two-year-old trees were approximately the same, the two-year-old trees still came into flowering more quickly following orchard planting.
• One possible reason for this effect is the larger/older root system on the two-year-old trees at the time of planting and the possible increased transplanting ‘check’ to growth.
• However, it was interesting to note that, in the subsequent years of this trial, the trees planted as one-year-olds grew away better and after several years were outcropping the trees planted as two-year-olds.

A severe transplanting check can be induced by stringfellowing the trees.

• This involves cutting off most of the roots to leave very short root stubs at the time of planting.
• Trials have shown that stringfellowed trees grow less but form more flower buds in the first one or two seasons following planting.

Another method of causing a temporary check to growth in the first season is to plant the trees late in the spring when higher soil temperatures induce rapid root development and shoot growth has less time to develop.

• This results in better quality fruit bud being laid down and improved cropping in the subsequent year.
• Successful establishment of trees planted as late as May requires very well prepared soil with a high organic matter content and immediate application of trickle irrigation after planting followed by an intensive fertigation regime.

The storage of trees for up to six months requires specialist cold stores with fogging systems to create a high humididty without wetting the trees and treatments to reduce development of diseases such as Phytophtoraand Nectria.

Soil management, irrigation and nutrition

Soils rich in nitrogen and with copious supplies of water induce strong shoot growth and reduced numbers of flowers per metre of shoot length. These flowers are often initiated late in the summer season on account of the prolonged period of extension shoot growth on the vigorous trees. Such late initiated flowers are inevitably poorer in ‘quality’ the following spring than flowers initiated earlier in the summer.

Young trees

• Obtain analyses of the soil mineral and organic matter contents prior to tree planting. Based on these analyses and the current recommendations concerning optimum levels (e.g. Defra recommendations in RB 209) any necessary base dressings should be applied.
• Supplementary nutrition of trees in their first or second years following planting should be applied in order to maximise early yields. This is best done by a combination of fertigation and regular applications of a balanced foliar feed.
• Where this is not possible, apply repeated small doses of a blended fertiliser or a slow release fertiliser in the planting hole.
• On deep fertile soils that have never been planted with apples these supplementary treatments may not be necessary.
• In all cases monitor the tree nutrient levels by leaf analysis during the establishment phase.
• Supplementary irrigation should be applied, preferably using trickle systems, only when soil moisture deficits reach 50 mm.
• Recent work has shown the benefit in improving establishment and early yields of using a shallow mulch of composted green waste. This should be used where irrigation is not possible and can give added benefits even where irrigation is applied.
• On varieties sensitive to bitter pit, care should be taken to only apply a light mulch just after planting as mulch material can contain high levels of potassium. Always ask for, or carry out, a mineral analysis of the compost and ensure that it conforms to the latest British Standards.

Mature trees

Avoid excessive use of nitrogenous fertilisers in the early part of the growing season. These fertilisers stimulate excessive growth and have an indirect negative effect on flower quality.

• However, studies have demonstrated that flower quality can be improved by sprays of urea applied either just before harvest or in the autumn following harvesting.
• The reasons for these benefits are thought to be increased meristematic activity in the floral cupula, increased longevity of the ovules and the associated longer EPP.
• It is also suggested that this encourages the spur leaves to develop almost at the same time as the flowers and that this helps flower quality (Stosser et al., 1996).
• Use nitrogenous fertilisers sparingly in the spring and summer prior to harvesting.
• Consider urea sprays in the autumn, following harvesting but before leaf fall to improve the quality of flowers in the subsequent spring.

 

Pruning and training techniques for young trees

Three types of pruning and/or training techniques are practised on young apple trees:

• Shoot pruning
• Shoot bending, training and brutting
• Root pruning or root restriction

Shoot pruning

Ideally newly planted trees should fill their allotted space at planting and, where this is the case, very little pruning is required. If trees are much smaller than the space allowed, early yields will be compromised as growth will need to be stimulated and this will be at the expense of crop. Growth should be encouraged by a combination of pruning, removing all fruit and applying extra nutrients.

• One of the potential problems with planting very large, well-feathered trees is the tendency to develop bare wood on branches.
• This is worse when trees are planted in the spring in cold soil conditions and/or when the tree undergoes a period of stress, especially during the early summer period.
• Where a well-feathered tree is planted and fills the space at planting, the leader should be left unpruned. It will then naturally develop short fruiting laterals and sufficient extension growth to achieve the required height without becoming too dominant.
• Only where the feathers are too few or too weak and do not fill the space should the leader be pruned.
• In severe cases, prune back to one bud above the top feather to give maximum growth stimulation to the feathers which also should be tipped lightly. Otherwise tip the leader approximately 20 -25 cm above the top feather.
• Six to eight of the optimally positioned feathers, if present, should be retained and the lowest and other unneeded feathers removed.
• Remove strong upright feathers which have narrow branch angles and may compete with the central leader.
• With scion varieties, which have a tendency to produce bare wood later in their life, it may also be wise to tip the retained feathers.
• Severe pruning of either the leader or the retained feathers will generally delay flowering on the trees and encourage strong vegetative growth.

Shoot bending, training and brutting

Horticulturists have known for many centuries that bending shoots of apples towards the horizontal reduces the vigour of extension shoot growth and stimulates increased production of floral buds in the following season. The quality of these flowers is also improved.

• With apple scion varieties that produce laterals with upright growth habit, the retained feathers should be bent down towards or below the horizontal and secured in this position.
• Where the upright growth habit is not too severe and the stems of the feathers are not too thick, this may be achieved using small weighted pegs.
• Alternatively, the young branches are secured using string attached either to the support stake or to W clips pushed into the soil.

Partially breaking small laterals on young trees (sometimes called brutting) can also reduce shoot growth and stimulate increased flowering on these shoots:

• Tie down lateral growth towards the horizontal or below this angle on very strongly growing varieties.
• Partially ‘snap’ short laterals formed from the central leader, so as to reduce shoot growth and encourage flowering and fruit set.

