Improving fruitlet growth – additional information

Ensure that fruitlets set with adequate numbers of seeds

Research conducted in New Zealand has shown that enhancing the pollination of the variety Braeburn not only increases fruit set but it also increases seed numbers and the calcium content of the harvested fruits (Volz et al., 1996).

Further research in New Zealand showed that, with increasing distance from pollinator trees, Braeburn fruits had lower seed numbers, increased poor shape, lower calcium concentrations and increased incidence of bitter pit and lenticel blotch (Brookfield et al., 1996).
Research conducted in Russia many years ago suggested that choice of pollinators could affect the size, shape and quality of the fruits produced.
However, attempts both at East Malling and Long Ashton to substantiate this effect (known as metaxenia) have proved unsuccessful.
The only likely effects of pollinator choice on fruit size or shape is possibly via their effects on seed numbers.
Ensure that apples set with adequate numbers of seeds as this will improve fruit size and shape and also increase calcium uptake on varieties such as Braeburn.

Optimise crop loading by judicious thinning of flowers and/or fruitlets

It is important to that reducing crop loads not only improves the final fruit size of apples.

At lower crop loads, the apples are often of improved quality; they ripen earlier, are sweeter, often firmer and have higher levels of dry matter.
Work conducted at East Malling many years ago showed the effects on fruit size, quality and storage potential of thinning the variety Cox’s Orange Pippin (Sharples, 1968).
More recent work on the same variety (Johnson, 1995) has shown that the ripening maturity of fruits is advanced by thinning treatments conducted at 5 days after full bloom but not by similar thinning treatments carried out 27 or 39 days after full bloom.
Advances in maturity, as judged by internal ethylene concentration, were as much as 16 days.
Similar effects of thinning on fruit quality at harvest time have also recently been demonstrated for Braeburn apples growing in Spain (Kelner et al., 1999).

Cell division

Thinning apples can significantly increase cell division in the fruits remaining on the tree.

However, only early thinning, either of flower buds or flowers, can achieve this objective.
Delaying thinning to the 12 mm fruitlet stage or later will often have only minimal effects on cell division in the persisting fruits, although it will result in increased cell expansion.
This is explained by the fact that cell division in fruits such as Cox is completed by about 6 or 7 weeks after full bloom (Denne, 1960).
Removal of excess flower buds in the winter months will also have benefits to cell division in the fruits forming on the remaining flower buds.

Fruit firmness

More recently, studies conducted at East Malling on the apple variety ‘Cox’s Orange Pippin’ (Johnson, 1992 and 1994) showed that the firmness of fruits at harvest was increased significantly if the trees were thinned, even when fruit size was increased by the thinning.

Subsequent studies showed that thinning to single fruits/cluster in the period between 5 and 15 days after full bloom gave the best results.
Thinning to similar levels at 25 days after full bloom gave no increase in firmness at harvest, but later thinning did improve harvest firmness.
All the recorded increases in fruit firmness were associated with increases in the percentage dry matter in the fruits.
The increases in harvest firmness were maintained after storage of the fruits in CA conditions (2% O₂and <1% CO₂) but not in CA with lower O₂ levels or in air storage.
In the low O₂ regime (1.25%) the enhanced softening is thought to have been attributable to increased sensitivity to core flush and senescent breakdown, which may have been related to the higher K and lower Ca status of these fruits.

Early thinning of apples by blossom thinning or very early fruitlet thinning will reduce the competition within the tree for assimilates and improve cell division in the persisting fruitlets.

Early thinning will improve fruit size and firmness and advance ripening.
However in some situations early thinned fruits may be more sensitive to storage disorders, such as senescent breakdown and core flush.

Climatic conditions in the first month after fruit set

Warm temperatures in the 4 to 6 weeks after fruit set will have a very beneficial effect in encouraging cell division in the young developing fruits. Such conditions will also aid the development of bourse and extension shoot leaves which will begin to contribute to the vital carbohydrates produced by photosynthesis which fuel the growth of young fruitlets.

