Most diploid apple varieties are cross compatible (i.e. self-incompatible). This means that they will not set fruit with their own pollen in most environmental conditions.
- They require the viable pollen of another variety to be transferred to their receptive stigmas and the subsequent germination and growth of this pollen down the style into the ovary.
- Once there, it should be able to grow, albeit relatively slowly, into the micropyle of an ovule and effect fertilisation.
However, some apple varieties are wholly or partially cross incompatible with each other.
- This has been attributed to a multi-allelic gametophytic locus S.
- In instances of cross incompatibility, the pollen grains germinate but their growth is arrested in the top one third of the style; this is known as gametophytic incompatibility.
Early work (Kobel et al. 1939) showed that by making various possible cross pollinations and observing the speed of pollen tube growth down the style it was possible to distinguish between compatible, semi-compatible (partially compatible) and incompatible crosses. On the basis of this work, S alleles were assigned to some 20 different cultivars.
Further research (Broohaerts et al., 1995; Janssens et al., 1995) isolated cDNAs responsible for the stylar ribonucleases associated with some of the S alleles first mentioned by Kobel et al.
- This led to the development of a molecular diagnostic technique for the identification of these alleles based on allele-specific PCR amplification and restriction digestion.
Recent research at East Malling (Bošković and Tobutt; 1999) has extended this work and characterised the S alleles in a range (56) of diploid and triploid apple varieties based on their stylar ribonucleases.
- Diploid varieties with the same pair of S alleles are in most instances incompatible with each other.
- Where the two varieties share one S allele in common, then only 50% of the pollen produced will be capable of germination and growth down the style to fertilise the ovule.
- This is known as partial compatibility or semi compatibility.
Self fertile clones of the apple varieties Cox’s Orange Pippin and Queen Cox have been produced using irradiation techniques of fruit breeding.
- These clones produce less viable pollen than their self-incompatible parents but are capable of setting fruits with this pollen and with no requirement for the planting of other pollinating varieties in the orchard.
- Work by Petrapoulu (1985) showed increased stylar receptivity in two of these self-fertile clones of Cox and the author concluded that this may be due to mutations at loci controlling stylar receptivity rather than at the S incompatibility locus.
Research in Switzerland (Kellerhals and Wirthner-Christinet, 1996) showed that hand pollination of Gala with its own pollen followed by cross pollination, using Spartan pollen, 24 hours later led to poor fruit set.
- The initial selfing in some way inhibited fertilisation by the Spartan pollen.
- It is not known whether this could have a role to play in poor fruit set observed with open pollination in the orchard.
Testing pollen compatibility
Growers wishing to test compatibility of varieties in the field can adopt the following simple method:
- Collect flowers of the variety chosen to provide pollen 24 hours before the planned pollination.
- Choose flowers at the balloon stage and remove petals before placing on trays at room temperature.
- On the tree to be pollinated choose and tag representative branches of a horizontal or semi-horizontal and similar orientation and height above ground level.
- Remove petals from the chosen flowers on the spur buds at the balloon stage; this makes them unattractive to pollinating insects.
- Remove surplus flowers from 25 spaced clusters to leave 4 flowers/cluster; remove the king flower and any potentially late opening flowers.
- Use only the pollen donor flowers that have begun to dehisce their pollen; each flower will usually be sufficient to pollinate four flowers on the variety to be pollinated.
- Thoroughly brush the stigmas with the anthers of the pollinating flower.
- Ideally, each flower is pollinated twice, once at the time of anthesis (flower opening) and again two days later.
- The reason for pollinating twice is to take advantage of the phenomenon known as ‘pioneer pollen’.
The above technique does not entirely exclude the possibility of cross pollination but it can give a quick idea of pollen compatibility.
- A modified strategy, used in the Netherlands, involves no removal of petals and only one pollination at the time of flower opening (anthesis) (Wertheim).
- Cross pollination is prevented by putting a very small dab of vaseline on the stigmas after they have been hand pollinated.