Root pruning or root restriction

Research in the USA and at East Malling Research has shown that pruning the roots of young trees, using a deep spade or root pruner reduces subsequent extension growth and increases the production of flower buds (Webster et al., 2000, see Further reading [hyperlink ]).

• Unfortunately, if supplementary irrigation and nutrition is not applied, fruit set per tree is not always increased and fruit size at harvest is generally reduced.
• Root pruning is not recommended for use on young newly planted trees.

Planting trees within root restriction membranes buried in the soil has similar effects to root pruning, although effects on final fruit size are usually not so severe.

• The most favourable responses have been achieved using membranes which only partially restrict apple tree root growth.
• Root restriction by planting trees within woven membranes is not thought to be economic, currently, under UK conditions.

Plant growth regulator treatments for young trees

Flowering on newly planted apple trees can be increased indirectly by using plant growth regulator sprays in the nursery year(s), in order to induce improved feathering on the trees.

• Once the young trees begin to establish and fill their allotted space in the orchard, use of a plant growth regulator might be warranted.
• Sprays of Cultar, at low concentration, may aid the production of flower buds and precocious yields on young trees that are growing too vigorously and forming few flower buds.
• Sprays of Regalis at petal fall have been found to encourage fruit set and reduce growth.
• No chemical plant growth regulators are approved for use as branching agents in the UK.
• Low concentration treatments with Cultar may aid flower production on young trees that are growing too vigorously.

Assessing fruit bud numbers and their quality on apple trees

Techniques for collecting a sample of buds and dissecting them to assess flower numbers have been developed. The time in the season when the dissections are made will determine how difficult it is to see the flower primordia and how powerful a microscope will be needed.

In the month or so prior to flowering it is quite easy to determine the presence or absence of flowers and the flower numbers in a floral bud. If, however, the determinations are made during the previous summer and autumn, the assessments are more difficult and require more time.

• Each floral bud will produce on average 5-6 flowers. In a normal year 1 in 6 flowers will set a good fruit, which means that one fruit bud will be needed for each fruit set to provide a good crop.
• A safety margin of 15%-20% over and above this level should ensure adequate fruit numbers at harvest.
• Some varieties e.g. Gala can set more heavily than this, producing on average 2 fruits per bud and, in orchards with a history of heavy setting, bud numbers should be reduced at pruning time especially in shaded parts of the tree and on weak wood.
• Generally, the size (diameter) of the bud is a reasonable guide to its quality. Plump buds are indicative of good quality.

Techniques for winter pruning

Flower numbers are influenced by:

• Light
• Tree vigour
• Food resources available within the tree

One of the main aims of winter pruning is to achieve the correct balance between fruiting and new growth.  New growth is required to replace old fruiting wood and provide sufficient leaf area to support the crop.

Poor light levels in the tree will reduce both flower bud numbers and bud quality.  Pruning should maximise light interception into all parts of the tree.

Tree shape

• The best tree shape is a well defined ‘A’-shape where the height of the tree is 1.5 times its width. (J.E Jackson et al). This shape is easily achieved in semi-intensive orchards but in wide spaced orchards the height will need to be reduced while still maintaining an ‘A’-shape.
• Within the tree, create a branch framework of evenly spaced branches which all point down to the edge of the tree space.

Treatment of the leader

Do not allow the leader to become too strong or dominant. This can be achieved by:

• Bending the leader into a gently ‘S’-shape.
• Not tying it too tightly to the stake and dead straight.
• Only pruning back into wood that will crop.
• Tearing off unwanted side shoots rather than cutting them off.
• Delaying any necessary pruning until early July.
• Keeping the leader narrow and with plenty of fruit bud.

A technique developed in Holland known as ‘klik’ (stub) pruning can be used in varieties where the leader naturally goes weak.

• In winter the leader is pruned back to a side shoot in the current year’s growth, and the side shoot is ‘klikked’ to 2 or 3 buds.
• In the following year the subsequent regrowth is then stubbed back again in the same way.
• The leader becomes stiffer and can be maintained at a constant height.
• In orchards with strong leaders the technique can also be used but the subsequent growth will be more vigorous.
• Although this technique does keep leaders to a standard height and is a simple method, it means that several cuts have to be made every year.

Treatment of lower branches

Improve light by:

• Removing strong upright shoots or bending them into a space.
• Removing heavy timber especially in the top of the tree.
• Where branches are overcrowded remove the dominant, vigorous ones first.
• Avoid ‘stubbing back’ branches as this promotes vigorous shoots further back into the tree, reducing fruit buds and creating more shade.  If branches have to be shortened: Cut to 2 to 3 mm above the set of growth rings between one- and two- or two-  and three-year-old wood. If a cut is needed further back along the branch always cut to a flat fruiting piece of wood and if this is not available crack the branch down to the correct  angle to reduce subsequent growth.

The above techniques refer particularly to Cox.  In Gala, Jonagold & Braeburn, cutting to upright shoots may be done without encouraging vigour, especially if the shoots have flower buds already present.

Where trees are too weak, where insufficient replacement wood is being produced or where too many flowers are being produced the following techniques can be adopted:

• Remove weak fruiting spurs and laterals particularly on the undersides or ends of branches.
• Reduce the number of branches, removing weak ones first.
• Stimulate the strength of branches by removing side branches and laterals.
• Check other causes of weak growth such as: poor soil structure, incorrect fertiliser regime, excess use of growth regulators, too heavy cropping.

Restructuring pruning of old and very large trees

Old and badly managed orchards are very difficult to get back into full production and it is usually better to grub the trees and replant. However, in a few cases it may be necessary to try and restructure old and very large apple trees.

• Reducing the large size of the trees by severe branch pruning is not to be recommended in isolation, as it will merely stimulate vigorous renewal shoot growth and minimal cropping.
• Hungarian research has shown that where severe branch pruning is essential to reduce the size of very large old trees, supplementary root pruning in the same season can help put the trees back into a better balance (see Brunner, 1990).
• Another alternative is to combine severe branch pruning with applications of plant growth regulators.