Any improvement of orchard site conditions, such that temperatures are raised slightly, may have a significant beneficial effect on cell division in the fruits.
Aim to increase temperatures in their orchards during the first six weeks following fruit set, so as to maximise cell division in the young apple fruits.
Choice of training systems has also been shown to influence fruit size, possibly via their effects on flower bud quality and cell division in the young fruitlets.
Training systems which maximise light exposure of the canopy, such as Tatura or other canopy systems frequently improve cell division and produce fruits of larger than average size.

Prune and train trees so as to optimise exposure of fruits to light and achieve good colour development

Prune and train trees so as to maintain a balance of flowering spurs and new extension shoot growth.

Lack of new growth, as may be experienced with trees of dessert varieties on M.27 rootstock, leads to high yield productivity but poor fruit size and grade-outs.
Stimulate growth on trees producing too little extension growth by making cuts back to vegetative buds in positions where new shoots/branches are desired and remove weak shoots and spurs.
The ends of weak fruiting branches should be tipped to an upward facing bud especially in varieties that crop on one year wood.
On trees with excessive shoot growth, reduce this by training shoots to the horizontal (or lower).
Where summer shoot growth results in shading of fruitlets and poor fruit colour at harvest use summer pruning techniques to open up the tree canopy and expose the developing fruitlets to better light conditions.

Observations on Gala trees growing in New Zealand suggest that large fruits are only produced on large calibre fruiting wood (Wilton, 1997 see Further reading [hyperlink]).

Fruit bud quality and the strength of carrying the buds is believed to be more important than the age of wood.
Buds formed on one-year-shoots (axillary flower buds) can set and size good fruits, if they are carried on wood of 10 mm or more in diameter.
The same author believes that bourse shoots, which terminate growth early in the season, are essential for maximising fruit growth potential.
Weak wood should be shortened to two or three buds.
This weak wood is usually the source of small fruits, especially if it is located in shaded parts of the tree canopy.

Maintain adequate and balanced tree nutrition and water supply

If fruitlets are to expand and develop into high quality large fruits at the time of harvest it is essential to maintain adequate supplies of water and nutrients to the tree throughout the growing season.

Irrigation

The influence of different amounts and timings of water supply to the growth and final size and quality of apple fruits has been studied in many regions of the world.

Unfortunately, the results are often variable, and are probably influenced greatly by local climatic conditions and soil types.
Reduced supply of water to trees growing in areas experiencing hot dry summers, results, as expected in a higher proportion of smaller fruits at harvest time.
As might be expected, application of increasing amounts of irrigation result in faster growth rates for fruitlets and larger fruit size at harvest.
However, several reports show irrigation of fruits also reduces fruit firmness and lowers the soluble solids content, in some but not all years (Bonany and Camps, 1998).

In most UK orchards and in most seasons it is not a problem in UK climatic conditions to maintain adequate supplies of water and nutrients to the tree throughout the growing season.

However, where trees are grown on shallow or sandy/gravel soils supplementary irrigation will be a necessity in most seasons.
This is best provided using a trickle irrigation system, which can supply water regularly and in low controlled amounts.

Water needs are estimated using calculations of water deficits based on evapotranspiration or are measured using various other systems (neutron probes, tensiometers etc.).

Research conducted on Braeburn trees has shown that witholding necessary irrigation water early in the season (up to 104 days after full bloom) reduced shoot growth and mean final fruit weight, although rates of photosynthesis did not appear to be affected.
In contrast, witholding water later in the season (i.e. 104 to 194 days after full bloom) had no influence on mean fruit size (Kilili et al., 1996a).
The same authors (Kilili et al., 1996b) noted that witholding water late in the season increased total soluble solids, soluble sugars (fructose, sucrose and sorbitol), flesh firmness and red skin colour intensity.
This late witholding of water, therefore, may have merits in certain situations. However, care must be exercised when using these Regulated Deficit Irrigation (RDI) systems of management.

Research in the USA (Ebel et al., 1993) has shown that whilst RDI systems produced apples with higher soluble solids, the fruit sizes at harvest were reduced.

