Most varieties of apple are self-sterile requiring pollination by another variety for successful fruit set. The exceptions are
- Self-fertile clones
- A number of varieties which seem partially self-fertile in specific climatic conditions
- Self-fertility induced by pollen mixtures
Self-fertile clones
A programme of breeding using irradiation techniques, begun by Long Ashton Research Station in the late 1960s and transferred to East Malling in the early 1980s, has been successful in producing self-fertile clones of the commercial apple varieties Cox’s Orange Pippin and Queen Cox.
- Two self-fertile clones have been released commercially. Self-fertile Cox Clone 8 is mainly sold into the home garden market.
- Of more interest to commercial fruit growers is self-fertile Queen Cox clone 18, which now accounts for a proportion of the sales of Queen Cox trees in the UK.
In seasons when climatic conditions at flowering time are unfavourable for pollen transfer by bee vectors, these self-fertile clones yield better than the more conventional self-sterile clones.
- Fruit size, colour and storage potential are similar on the self-fertile and self-sterile clones of Queen Cox.
- Self-fertile Queen Cox clone 18, which is available from UK nurseries, gives more reliable cropping than the traditional self-sterile clones, in years unfavourable for pollen transfer between varieties by bees.
- This self fertile Queen Cox clone should not be used to pollinate other varieties, as it produces insufficient viable pollen.
- Growers considering purchasing Self-Fertile Queen Cox clone 18 are recommended to obtain this only from a verified UK source.
Partially self-fertile varieties
Several varieties, such as Braeburn and Fiesta, although not self-fertile in the true sense, seem able to set adequate crops of fruits with their own pollen (Volz et al., 1996; Petropoulu and Alston, 1998).
- Such varieties are thought to combine high stylar receptivity and support strong pollen tube growth, even at relatively low temperatures (e.g. 15 degrees C).
- When planting such varieties on sites favourable to consistent fruit set, use of fully compatible varieties as pollinators should be avoided if overset is not to be a problem.
- However, relying on this partial self-fertility and planting no other pollinators is not to be recommended.#Studies on Braeburn in New Zealand have shown that, when bees were excluded from trees and set was only by selfing, although yields were not reduced, seed numbers and calcium concentrations in the fruits at harvest time were reduced.
- This could have severe implications in terms of bitter pit incidence in some situations and planting Braeburn without pollinating varieties cannot be recommended.
Work in Switzerland (Kellerhals and Wirthner-Christinet, 1996) has shown that Gala may set substantial crops of fruits when pollinated with its own pollen or even when left unpollinated.
- However, most of the fruits had no seeds and fruitlet drop was much more severe than on trees receiving cross-pollination.
- Most of the few fruits that persisted were thought to be parthenocarpic.
- However, a few fruit with seeds were produced and partial self-fertility has been reported elsewhere with Gala (Kemp and Wertheim, 1992).
- Also, experiments in the Czech Republic (Papštrein and Blažek, 1996) have shown that although a few varieties, such as James Grieve, Wagener and Ontario are partially self-fertile, they rarely set a full crop of fruit without cross-pollination.
The variety Golden Delicious, which is often considered partially self-compatible in other countries, set fruits very poorly when selfed in trials at Long Ashton Research Station (Bennett et al., 1973).
- However, it was noted that some clones of Golden Delicious may prove more self-compatible than other clones and that virus infection may influence this.
Even the variety Cox and its clones seem capable of a level of self-fertility if given suitable climatic conditions.
- Trials many years ago at Long Ashton Research Station (Williams and Maier, 1973), showed that at temperatures of 20-25ºC Cox flowers were capable of setting fruits with their own pollen.
- Using detached flowers held at 20ºC, 8% of styles had pollen tubes from selfed pollen that had penetrated their full length and into the ovary after only 2 days and by the third day 70% of the styles had been completely penetrated.
- Pollinating the flowers 2-3 days after anthesis and use of high pollen densities on the stigma were also beneficial in inducing this self-fertility.
- The term ‘pseudocompatibility’ was coined for this phenomenon.
- In areas of the world with more favourable spring temperatures, such as North Island, New Zealand, it is quite common for Cox orchards to be planted with few, if any, pollinating varieties.
Although several popular apple varieties, such as Red Pippin and Braeburn, show a level of self fertility if climatic conditions at flowering time are favourable, this cannot be relied upon to ensure consistent and high yields of fruits.
- Although Braeburn planted without pollinators will set good yields of fruits, these will contain few seeds and will have low levels of calcium.
- The seeds are essential in the uptake of calcium into the fruits and the reduction in bitter pit incidence.
- Varieties such as Gala and Golden Delicious also often set fruits with their own pollen when weather conditions are particularly favourable.
- However, most of the selfed fruits usually drop off at the time of June Drop.
Self-fertility induced by pollen mixtures
Research conducted in the 1970s showed that the variety McIntosh was fully self-sterile and, when pollinated with its own pollen, no fruit set was achieved.
- However, if dead pollen of the variety Delicious was mixed with the McIntosh self pollen, successful fruit set was achieved (Dayton, 1974).
- This dead pollen, termed ‘recognition pollen’ overcomes in some way the normal rejection of the self-pollen both on the surface of the stigma and in the style.
- Seed numbers in the fruits are, however, generally less than if the flowers were pollinated with live pollen of another variety.
- The self-fertile clones of Cox and Queen Cox both produce much dead pollen and it is possible that this may account partially for their self-fertility.
Parthenocarpic fruit set
Parthenocarpy is the ability of a variety to set and develop fruits without the need for flower pollination or fertilisation.
- No commercially important varieties of apple show natural parthenocarpy.
- A few varieties of apple are able to set fruits and grow these to maturity without need for fertilisation of the female egg cells (ovules) in the flowers and the formation of seeds.
- In some instances, pollination is necessary, but not fertilisation; in others no pollination stimulus is required. This fruit set without ovule fertilisation is called ‘parthenocarpy’.
- Parthenocarpy in varieties could have significant benefits to fruit growers, as the pollination and fertilisation of flowers is not necessary for fruit set.
- This could be a significant advantage in springs when climatic conditions are unfavourable for bee activity and pollen transfer.
Also, trials with a naturally parthenocarpic variety, Spencer’s Seedless, show that when it sets fruits parthenocarpically without seeds there is no problem with return bloom (i.e. bienniality).
- In contrast, if this variety is hand pollinated with pollen from another variety seeded fruits and biennial cropping are induced.
- This supports the long held hypothesis that it is the seeds in apples not the fruits per se that cause the reductions in flowering in the subsequent season when yields are too high.
Breeding for parthenocarpic fruit set has been undertaken as a small part of a programme based at East Malling (Tobutt, 1994).
- Trials conducted in the 1970s showed that parthenocarpy could be induced in Cox and other UK apple varieties using sprays of growth regulators.
- The seedless fruits sometimes produced following self-pollination of the apple variety Kent are thought to be due to a phenomenon called ‘stenospermocarpy’ (Spiegel-Roy and Alston, 1982), where fertilisation, followed by early seed abortion, results in seedless fruits of parthenocarpic appearance.
Parthenocarpic fruit set can be induced by spraying mixtures of a gibberellin and an auxin. These mixtures are not, however, approved for use in UK orchards.