Application of plant growth regulator treatments

The application of approved plant growth regulators can be a valuable aid to tree management and can lead to a significant increase in flower numbers. Where continued reliance on growth regulators is made year after year, other tree management strategies should also be incorporated.

• It is generally accepted that treatments that retard shoot growth on apple trees usually result in an increase in flower bud initiation.
• The gibberellin-inhibiting chemical paclobutrazol (Cultar) can increase flower induction.
• Trials in the UK and abroad have demonstrated increases in floral abundance following use of Cultar (Shearing, et al., 1986).
• Providing the label recommendations are adhered to, no deleterious effects of the sprays on fruit set or fruit quality should be experienced.

The plant growth regulating chemical, prohexadione-Ca is also available for controlling shoot growth.

• In some, but not all, of the trials the treatments have also improved flower production.
• The product is less persistent than Cultar and the risks of overdosing are minimal.
• Where apple trees are making excessive shoot growth and are flowering poorly, treatments with Cultar will reduce subsequent shoot growth and increase flowering.

Trials over the years have shown the benefit of applying repeated low doses of growth regulators rather than fewer high doses.

• Suppressing growth early in the season before it gets away is also advantageous.
• Exact timings and rates of application are varied subject to individual orchard conditions and the need to reduce any potential side effects and should be discussed with a suitably qualified and experienced advisor.

Summer pruning

The effects of summer pruning on flower bud formation are extremely variable. This is partly explained by the different techniques of summer pruning employed, the different timings of the pruning and the different responses of scion varieties.

• On spur bearing varieties, any summer pruning which removes entire strong extension shoots back to their base, should have no negative effect on flowering.

• Indeed, in cases where shoot growth on the tree is very dense, such summer pruning should expose the spur buds to light and improve their quality.
• The aim usually is to remove strong upright shoots, which are shading fruits from light.
• In contrast, summer pruning that involves various degrees of heading back (tipping) of extension shoots may have a deleterious effect on flowering.

• Heading back shoots to half or one third their length will stimulate regrowth on the shoots and possibly depress the number and quality of spur flowers produced.
• Tipping or lightly heading extension shoots of some varieties may, however, aid flowering.
• This is mainly beneficial on tip bearing varieties, such as Worcester Pearmain.
• On these varieties, the aim should be to encourage as many as possible short extension shoots (dards or brindles) with strong terminal flower buds.
• Summer pruning of shoots, if carried out at the optimum stage of growth, can encourage this production of short shoots.
• On spur bearing varieties, only summer prune to remove strong upright shoots and those shading fruits within the canopy.
• Remove whole shoots back to their base. Do not head back.
• On tip bearing varieties, light tipping of shoots during the summer may aid the production of short laterals and terminal flowers.
• Where excessive growth is produced regularly and summer pruning is necessary each year to enhance fruit colour development, the need for a change in tree management practices is indicated.

 

Trunk girdling (ringing)

It has been known for many decades that partially girdling (or ringing) the trunks of apple trees can increase the numbers of flowers produced in the subsequent season.

• The girdling treatments may comprise a single narrow diameter knife cut, made through the bark into the cambium of the tree for the whole circumference of the trunk.
• Alternatively, it may comprise two slightly wider cuts, usually made one slightly above the other on each side of the tree each for half of the trunk circumference.

Trials at East Malling over the years have shown variable effects of partial girdling (bark ringing) on the subsequent vigour of extension shoot growth.

• Whilst growth is reduced in some trials, in others it is not.
• However, in most trials flowering abundance has been improved following girdling.
• Research in the USA showed that girdling six-year-old trees of the tip bearing variety Cortland 19 days after full bloom increased flowering in the subsequent season but had no effect in the season after that (Greene and Lord, 1983).
• Where trees have become unbalanced, in terms of their excessive shoot growth and poor flowering, girdling (bark ringing) of the trunk may provide a means of improving floral abundance and reducing excessive shoot growth.
• However, this is not a reliable technique and the use of growth regulators and/or root pruning should be used in preference.
• Care must be taken, however, when bark ringing varieties which are sensitive to diseases such as apple canker (Nectria galligena).

 

Root pruning

Although root pruning has been shown, in trials at East Malling, to increase the abundance of flowering on young apple trees (Webster, et al., 2000), the effects on mature trees are much more variable.

• Trials in the USA on semi-mature and mature apple trees have occasionally increased floral abundance but more often have had no effect on flowering.
• The effects of root pruning on the quality of flowers produced are also variable, irrespective of tree age.
• Severe root pruning of five-year-old Braeburn, Royal Gala, Oregon Red Delicious, Splendour, Granny Smith and Fuji apples on MM.106 rootstock planted in New Zealand trials also increased flowering abundance (as in the trials at East Malling) but reduced yields (Kahn, et al.,1998).
• The authors suggest that this yield reduction is brought about by reduced bourse shoot growth following the treatment and the reduced photosynthesis and carbohydrate supply to the roots resulting from this.
• The theory is that it is these root carbohydrates that are so important to fruit set and retention in the subsequent spring.
• Root pruning may be resorted to when other measures have failed to bring over-vigorous apple trees in a balance of shoot growth and fruiting.
• Root pruning on one side of the tree should be tried initially and the effects observed.
• To be effective the treatment should be applied approximately one third to one half of the distance from the trunk to the edge of the tree canopy but no closer than 50cm from the trunk.
• Tree anchorage may be a problem following severe root pruning.

Crop loading and flower or fruitlet thinning/abscission

The number of floral buds initiated can be significantly influenced by the crop load set on the trees at the time of initiation and in the few weeks following.

