Pollenizer management in a hazelnut orchard

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.

The hazelnut tree is a wonder

Jeff Olson, Extension agent, Oregon State University

HGA newsletter, Autumn 2003

Horticulturally speaking, the hazelnut tree is clearly out of the ordinary. It is more than just nutty. It is unique and wonderful. The way in which it achieves pollination in the winter and completion of nut set in the spring, is like no other horticultural crop that I have ever heard of. It is a “one of a kind”, just like some of the people in our industry!

Over the years, many researchers have investigated the growth and development of the hazelnut, in an attempt to unlock some of the secrets of this unusual plant. In fairly recent times, 1979, Dr. Maxine Thompson, of OSU, published a very informative article about the growth and development of the hazelnut flowers and nuts. It is one of those information-packed articles that is peppered with words like: megasporocytes, achesporial cells, funiculus of the anatropous ovule…you know what I mean, light reading.

Maxine followed the development of the flower and nut from the time of pollination to harvest. And she found some fascinating things. Pollination takes place in January and February, with the wind being the pollinator. There are a pair of styles that are joined at the base by a tiny ovarian meristem. When the pollen lands on the stigmatic surface of the style, hopefully it germinates and forms a pollen tube. The pollen tube grows to the base of the style in 4 to 7 days and then it rests there until May. Four to five months lapse between pollination and fertilization of the ovary.

This research project looked at how long the stigma would remain receptive to pollen. Did you know that the stigmatic surfaces of hazelnut female flowers remain receptive for up to 3 months? This study used a controlled hand pollination to test the length of receptivity at five different stages of flower development, and found them to be viable all the way up to March 13. Those old flowers were withering and necrotic, but the stigmas were still receptive.

The clusters that had not been pollinated successfully all dropped to the ground by the end of May. The flowers that were successfully pollinated continue with the development of the ovaries that become mature by the middle of June. Fertilization of the ovaries can then take place. The nut can now develop, and it reaches full size by the beginning of August. By July the shell begins to harden, and is hard by early August.

The cells that will form the following year’s crop also form in the spring and summer, while the present crop is being set. The male flower, the catkins, differentiates at the end of May and beginning of June. The female flowers for the next year differentiate between July and September. So, you can see that there is an awful lot going on inside that hazelnut tree in the spring and summer.

Way back in 1925 Joseph Newell did a study of the bearing shoots of the filbert. He found that the number of fruiting buds was directly proportional to shoot length, with the number of fruit buds increasing as the shoot length increased. The diameter of a shoot was a good index to its length. In most cases he found that there is very little degree of negative correlation between a shoot`s angle and the number of fruit buds. He noted that better light illumination results in better production in the upper third of the tree. Of course, this observation has been verified quantitatively by the more recent work of Dr. Anita Azarenko.

Newell found that in most cases the variations of illumination on different sides of trees was insufficient to cause any consistent variability in the number of fruit buds formed on the shoots. He observed that in Barcelona, a fair degree of correlation exists between the male catkins and the number of fruit buds per shoot.
In 1957 John Painter and Henry Hartman reported that the greater the length of the shoot: 1) The greater the number of female flower clusters; 2) the greater the percentage of female flowers that had nuts set in May; 3) the greater the number of nuts in a cluster; 4) the greater the number of nuts matured; 5) the larger the nuts. Overall evidence suggests twigs greater than 6.25 inches long bore the largest and greatest numbers of nuts. Twig length did not effect distribution of blank nuts. They noted that filberts can not produce both extensive twig growth and extensive nut production in the same year, hence the alternate year effect.

In one of the few research studies on the root systems of hazelnuts that I have found, scientists in Italy exposed two adjacent halves of two neighboring trees. The trees were closely spaced at 13′ x 13′, so the roots tended to extend diagonally to the corners of a square but laterally only as far as the roots of the next tree. The bulk of the roots occurred in the 8-31 inch depths of the soil. Many feeder roots occurred in the soil directly under the trunk. The study was conducted on deep fertile soil and that had a practice of cultivating to a 10-inch depth, which were the factors mainly responsible for this type of vertical root distribution.

So, you see that the quest for understanding the marvelous hazelnut tree have been going on for decades. Significant discoveries have been made, and certainly more await discovery.