If fruit size is not to be reduced deficits should only be imposed late in the growing season.

Recent work in New Zealand showed that irrigating Braeburn trees on MM.106 rootstock improved the concentrations of calcium in the fruits at harvest (Mills et al., 1994).

Fruits from non-irrigated trees developed sugars slightly earlier than fruits from irrigated trees.

Nutrition

Nutrient deficiency is unlikely to influence fruitlet growth in most conventional orchards, which are fertilised to established recommendations. However, nitrogen and potassium deficiency in organic orchards is likely to be a problem.

Ensure that the trees are supplied with adequate water and nutrients so as to maintain the cell expansion and growth of apple fruitlets.
Monitor water requirements regularly, using calculations of evapotranspiration or other equipment marketed to measure soil water levels.
Check soil and leaf nutrient levels regularly and adjust nutrition accordingly.

Avoid use of crop protection or other chemical sprays which may cause damage to the skin of fruitlets (russeting)

Most pesticides currently approved for use with UK apples will cause no problems of skin russeting, if applied according to the label recommendations.

However, growers should take care when using tank mixes of several different products.
Only use the tank mixes recommended by the manufacturers.
Occasionally, applications of two or more products may cause russeting.
This is often the result of incompatibility of the mixtures and/or enhanced uptake due to the combination of several surfactants.
Adhere to label recommendations for pesticides, concerning rates, water volumes, compatible tank mixes and weather conditions, so as to avoid phytotoxicity damage and loss of fruit quality due to russeting.

Apply gibberellin sprays to improve final skin finish of fruits

Trials conducted at HRI-East Malling in the early 1980s showed the benefits of applications of GA₄₊₇ (Regulex) on the skin finish of Cox, Discovery and Golden Delicious (Taylor et al., 1985).

Four sprays at low concentrations (2.5-10 ppm), commencing at first open flower and applied at 10-day intervals, significantly improved the fruit grade out by reducing the russeting on fruits.
Only the sprays applied at the highest concentration (10 ppm) had any significant effect on return bloom the following season.
When pure formulations of GA₄ and GA₇ were compared the former produced the best response, in terms of russet prevention and also had the least effects upon return bloom.

Trials abroad have shown that sprays of Promalin (16 ppm of a mixture of GA₄₊₇ and benzyl adenine) can reduce russeting on varieties such as Golden Delicious (Eccher and Maffi, 1986).

Promalin is approved for use on some varieties of apple, especially in the USA.
These sprays improve fruit shape by increasing the length:diameter ratio of varieties such as Red Delicious (Curray and Williams, 1983; Unrath, 1978) and may also increase fruit weight (Unrath, 1978).
Applications of Promalin at 25 ppm at full bloom gave the most beneficial effects.
Trials with Promalin on Cox, conducted at Long Ashton in the late 1970s, showed no effects on fruit length:diameter ratios (Child et al., 1980).
On varieties such as Cox and Discovery apply sprays of gibberellic acid (e.g. Berelex) at label recommendations (4 times at 5-10 ppm, at 10 day intervals commencing at first open flower) so as to improve skin finish at the time of harvest.

The physiology of fruitlet growth

After fruit set, the growth of the persisting fruitlets is brought about by a combination of cell division and cell expansion.

Research undertaken many years ago showed that the majority of cell division ceases within 4 weeks of pollination (Bain and Robertson, 1951), whereas cell expansion continues throughout the growth period of the fruit.
In this Australian research conducted on Granny Smith apples, final differences in fruit size were associated more with cell number than with cell size.
Even earlier research on the variety Bramley’s Seedling similarly showed cell division to cease only a few weeks after fruit set (Tetley, 1930 and 1931).

Research in New Zealand on the variety Cox showed that fruit weight increased slowly for the first few days following pollination then increased exponentially at a very rapid rate for three weeks and, thereafter, increased at diminishing rates (Denne, 1960).

Cell division was very rapid in the three week phase of exponential growth and then declined but some cell division occurred until 6 or 7 weeks after pollination.
Cell expansion was also very rapid until 7 weeks after pollination but then occurred at diminishing rates until harvest.