• Flower quality, as well as flower abundance, in the subsequent season is strongly influenced by crop loading and the time of thinning.
• Research conducted at Wye College in Kent (Buzzard and Schwabe, 1995) showed that Cox’s Orange Pippin trees, that were cropping very heavily in the subsequent spring, developed smaller flowers which had shorter EPPs and lower fruit set than trees with lower crop loads in the previous season. Anatomical differences in the stigmas of flowers from trees of the two different crop loads were demonstrated.
• Flower bud numbers and their quality can also be influenced by the dates of fruit harvesting in the previous season.
• Research conducted at Long Ashton Research Station in the late 1970s (Williams et al., 1980) showed that delaying harvesting of Bramley’s Seedling fruits reduced flower numbers in the following spring and also the ability of these flowers to set fruits (i.e. their quality).
• Picking in late August gave the best return bloom but reduced total yields significantly; picking in late October reduced flowering number and quality significantly.
• However, in the year of this experiment, the autumn was atypically mild and shoot growth continued very late in the autumn.
• For optimum flower bud quality trees should be thinned early in the previous season to optimum crop loads.
• Fruit harvesting should not be delayed too long as late picking may reduce flower quality on varieties such as Bramley and Braeburn.

Tree pruning/training and flower bud position on the tree

Severe branch pruning in the winter and early spring reduces flower quality as well as floral abundance on apple trees.

• The pruning stimulates vigorous shoot growth which often continues late into the autumn.
• This shoot growth competes strongly with the spurs and other sites of floral bud production for photosynthates and minerals
• The consequence is the production of limited numbers of floral buds, most of which are of relatively poor quality.
• Light pruning generally produces the best flower bud quality on spurs and short terminal shoots.
• Pruning that exposes the sites of floral bud initiation to adequate light is most beneficial.

Bending branches towards the horizontal, or below the horizontal, reduces the rate of extension shoot growth on the branch.

• This in turn results in photosynthates being partitioned more towards the spurs and other sites of flower initiation and away from extension shoot growth.
• These changes have the effect of improving the quality of flowers produced.

Flowers produced on one-year-old extension shoots produced in the previous season (axillary blossoms) tend to have shorter EPPs and hence lower quality than flowers formed on spurs or short terminal shoots.

• These differences are partly attributable to the time of flower initiation in the previous season.
• Flowers on axillary shoots are initiated much later than flowers on spurs or short terminals and develop much less by the onset of dormancy in the late autumn.
• Flowers formed on spurs situated on two- or three-year-old wood often produce better quality flowers than spurs formed on older wood.
• Flower bud quality is usually aided by light, rather than severe winter pruning.

• Summer pruning can aid flower bud quality on spurs by removing shading.
• However, where summer pruning entails heading back extension shoots, this may reduce flower bud quality by stimulating late shoot growth.
• Shoot bending improves flower bud quality, but should not be carried out to excess or the optimal balance of new renewal shoot growth and flower bud production will be lost.
• Pruning or training, which encourages the production of more one-year-old shoots and the associated axillary blossoms, is generally detrimental.
• Only on tip-bearing varieties can such techniques be warranted.

Exposure of the sites of flower initiation to adequate light

Research at East Malling and in several other countries has demonstrated the importance of good light exposure to the production of high quality flower buds.

• This is particularly relevant in climates such as those experienced in the UK.

• Growers should prune and train their trees so as to ensure good light penetration into all parts of the tree canopy.
• A combination of winter and summer pruning is recommended.

Winter chilling and satisfying the dormancy requirements

Apple trees are thought to require an accumulation of an approximate number of hours during the dormant period when temperatures are above freezing but less than 8^oC. Figures of 1000 to 1500 hours are often given in the literature.

• Lack of sufficient chilling units is unlikely, on the above evidence, to be a problem in UK apple orchards.
• In most seasons sufficient chilling units have been accumulated by sometime in February.
• Recent studies have been undertaken in Israel, where winters are warm and spring temperatures very hot.
• These have shown that sprays of urea (8%) applied at the stage of green bud swell advanced initial bud break, elevated the total number of flowering buds and significantly increased subsequent yields.
• However, in climatic conditions more suited to apple production, similar benefits of urea sprays have not been recorded.
• Climatic conditions in the UK should provide sufficient chilling units to satisfy the dormancy requirements of traditional apple varieties.
• Problems of insufficient chilling could be experienced if growing within protective structures is contemplated.

Spring temperatures prior to bud burst

Although UK apple trees are thought to receive adequate chilling units to satisfy their dormancy requirements, it has been shown that unfavourable temperatures in the spring, prior to bud break, can have a negative effect on subsequent fruit set and yield.

• These negative effects were not caused by very low temperatures and frost damage to blossoms, as might have been expected, but by atypically warm temperatures in February and March and to a lesser extent April.
• The authors of this work, which was mainly carried out by scientists based at East Malling, constructed a model which predicts yields of Cox based on these early spring temperatures.
• Subsequent work in the USA showed that the yields of many of the varieties growing in New York State could be predicted using a similar model.
• Data collected over a number of years indicates that the fruit set of Cox (and its clones) remains high for this variety (30%) in years when the average maximum temperature in February, March and April was no more than 9^oC.
• Average temperatures of 10^oC or 11^oC were associated with much reduced levels of set (<10%).
• The reasons for the negative effect of these early spring temperatures on flower quality are not yet fully understood.
• Subsequent experiments conducted using semi-controlled environments showed that trees experiencing spring temperatures, such that they received 106 day degrees above ambient, set fruits much more poorly than trees receiving 40 day degrees below the ambient (Miller et al., 1986).
• Fruit setting percentages were lower in the ‘warmed’ trees irrespective of how soon after opening the flowers were pollinated.
• The EPPs were also very much poorer in the flowers of these trees.

Growers should take note of the average maximum temperatures in February, March and April in their Cox orchards.

• Where temperatures are higher than desired average maxima (i.e. 10^oC or higher), they are advised to intensify their efforts to secure good pollination and flower fertilisation.
• This can be achieved by supplementing bee populations, providing increased shelter and boosting supplies of compatible pollinating varieties.
• Where temperatures are of the desired average maximum, or lower, growers will need to consider implementing appropriate thinning strategies.

Overcoming biennial bearing

It is fortunate that most of the commercial dessert and culinary varieties of apple now grown in the UK have only weak biennial bearing tendencies.

