Key Points and Detailed Methods of Walnut Hybrid Breeding Techniques

    Walnut is an important oilseed fruit tree. The oil content of walnut is generally 60%-75%, and its protein content reaches over 15%. It also contains calcium, phosphorus, iron, vitamins and other nutrients. It is both a nutritious food and has great medicinal value. Walnut is light-loving, cold-tolerant, and highly drought- and disease-resistant. It adapts to a variety of soils and has low requirements for environmental conditions, making it especially suitable for planting on barren hills and slopes. It is currently an ideal economic tree species for returning farmland to forest. Carrying out hybrid breeding to cultivate superior tree varieties suitable for local growth, enabling walnut to bear fruit early and abundantly, enhancing growth vigor, productivity and stress resistance, and improving economic benefits per unit area, has very important practical significance for the development of the walnut industry. 

    I. Flower Structure and Blooming Habits

    The male flowers of walnut are borne at the tip of the previous year's branches, while the female flowers are borne at the tip of the current year's branches. In the previous spring, from April to May, the male flower buds have already formed in the leaf axils, initially appearing as 1-2 small protrusions, taking on a mulberry-like shape in autumn, and blooming and shedding pollen in April to May of the following spring. The male inflorescence is catkin-like, with each inflorescence bearing 100-170 small flowers, and each small flower having 12-26 stamens with short filaments and plump, yellow-green anthers. The female flowers are clustered, mostly 1-3 per cluster, with a 2-lobed stigma that is feathery with papillary protrusions, and an inferior ovary.

    Walnut is a monoecious species with unisexual flowers and cross-pollination. However, the blooming periods are often not synchronized, with very few blooming at the same time. The opening dates of female and male flowers on the same branch are generally 7-10d apart. 

    II. Hybrid Combinations and Parent Selection

    The selection and pairing of hybrid combinations must be based on the breeding objective. For example, to breed high-yield varieties, high-yield parents must be selected; to breed cold-resistant varieties, parents distributed in high-altitude, high-latitude areas should be selected. Generally speaking, the best parents are carefully selected superior individual trees. A comprehensive and systematic understanding of the economic traits, biological traits, and resistance of superior individual trees should be obtained to meet the requirements for further breeding of improved varieties. To enhance heterosis, different ecotypes can also be used for hybridization.

    In addition to intraspecific hybridization, walnut can also be hybridized with different genera and species within the same family. For example, mixed pollination using pollen from Manchurian walnut (Juglans mandshurica) and pecan (Carya illinoinensis).

    Whether selecting superior individual trees or ordinary fruiting trees as hybrid parents, middle-aged plants that are vigorous in growth, free from pests and diseases, with large crowns, spreading branches, and numerous female flowers should be chosen. Middle-aged fruiting trees are well-developed, have strong vitality, vigorous metabolism, and produce plump kernels after hybridization.
 
    III. Hybridization Techniques 

    1. Pollen Collection

    Walnut pollen can be collected using the following two methods: one is during the peak pollen-shedding period of male flowers, from 8-9 AM to 2-3 PM, climbing the tree to shake the pollen into a collector; the other method is to collect male inflorescences that are about to open or are in full bloom, place them in a dry place or desiccator to dry. After the anthers lose water, they crack and shed pollen. The pollen is collected, cleaned of impurities, placed in test tubes, plugged with cotton at the tube mouth, stored in a desiccator, and preferably kept in a refrigerator for later use. 

    2. Bagging

    Isolation generally requires bagging 3-5d before the male flowers shed pollen. Bagging too early will affect female flower development, while bagging too late risks contamination by other pollen. Parchment paper or transparent paper is best for bagging, with bag dimensions of about 45cm long and 25cm wide. When bagging, first cut off the larger leaves at the lower part of the branch, then cut off half of the young leaves near the female flowers. Place a small amount of cotton at the junction of the bag opening and the branch, then tie it tightly with string to prevent other pollen from entering.

    3. Pollination

    After bagging for isolation, observe daily to pollinate at the appropriate time. Usually, the temperature and humidity inside the bag are higher than outside, and female flowers develop 1-2d earlier than those outside the bag. The most suitable pollination period is 2-3d after blooming. At this time, the walnut stigma has changed from blue-green to yellow-green, opening horizontally to both sides, glossy and covered with mucus. Pollination should be carried out before 10 AM. At noon, the temperature is high, and the mother tree sheds large amounts of pollen, which can easily be mixed with other pollen and affect the accuracy of hybridization.

    When using a dusting device for pollination, cut a small opening at the upper part of the bag and insert the nozzle of the duster to pollinate. When using a brush or cotton ball to apply pollen for pollination, first remove the bag or cut off the upper part of the bag and then pollinate. After pollination is completed, fold the bag opening several times and fasten it with a paper clip or re-tie the bag. To improve the fruit-setting rate, pollination should be repeated 2-3 times, that is, once a day or every other day.