Studies on the variety Cox in the early 1980s showed that differences in cell size at harvest were not an important factor influencing final fruit size (Skene et al., 1984).

The indications were that fruit size was more influenced by cell numbers and that few new cells were produced after the fruitlets had reached approximately 18 mm in diameter.

The main bulk of any apple fruit, which is thought to develop from the receptacle in the flower, consists of a parenchymatous tissue forming the flesh, of which the cortex constitutes the greatest percentage by volume. The cells of the cortex are of spongy tissue, large, thin-walled with a thin lining of cytoplasm and a large vacuole.

A number of factors, other than crop loading are known to influence cell division and expansion. These are:

Photosynthesis and carbon supply to the fruitlets as influenced by temperature and light.
Production and movement within the tree of natural plant hormones.
Tree health.

Photosynthesis and carbon supply to the fruitlets

Work conducted in the USA, using equipment capable of measuring the light interception of the various types of leaves and the gas exchange associated with a whole apple tree, has enabled scientists to work out the photosynthetic potential and the tree’s assimilate needs to sustain a crop load at various times during the season (Lakso et al., 1998).

This works shows that the demand for assimilates increases rapidly following bloom, peaking at about 4-6 weeks after full bloom.
Thereafter, it remains fairly stable through until harvest.
Comparing this demand with the tree’s potential to supply showed that the carbon production by the apple tree leaf canopy cannot support all of the fruits, which potentially could set, for more than a week or more after bloom.
The work shows that, during the season there are two periods of potential limitation of carbon supply; one about 2-4 weeks after blooming and the other in the last few weeks prior to harvest.

This identification of a critical period for assimilate supply is supported by other recently published research.

Researchers in Michigan showed that applications, 15 to 30 days after full bloom of chemicals known to limit photosynthesis temporarily (e.g. terbacil) caused fruitlet abscission (Flore et al., 1998).

Research at East Malling has shown that shading of apple trees also reduces photosynthesis and may cause reductions in fruit size, as well as reducing flowering and fruit set (Jackson and Palmer, 1977).

However the shading needed to significantly reduce fruit size was 63% or more.
Severe shade (only 10% of full sunlight) reduced fruit size by 30%.
Further work by the same team (Jackson et al., 1977) showed that the shading reduced fruit size by reducing both cell numbers and individual sizes within fruits.
Shading also reduced fruit colour, russeting, cracking, and the dry matter and starch per unit weight.
Research in Japan has shown that glucose, fructose and sucrose contents of apple fruits are all decreased by shading.

It is argued that the high productivity of compact trees, such as those produced on dwarfing rootstocks, is attributable to a high level of light interception within the tree canopy; this in turn improves photosynthetic capacity (Robinson and Lakso, 1991).

Training systems also influence light interception and hence photosynthesis and fruit size.
German research has shown that trees trained to a Y trellis system bore more fruits and larger fruits than trees grown on spindles (Chen et al., 1998).
Fruits from the Y trellis system were also higher in dry matter, total soluble solids, starch, glucose, fructose, sorbitol and total non-structural carbohydrates.

Production and movement within the tree of natural plant hormones

Gibberellins have been strongly implicated in the growth of fruits. Many gibberellins have been identified in the seeds of apple fruits (Hedden et al., 1993). Soon after anthesis (first opening of the flower), high concentrations of the active gibberellins A₁, A₃, A₄, and A₇ are found in the apple seeds.
These are believed to provide an important stimulus to fruitlet growth (Hedden and Hoad, 1985).
Gibberellins are also essential for the development of apples with good skin finish and freedom from russet.
Sprays of GA₄₊₇ are applied to improve skin finish.
Trials in the USA indicate that gibberellins are also implicated in determining the shape of fruits. Fruits of the variety Red Delicious which are high in gibberellins at or just after flowering develop a more angular shape at their basal end.

A review of recent studies focused on the importance of gibberellins in fruit set and development has been produced by Garcia-Martinez (1997).

Apple Best Practice Guide