• This is in contrast to several of the popular varieties of cider apples (Tremlett’s Bitter, Vilberie, Reine de Hâtives and Néhou) and to some of the older dessert varieties, such as Blenheim Orange, Miller’s Seedling and Laxton’s Superb, all of which suffered from this problem.
• Amongst popular dessert varieties that are currently grown, only Elstar shows pronounced biennial bearing tendencies.
• Biennial bearing varieties characteristically produce abundant flowers in what is called the ‘on’ season and set heavy crop loads.
• In the subsequent season, the ‘off’ season, they produce few flowers and fruits.

As the initiation of what are likely to be the best quality flowers begins only 3 to 6 weeks after petal fall, the importance of early blossom (flower) thinning is obvious. Whilst the more popular thinning of fruitlets at the 12 mm fruitlet diameter stage will give good benefits in improving flowering in the subsequent season, even better flower quality should be achieved by earlier blossom thinning.

It is interesting to note that the variety Cox and its clones is described as having slight tendencies to biennial bearing in several countries (e.g. New Zealand).

• If the UK climate continues to get warmer, resulting in heavier crops that are not thinned correctly, biennial bearing may become more of a problem.

The usual methods employed to overcome biennial bearing are:

• Pruning of flower spurs in the winter prior to the ‘on’ year.
• Thinning of flower using ATS.
• Thinning of fruitlets early in the season.

 Other strategies that have been tested in research trials involve:

• Treatments with plant growth regulators.

Pruning of flower spurs in the winter prior to the ‘on’ year

The strategy employed here is to remove surplus flower clusters, usually spurs, so that excessive numbers of fruits cannot set in the subsequent spring.

• Knowledge of the variety is essential and the correct degree of spur pruning is only achieved with experience.
• It is also important not to prune the trees so severely so as to induce excessive regrowth in the following year.
• The aim is to achieve the optimum balance of fruit set and growth.
• Prune away excessive spurs in the winter prior to an ‘on’ year.

Treatments with plant growth-regulating chemicals

Most of the growth regulating chemicals that have been tested for their efficacy in reducing the problem of biennial bearing have brought about their effect by flower or fruitlet thinning. However, there are a few isolated cases of other chemicals inducing favourable responses.

• Trials conducted at Long Ashton Research Station in 1977 showed that sprays of the chemical bromouracil (50-75 ppm) to biennial-bearing cider apple varieties in the ‘off’ year reduced biennial cropping.

Sprinkling apple trees in the spring with water

The evaporative cooling effect of water can be used to delay the flowering of apple trees in the spring. Research conducted at East Malling in the 1970s showed that, by using overhead sprinkler systems, blossoming of Cox’s Orange Pippin could be delayed by up to 14 days (Hamer and Boustred). In further research, the authors attempted to construct a computer model to evaluate the effects of evaporative cooling on apple bud dormancy (see Hamer and Boustred, 1980).

• In these trials, Cox trees were misted with water via overhead sprinklers when ambient air temperatures exceeded 4.5^oC.
• The misting commenced in mid February and continued until mid April.
• The buds were cooled by as much as 5^oC and this induced a delay in flowering of eight days in comparison with non-misted controls.
• Fruit setting potential (as determined by hand pollination) was significantly improved on the misted trees and these trees produced approximately 30% more yield.
• The misting was shown to have beneficial effects on ovule maturity in the flowers, by reducing the proportion of flowers with over-mature ovules.
• Care is needed when using this strategy.
• However, an application of excessive misting, whilst maximising bloom delays, increases the water content of the buds, such that they are more, rather than less, sensitive to frost damage (Hamer and Newman, 1981).

Further research may be warranted to examine misting techniques to delay flowering times of apple varieties.

Physiology of flower initiation and development

The processes of flower initiation in apple

Flowers on apple trees are formed on three types of bud:

• Axillary buds, formed in the basal axils of shoots made in the previous season
• Spur buds formed often in clusters on two-year-old or older wood
• Terminal flower buds, which are formed on the ends of short shoots.

The processes by which these buds are formed are complex.  The beginning of floral differentiation is seen as a flattening of the apical dome in a bud which, up to that point, could be considered vegetative.

• Before any visible change in the bud morphology is seen, the meristem of the bud is programmed to form flowers by some, as yet unknown, signal or biochemical stimulus.
• Once the bud transforms from vegetative to floral this process is believed to be irreversible.
• Management or climatic factors occurring after this event may change the quality of the bud but not whether it is floral.

To be sensitive to the inductive stimulus (whatever that may be) and to change to floral type, a bud must be in a certain phase or stage. This is characterised by a critical number of nodes, usually 16-20 (Faust, 1989; Buban and Faust, 1982).

• Additional requirements are a minimum duration (possibly 7 days) of what is known as the plastochron. This is the rate of production of new primordia in the apple bud.
• This plastochron remains very stable throughout long periods of the season, irrespective of climatic conditions.
• However, research at East Malling showed that the length of the plastochron did change, relatively abruptly, during the season and it was usually 5, 7 or 18 days.
• These values are thought to be appropriate for many apple varieties.
• The plastochron is controlled by the younger leaf primordia in the bud, which may themselves be controlled by the foliage.
• It is these changes in plastochron which help to determine whether the buds formed are vegetative or floral.
• A final necessity for buds to become floral is the presence of bracts.  For further details see Fulford, (1962); Fulford, (1965b) and Buban and Faust, (1982).

The presence of leaves is a prerequisite for flower bud production and it is believed that increasing numbers of leaves on spurs will increase flower initiation. This effect is probably indirect and operates via its effect on available photosynthates.

The influence of rootstock and/or interstock on flower initiation and development

Rootstocks and interstocks have been shown to have a significant influence on flower initiation and development in apple scions. How they bring about these effects is not fully understood.

• It is hypothesised that rootstocks in some way change the partitioning of assimilates (photosynthates) and minerals, such that the sites of floral initiation (e.g. spurs) become strong sinks and the development of increased numbers and better quality floral buds is a consequence of this.
• Part of this change of assimilate partitioning may be explained by rootstock effects on shoot vigour.
• Dwarfing rootstocks bring about slower rates of extension shoot growth during the summer months, but, more importantly, they bring about an earlier cessation of active shoot extension.
• Once the active extension of shoots has stopped and a terminal resting bud is formed, alternative sinks for assimilates in the tree will become more dominant.
• Assimilates will move much more readily to the sites of floral bud initiation in spurs or to roots once active shoot growth ceases.

