Compatibility explained

The details matter for optimising yields and quality in pome and stone fruit. By Anna Mouton.

Pollination is essential for fruit set in most pome and stone fruit. The transfer of pollen to the stigma is the first step toward fertilising the ovule and developing seeds that produce auxins and gibberellins to stimulate early fruit growth.

Unfertilised flowers or non-viable seeds will usually end as dropped fruitlets. Application of plant growth regulators such as gibberellins stimulates the parthenocarpic set of unfertilised ovules in especially pears. However, parthenocarpic pears can have poor size and shape compared to seeded pears and may be more prone to drop when faced with shoot competition.

Cross-pollination is crucial in pome and almost all stone fruit as they are self-incompatible — pollen cannot fertilise ovules of the same cultivar. Peaches and nectarines are self-compatible and do not need cross-pollination by a different cultivar.

Understanding self-incompatibility

Nearly half of all flowering plants are self-incompatible. Self-incompatibility helps to prevent inbreeding and to maintain genetic diversity in plant populations. How does this work in deciduous fruit?

Take the example of apples. Apple trees have S-genes — S stands for self-incompatibility — that code for proteins made in the pollen and female flower parts. A tree will have one copy of these genes on each set of chromosomes. Most apple cultivars have two sets of chromosomes and two sets of S-genes.

S-genes come in many variations — called alleles — coding for slightly different proteins. Researchers have identified more than 50 of these variations in apple and crab-apple species. An individual tree with two sets of S-genes will have two of these 50 variants. Pollen has only one set of chromosomes and, thus, a single S-gene variant.

Germinating pollen expresses proteins according to the instructions in its S-gene. The flower receiving the pollen can tell from these proteins whether the pollen is from a tree with the same or different S-genes to itself. Pollen tubes with the same S-genes are destroyed before they reach the ovule.

Degrees of compatibility

Many modern apple cultivars are closely related and share S-alleles. For example, Cripps Pink, Golden Delicious, and Gala all have S₂, and Granny Smith and Gala both have S₃. A study of 150 apple cultivars found that S₂, S₃ and S₉ each occurred in roughly a third of cultivars.

How this affects growers is best illustrated with an example. Say a grower plants a Cripps Pink orchard — Cripps Pink has S₂ and S₂₃ alleles — and consults Google for potential cross-pollinators. Google suggests Granny Smith, Royal Gala, Fuji, Red Delicious, and Hillieri.

Granny Smith has S₃ and S₂₃ and Royal Gala S₂ and S₅ alleles. S₂ and S₂₃ pollen is incompatible with Cripps Pink, so only half the pollen produced by Granny Smith and Royal Gala can fertilise Cripps Pink ovules. This is called partial compatibility.

On the other hand, Fuji has S₁ and S₉ and Red Delicious S₉ and S₂₈ alleles, so all their pollen is compatible with Cripps Pink — they are fully compatible with Cripps Pink.

Hillieri is interesting because it’s a crab apple. Crab apples share few S-alleles with commercial apple cultivars and are consequently often fully compatible.

The cost of incompatibility

A study in commercial apple orchards in Israel compared pollination and yields in different combinations of full-bearing fully or partially compatible cultivars. In each orchard, two rows of the cross-pollinator alternated with two rows of the main cultivar.

Table 1 shows the fruit set obtained in the main cultivar after open pollination with fully or partially compatible cross-pollinators. Fruit set in Golden Delicious and Royal Gala was significantly lower with partially compared with fully compatible cross-pollinators. Fruit set in Granny Smith did not differ between partially and fully compatible cross-pollinators.

Table 1: Fruit set achieved in the main cultivar with fully and partially compatible cross-pollinator combinations tested in an Israeli study. The S-alleles of each cultivar are in parentheses.

Genetic analysis revealed that pollination rates by partially compatible cross-pollinators were even lower than the results in Table 1 suggest — up to 45% of the pollination in blocks with partially compatible cultivars was contributed by compatible cross-pollinators in other blocks.

In contrast, in blocks with fully compatible cross-pollinators, the compatible trees were responsible for 80%–90% of the pollination of the main cultivar.

The researchers also tested hand pollination using the same cultivar combinations and found that manual application of excess pollen compensated for partial compatibility. This suggests that planting a high proportion of cross-pollinators and bringing in extra hives would overcome the drawbacks of partial compatibility — but fully compatible cross-pollinators remain the more cost-effective and low-risk option.

References

Broothaerts W, Van Nerum I and Keulemans J. 2004. Update on and review of the incompatibility (S-) genotypes of apple cultivars. HortScience 39(5) pp943–947.

Orcheski B and Brown S. 2012. A grower’s guide to self and cross-incompatibility in apple. New York Fruit Quarterly 20(2) pp25–28.

Schneider D, Stern RA and Goldway M. 2005. A comparison between semi-and fully compatible apple pollinators grown under suboptimal pollination conditions. HortScience 40(5) pp1280–1282.