Cuttings

Stem, leaf, or root cuttings are most commonly used to propagate herbaceous plants; however, stem cuttings have been used to propagate clonal rootstocks of Prunus species and also self-rooted trees, especially for peaches. Depending upon the species, hardwood cuttings collected during the dormant period, softwood cuttings in spring, or semihardwood cuttings in fall may be used. For semihardwood cuttings, for example, the process requires collecting the current season's growth in late summer, cutting the shoots into sections approximately 15 centimeters long, and removing all but the four or five leaves at the upper portion of the cutting. The bottom of the cutting should be lightly wounded, treated with a rooting hormone, and then planted in moistened growing media. The leafy cuttings need to be placed in a mist bed that provides intermittent misting. Once the cuttings are rooted, they can be planted outdoors.

Layering

Layering occurs in nature in a number of plants (e.g., forsythia, grape, and many brambles) where the flexible branches make contact with the soil. The primary use of layering in the fruit industry is in the production of clonal rootstocks. This is achieved either through mound layering or trench layering. In the former, which is used especially with apples, young plants of the rootstock grown in a stool bed are cut to within a few centimeters of the ground. When the new shoots appear, they are partially covered with soil, and this process is repeated during the growing season as the shoots continue their growth. At the end of the season, the mound of soil is removed and the rooted shoots harvested. The process is repeated in subsequent years, and well-maintained stool beds have a long life. The other method, referred to as trench layering, is essentially similar to mound layering, except in the initial step the plants of the rootstock are laid in a shallow trench and pegged in place. This forces the plants to generate new shoots, which are treated similar to those in mound layering.

Grafting

Grafting is the process of uniting two compatible plants—the scion (the cultivar) and the rootstock—to produce a single desirable plant. This is valuable from the standpoint of exploiting the benefits provided by clonal rootstocks or simply for vegetative propagation of a desirable scion (where a seedling rootstock is being used). With temperate fruit trees, grafting is done in winter or early spring, using dormant scion wood. Although many types of grafts may be used, one common type is the whip-and-tongue graft. In this, a diagonal cut is made through the rootstock, and then a vertical slit is made at the upper end of the rootstock. Matching cuts are made to the scion, and the two components are interlocked with each other, ensuring good cambium contact between the stock and the scion. The union is wrapped with airtight material to keep it from desiccating and to hold the two parts together during the healing process. A number of materials, including grafting tape and polyethylene strips, can be successfully used to wrap the union.

284 CONCISE ENCYCLOPEDIA OF TEMPERATE TREE FRUIT Budding

With budding, also referred to as bud grafting, a single bud from the scion is inserted into the rootstock, and this grows to produce the top of the tree. The advantages of budding over grafting are that it permits more efficient use of bud wood and allows for greater flexibility as regards time of conducting the operation. Although many types of bud-grafting techniques have been developed, two have been more widely used with temperate fruit: chip budding and shield budding (also referred to as T-budding).

Shield budding is conducted during the growing season when the cambium is active. A vertical incision about 2.5 centimeters long is made on the rootstock, followed by a horizontal second cut at the top of the first to produce a "T"-shaped incision. A bud is removed from the bud stick by making a cut starting about 1.3 centimeters below the bud and extending about 1.3 centimeters above the bud. For June and fall budding, the bud stick is obtained from an actively growing tree, so immediately after obtaining it, the leaves must be removed. The shield-shaped bud from the scion is inserted into the T-shaped cut in the rootstock, and cambium contact is readily achieved. The bark flap of the rootstock will cover the bud and keep it from desiccating, and tying the union with a rubber budding strip ensures that it is held securely until it has healed. Once the bud has formed a union with the rootstock, the top of the rootstock needs to be decapitated to allow the bud to grow.

Chip budding has a major advantage over shield budding in that it is not limited by the season and does not require that the cambium is actively dividing. Further, this method results in better graft union formation and is therefore widely used by commercial nurseries. The procedure requires making two cuts in the rootstock—a "down-stem" cut about 2.5 centimeters that is angled inward and a second, shorter, "up-stem" cut at an angle of approximately 45 degrees into the stock that meets with the first cut. This results in a triangular-shaped chip of bark and wood being removed from the rootstock. This is replaced by a similar-sized chip from the scion that contains a bud. Unlike a shield bud, a chip bud is not held in place or kept from desiccating by a bark flap, and the union must be wrapped with an airtight material.

Tissue Culture

Tissue culture (or micropropagation) provides a technique for rapid multiplication not only of herbaceous plants but also temperate fruit species. The procedure is relatively simple. Actively growing shoot tips, about 2.5 centimeters in length, are excised from the plant to be multiplied and implanted on sterile medium in a test tube or other appropriate container. The culture medium, usually solidified with agar, contains nutrients, sugar, and growth hormones. Supplementing the culture medium with a cytokinin (benzyladenine at 1 to 2 parts per million) induces the shoot tip to produce new shoots. These shoots can be excised and recultured on fresh medium at periodic intervals. Shoots can be rooted either in vitro (in culture medium supplemented with an auxin) or ex vitro (where they are treated as micro-cuttings). Rooted plantlets are propagated under conditions of high humidity and need to be gradually acclimated to ambient conditions.

Decisions made during the initial establishment of an orchard will have far-reaching ramifications. It is important that high-quality plants are obtained from a reputable nursery because they will have a significant impact on orchard operations and profitability.

Related Topics: CULTIVAR SELECTION; ROOTSTOCK SELECTION SELECTED BIBLIOGRAPHY

Garner, R. J. (1976). The grafter's handbook. New York: Oxford Univ. Press. Hartmann, H. T., D. E. Kester, F. T. Davies Jr., and R. Geneve (2002). Plant propagation: Principles and practices. Englewood Cliffs, NJ: Prentice-Hall. Singha, S. (1986). Pear (Pyrus communis). InBajaj, Y. P. S. (ed.), Biotechnology in agriculture and forestry, Volume 1 (Trees 1) (pp. 198-206). Berlin: SpringerVerlag.

Singha, S. (1990). Effectiveness of readily available adhesive tapes as grafting wraps. HortScience 25:579. Zimmerman, R. H., R. J. Griesbach, F. A. Hammerschlag, and R. H. Lawson, eds. (1986). Tissue culture as a plant production system for horticultural crops. Dordrecht, the Netherlands: Martinus Nijhoff Publishers.

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