This explanation is partially flawed, however, as a few invigorating clonal rootstocks (e.g. M.25) stimulate the initiation of many more floral buds on scions than other rootstocks of similar vigour potential.

• This suggests that rootstocks control floral initiation in the scion directly, as well as indirectly via their effects on shoot growth. How they do this is not understood.

Rootstocks do influence the concentrations and movements of plant hormones (auxins, gibberellins, cytokinins and abscisic acid) within the tree (Soumelidou, et al., 1994; Kamboj, et al., 1997).

• It is likely that these differences are in some way involved in their influence on floral bud initiation in the scion.

Another possibility is that as many rootstocks improve the branching angle of scion trees, making branches more horizontal, this could contribute indirectly to their effects on floral initiation.

Effects of pruning and training techniques on flower initiation and development

Both shoot pruning/training and root pruning/restriction techniques may influence flowering on apple trees.

Shoot pruning and training

• It is well known that shoot bending towards or below the horizontal is very effective in inducing increased numbers and quality of floral buds in apple trees.
• What is less well understood is why shoot bending has this effect. It can be suggested that shoot bending slows down new shoot extension, and because new extension leaves form abundant supplies of gibberellins, the result is to reduce the effect of gibberellins on flower bud inhibition.
• However, shoot bending stimulates improved flowering, even when there is no active shoot growth on a branch, indicating a more complex relationship than may at first be apparent (Tromp, 1972).

Research on clones of Red Delicious grown in the USA has shown that summer pruning 30 days after flowering increased spur length and the number, size and area of spur leaves but did not influence flowering spur diameter (Rom and Barritt, 1990).

Root pruning and restriction

In trials conducted at HRI-East Malling, root pruning or root restriction of young apple trees has been shown to increase the abundance of flower production (Webster, et al., 2000).

• However, flowering is not always increased by root pruning, as shown by trials on mature Golden Delicious trees growing in the USA (Schupp et al., 1992)
• These USA trials did report an advancement in the time of flowering following root pruning.
• Root pruning causes reductions in shoot growth, a temporary negative water potential, reduced stomatal conductance, transpiration and photosynthesis.

The effects of defoliation on flower initiation

Removal of spur leaves on temperate fruit trees is known to inhibit flower production for the subsequent season (Huet, 1973).

• This inhibitory effect may be due to the removal of sites for cytokinin synthesis, which is believed to be important in flower initiation (Hoad, 1980; Ramirez and Hoad, 1981).

The importance of spur leaves has implications concerning the use of certain chemicals for flower thinning.

• Use of urea as a flower thinner may result in reduced flower numbers in the subsequent year.
• Urea often thins by damaging the spur leaves surrounding the flowers.

Work on spur types of Red Delicious growing in the USA has shown that reducing the spur leaf area on non-cropping (vegetative) spurs in August caused a reduction in flower number but not flower size in the subsequent season (Rom and Barritt, 1990).

• Reducing leaf area on the fruiting spurs caused a reduction in the growth of the bourse shoot in the subsequent season.

The influence of crop load on flower initiation and development

Attempts to understand how crop load influences flower initiation in apple were made at Long Ashton Research Station in the 1970s (Abbott et al., 1975).

1. The results are shown below:

Treatment No.	Total fruit number/tree 1973		No of short shoots/tree (>5  cm) 1973	Length of longest shoot/tree (cm) 1973	Fruit buds /tree 1974
	<60 mm	>60 mm
1	90	79	6.5	46	0
2	30	89	7.6	46	0.2
3	5	86	6.9	42	1.0
4	0	50	10.1	51	7.4
5	–	–	12.0	85	10.3

• It can be seen that high fruit numbers on these small trees reduced shoot growth as well as almost completely inhibiting flowering in the subsequent season.
• However, although return bloom was closely correlated with fruit numbers in the previous year, in this experiment no such correlation was found with the development of flowering buds.
• Anatomical observations of buds harvested from branches sampled throughout the season showed bud development at roughly the same rate.
• The first visible flower initiation was not noted until mid-August and buds from all the treatments achieved this at approximately the same time.
• The experiment indicated that fruit number on trees (crop loading) influences the intensity of flower initiation through some mechanism other than rate of bud development.

However, other work at Long Ashton (Abbott, 1984) indicated that cropping on trees extended the ‘plastochron’ (the time taken for each node on bud primoridia to form) from 8 to 19 days from mid-June onwards; i.e. it slowed down bud development.

• Another effect of heavy crop loads is reduced root development.
• If one accepts the hypothesis that root activity associated with nitrogen assimilation is a necessary stimulus to flower initiation, then this could be an added reason for heavy cropping reducing flowering.
• This might also explain why root pruning, which stimulates root growth at the expense of shoot growth, also stimulates flower production.

Timing of the thinning operation can be critical in influencing return bloom on apple trees. This has been shown to be particularly relevant with varieties such as Boskoop where biennial bearing may be a problem in some seasons.

• Thinning at full bloom or up to two weeks after had a very positive effect on return bloom in the subsequent season.
• However, thinning three weeks after bloom was too late and no more flowers were initiated than on the unthinned controls (Tromp, 2000).
• This is perhaps surprising, as the formation of significant amounts of gibberellins by the fruits (which are believed to be a major cause of flower bud inhibition) had not begun during this three-week period.
• Competition for vital assimilates may play an important role in flower bud initiation in this early post-flowering period.

Climatic effects on flower initiation and development

The main climatic effects on flower initiation and development are as follows:

• Climatic effects on flower initiation in the season prior to flowering.
• Climatic effects on winter dormancy.
• Climatic effects on pre-budburst flower quality.

Climatic effects on flower initiation in the season prior to flowering

In research conducted many years ago at Long Ashton Research Station, the effects of different day/night temperatures on flower development were assessed (Abbott and Bull, 1973a).

