S.A. Mehlenbacher and A.N. Miller – Oregon State University, 1988
HGA newsletter, Winter 2003
Three factors must be considered in choosing pollinizer cultivars: 1) the amount of viable pollen produced, 2) compatibility, and 3) time of pollen shed.
The amount of viable pollen produced by a hazelnut tree is largely a function of the number of catkins on the tree and the viability of the pollen produced. Some cultivars set pollen in abundance Others typically set very few catkins. Some cultivars drop their catkins prior to pollen shed. Since one good Daviana catkin is estimated to produce 4 million pollen grains, the amount of pollen produced by a single pollinizer tree is tremendous.
Incompatibility occurs when plants having functional pollen and functional female flowers are unable to set seed when self-pollinated or crossed with some of their relatives. Incompatibility is a physiological between pollination and fertilization. Pollinizers are required in hazelnut orchards because cultivars are self-incompatible. They are also cross-incompatible in certain combinations. Hazelnuts are similar to Brassica species (cabbage, broccoli, and their relatives) in that all have the sporophytic incompatibility system All pollen produced by a tree exhibits the same incompatibility reaction. The reaction is under simple genetic control. There is one locus (one gene), the S-locus (for self-incompatibility) with 22 known alleles. Don’t let this terminology scare you. In humans, there is a locus controlling eye color, an allele for brown eyes, and an allele for blue eyes. Humans have a locus controlling hair color, an allele for brown hair, and an allele for blond hair. In hazelnuts, there is one S-locus which controls the incompatibility reaction, and alleles S1, S2, S3,…..S22. Hazelnut trees, like humans, are diploids. Thus at every locus, they have two alleles. Brown eyed people may have two alleles for brown eyes or one allele for brown eyes and another for blue eyes. The allele for blue eyes is recessive. In hazelnuts, both alleles are always expressed in the female flowers. One or both may be expressed in the pollen. If a given allele expressed in the pollen is met with the same allele in the female flower, the cross is incompatible. In simpler terms, if like meets like, the reaction is incompatible. If a given allele expressed in the pollen is met with different alleles in the female flower, the cross is compatible. Because of the dominance relationships among S-alleles, some crosses are compatible yet the reciprocal cross is incompatible.
A third factor to consider in choosing pollinizer cultivars is time of pollen shed. It is essential that the pollinizer shed pollen when the female flowers of the main crop cultivar are receptive. Pollen which is shed before the female flowers emerge is wasted. Unpollinated female flowers remain receptive for up to 3 months, so a late pollinizer can be very effective. Although actual dates vary from year to year, relative dates of pollen shed are consistent. Female flowers and catkins respond differently to temperature. In colder winters, female flower development is accelerated relative to catkin elongation. Thus a pollinizer which sheds at the ideal time one year may shed too early or too late the next year. Some cultivars shed their pollen over a very short time while others shed over a much longer time. By planting 2 or 3 rather than a single pollinizer, growers increase the odds of having pollen shed at the optimum time.
Another dehiscence and pollen release from catkins requires lower relative humidity and warmer temperatures. If temperatures are too cold (320F or less) and humidity is too high (85%+) then pollen will not be shed.
Temperature will also have an effect on pollen viability. If temperatures exceed 730F then pollen viability decreases. Optimum pollen germination in artificial culture has been obtained at 68-720F, but will also germinate at 34-390F.
Once pollen is released from the anther, wind is required for its distribution. The absence of wind could result in pollen dropping to surfaces below the catkins. Theoretically, pollen could travel about 250ft. in 36 seconds and drop 3 feet. if a 15mi/hr wind was blowing. However, hazelnut pollen is moved by eddy diffusion where the air movement rolls and swirls. Pollen consentration drops very quickly to relatively low amounts around 46-72 ft. distance away from the orchard edge. At this time we do not know what the required amount of pollen is to guarantee optimum nut set and development. However, in 2 research articles Schuster (5,6) states, “In observation in the field is shows that trees planted 40-60 ft. from a pollinizer bear good crops though the ones at 60 ft. sometimes appear light.” and “In the field it has been noted that trees more than 50ft. from the pollinizer yield smaller crops than closer.”
The above article has been abridged to conserve space in our news letter. Data tables showing compatible crosses, compatible cultivars, Cultivar pollen shed compatibility, Pollen concentrations at different distances, and charts showing pollinizer planting options within orchards have been left out. If a full copy of the article is wanted, please contact the Editor.