• Trees of Cox’s Orange Pippin, which were subjected to temperatures of 9^oC during the day and 5^oC during the night during August to December, reached full bloom at the beginning of April in a glasshouse.
•  However, only 23% of the buds were floral and the buds were of what was described as of ‘young’ type.
• The fruitlets on these trees dropped steadily and at harvest only a few ‘king’ fruits remained.
• In contrast, trees retained at 17^oC during the days and 13^oC during the nights from August to December were slow to break bud in the spring and did not flower until the end of April (three weeks later than the trees subjected to cool autumns).
• With these trees, 70% of the buds were floral and the clusters were typically of the ‘old’ type.
• Fruit drop was less on these trees and concentrated mainly in the June drop period, but the fruits formed were slightly flattened with short stalks.
• The best yielding trees were those subjected to 13^oC during the day and 9^oC during the night between August and December.
• It is interesting to note that these temperatures were closest to those normally experienced in the orchard.

In research conducted by the same Long Ashton team, the influence of temperatures between the end of April and mid-October on flower initiation of Cox was also studied (Abbott and Bull, 1973b). 

• Cox trees subjected to day temperatures of 13.5^oC and night temperatures of 7.5^oC developed necrotic spots on the primary (spur) leaves which were similar to ‘Cox Spot’ and the new unfolding leaves showed symptoms similar to zinc deficiency.
• Flower numbers in the subsequent spring were average for this treatment but the flowers were variable in their time of opening.
• Other characteristics of this treatment were large spur leaves, a high rate of flower abortion, long bourse or cluster axes, and short flower stalks.
• This all suggests that bud development was slow and flower initiation was late on these trees. The fruits formed were mostly kings.
• Trees subjected to daily fluctuations of 13.5^oC and 7.5^oC developed bourse shoots, which grew out, and then rosetted and formed short shoots a few cm long.
• This second flush occurred too late for flower buds to develop properly within these spurs and flower numbers were low in the subsequent spring.
• Trees maintained in temperatures of 21.5^oC day and 15.5^oC night produced ‘old’ type flower clusters with small spur leaves and uniform flowers with long stalks and short bourse shoots.
• Unfortunately, the blossoming abundance on these trees was very variable being inversely related to the fruit numbers/tree carried in the previous summer.

The general conclusion is that higher than average summer temperatures improve flower initiation on apple trees grown in the UK.

• This is thought to be due to the temperatures inducing flower initiation early in the season, allowing more time in the season for successful development of flowers.
• However, care must be taken when trying to use increased temperature to stimulate flowering.
• Trees experiencing high temperatures under glass or polythene structures usually show reduced flower initiation and development.
• This is caused by the stimulation of competitive extension shoot growth and bourse shoot flushing, as well as the lower light levels under protective structures.

Research conducted in Canada has shown that high temperatures (30^oC or higher) in June during flower initiation have a deleterious effect on flowering in the subsequent season, as do temperatures over 26^oC in August during flower development (Caprio and Quamme, 1999).

• Such high temperatures are unlikely to be a limit on flower initiation in the UK climate.

Climatic effects on winter dormancy

• Apple trees of most scion varieties are believed to require an accumulation of approximately 1000 hours of temperatures of less than 8^oC but higher than freezing in order to satisfy their winter chilling (dormancy) requirement.
• Temperatures of above 12^oC are thought to result in a loss of some of the accumulated chilling units.
• Many of the ornamental Malus species require several hundred hours less chilling units.

Studies conducted in New York State in the USA in the early 1990s (Hauagge and Cummins, 1991) showed the following chilling unit requirements. The values are means of three years data:

Cultivar name	Mean chilling unit requirement (hours)
Anna	218
Delicious	1093
Elstar	1027
Empire	1079
Fuji	1077
Gala	1064
Golden Delicious	1050
Granny Smith	1049
Idared	1017
Prima	1072

In the 1970s, USA researchers developed a chilling unit model (Richardson et al., 1975) for use on peaches and several attempts have been made to use this model with other temperate fruit crops including apple (Shaltout and Unrath, 1983).

• Information more specific to UK conditions was first presented by Landsberg, (1974) and Williams et al., (1979).
• The chilling requirement is primarily needed to the buds themselves (Samish, 1954) but studies in the USA also indicated that for effective bud break in the spring the tree roots also need to be chilled (Young, 1988).

Once the dormancy requirement has been satisfied, the buds are capable of developing into flowers or vegetative shoots if subjected to suitable ‘forcing’ (>15^oC) temperatures.

Climatic effects pre-budburst on flower quality

It is generally accepted that once the chilling unit requirement has been satisfied and dormancy broken, buds will develop through to flowering if subjected to what are known as forcing temperatures.

• Buds develop at any temperatures over about 10⁰C, but development is speeded up by higher temperatures.
• The time taken for the flowers to break bud is variable depending upon the scion variety, the bud type and how well the dormancy requirement has been satisfied.
• Temperatures that are unseasonably high during February, March and April (average maxima of 10^oC or above) can have a negative influence on flower quality.
• The possible reasons for this effect and its implications are described in the previous section.

The influence of water supply to the tree on flower initiation

In experiments conducted in Germany under very controlled environments and water supply, it was shown that when water supply was reduced to 50% or 25% of the trees estimated needs, the numbers of flower clusters and flowers per tree were reduced (Sritharan and Lenz, 1988).

• However, shoot growth was also greatly reduced and when estimates were made of flowering or fruit set per metre shoot length, the drought treatments caused increases in flowering.
• Unless drought is very severe, it is unlikely to influence the efficiency of flower bud production or flower bud quality.

Effects of tree mineral nutrition on flower initiation and development

The evidence for mineral supply having a significant influence on flower initiation is only strong when considering nitrogenous fertilisers.

• Applications of high rates of nitrogenous fertilisers, together with sufficient water to facilitate their uptake by the tree, will increase the rate and duration of active shoot growth.
• This in turn will delay the initiation and development of floral buds.
• One apparent anomaly is that whilst nitrogen is antagonistic to flowering, as stated above, root growth and associated nitrogen assimilation is thought to be vital for flower initiation (Abbott, 1984).
• This might be explained by the fact that nitrogen applications whilst extension shoot growth is still active are indeed antagonistic to flower initiation, whereas nitrogen applications after termination of shoot growth often improve flower quality.

For many years there was a theory that flower bud initiation was dependent upon the establishment of a critical ratio between the amounts of carbohydrates and the amounts of nitrogen present; the so-called C/N ratio.

• When the ratio is high in favour of carbohydrates then flower bud initiation was thought to occur. However, this theory has now been largely discounted.

The evidence for other elements having a strong effect on flower initiation is poor.

• Indeed, in experiments conducted in controlled environment conditions, scientists at Long Ashton Research Station found no significant effect of mineral supply on flower initiation on Cox.
• However, these experiments were conducted under glass, where temperatures were higher and light levels lower than those experienced in the orchard.
• Subsequent trials at Long Ashton indicated that flower initiation was favourably influenced by an optimal balance of high light, cool temperatures and good nutrition.

Effects of natural and applied plant hormones on flower initiation and development

Apple trees produce several chemicals, often called endogenous hormones, which are important in signalling within the tree and in stimulating many vital processes associated with growth and cropping.

• Some of these natural hormones, especially the gibberellins, but also auxins and cytokinins, have been implicated in the control of initiation and development of flowers on the apple tree.

Alternatively, chemicals that inhibit plant processes may be used in flower initiation. These are mostly plant growth retardants, which inhibit the tree’s production of gibberellins. The most important of these are:

• Paclobutrazol e.g.Cultar
• Calcium Prohexadione e.g. Regalis

Gibberellins

It has been known for many years that gibberellins have an important influence on the formation of reproductive organs in apples and many other seed plants (Pharis and King, 1985).

• Applications of gibberellins in spray treatments have been shown to induce parthenocarpy and improve fruit set in apples (Goldwin, 1981) and also to inhibit flowering (Hoad, 1984).
• Strong competing shoot growth or excessive numbers of fruits inhibit flowering on apple trees and it can be speculated that gibberellins, formed either in the fruit seeds or in the shoot tips, are the signal hormone responsible for this effect.

Research in Holland showed that both GA₃ and GA₇ strongly inhibit flowering whilst GA₄ has no inhibitory effect on flowering (Tromp, 1982).

• Indeed, research conducted in Canada suggests that GA₄ sometimes promotes flowering in apple (Looney et al., 1985).
• Work in Germany has supported these findings to some extent. It has shown that, whilst gibberellic acid (GA₃) is very inhibitory, GA₄₊₇ has very little inhibitory effect upon flower initiation (Prang et al., 1997).
• This work also showed that there was a peak in endogenous (natural) levels of gibberellins (200-450 pg/fruit) in the trees of apple scion varieties at approximately 4-6 weeks after full bloom.
• This timing is known to be critical for flower bud initiation.
• The problem with this hypothesis, that gibberellins are responsible for inhibiting flowering in apples, is that both biennial varieties, such as Elstar, and non-biennial varieties, such as Golden Delicious, appeared to export approximately the same amounts of gibberellins.

The beneficial effects of GA₄ on flowering, reported by Looney et al., (1985) are not consistent.

• In research conducted in the USA (Greene, 1993b), GA₄ has stimulated and increased, as well as reduced, flowering in Golden and Red Delicious trees.
• Nevertheless, even when GA₄ induced negative effects on flowering these effects were less severe than those induced by GA₇ at the same concentration.

Auxins

Recent studies in Germany (Callejas and Bangerth, 1997) have suggested that the downward (basipetal) transport of the auxin IAA in the apple shoot could play a role in flower initiation.

• This hypothesis is based on observations that showed that applications of gibberellins to apple shoots (which are known to suppress flower initiation) increased the amounts of IAA diffusing down from the tips, during the critical stage for flower initiation.
• Also, seed number in fruits is known to influence flowering.
• The high numbers of fruits, and hence seeds, produced on biennial bearing varieties in their ‘on’ year is thought to be a major cause of the reduction in flowering in the subsequent ‘off’ year.
• The German work showed that increased seed numbers in fruits were associated with increased auxin transport from the fruits towards the spurs and the sites of potential flower initiation.

Cytokinins

Leaves, as well as fruits, have been shown to be the sites of manufacture by the plant of cytokinins.

• The observation that removal of spur leaves from apple trees inhibited flowering in the subsequent season led some researchers to hypothesise that this reduction in flowering was due to the associated reduction in cytokinin supply to the spurs.
• Research by Hoad (1980) lent credence to this hypothesis.
• In this work and in that of Ramirez and Hoad, (1978) it was shown that the inhibitory effect of spur leaf removal on flower initiation could be reversed by local applications of cytokinins to the spurs.

In research conducted by McLaughlin and Greene (1984) the authors suggested that the main influence of cytokinins on flower initiation was in overcoming the negative influence of diffusing gibberellins.

• Applications of the cytokinin benzyl adenine (BA) to fruiting shoots increased the numbers of flowering spurs in the subsequent season whilst similar applications to non-flowering shoots did not.
• The same research showed that BA applications increased the numbers of flowers per spur on both fruiting and non fruitling limbs and the effect seemed to be one of stimulating lateral flower bud formation.
• The effect of BA thinning sprays on return bloom of apples is thought to be entirely due to the effect of the sprays on fruit, or more importantly seed numbers (Greene and Autio, 1994b).

Other cytokinins used for thinning apples, such as CPPU and thidiazuron do not promote return bloom, suggesting that, in comparison with BA, they have some direct negative influence on flower initiation (Greene, 1993).

Paclobutrazol (PP333 or Cultar)

Trials at East Malling in the 1980s showed that apple trees treated with paclobutrazol frequently formed more flower buds/tree and many more flower buds/metre shoot length than untreated trees.

• It is thought that this effect is stimulated by paclobutrazol’s effect in reducing internode length and shoot length and the partitioning of additional assimilates into the formation of flower buds.