Specialtyapplications In Targeted Situations Tree Development

Since endogenous auxin and cytokinin regulate lateral bud outgrowth in green shoots (apical dominance) and in woody shoots (apical control), they have been used successfully to improve tree architecture. In large top-dominant trees that require heavy pruning, water sprouts can be difficult to control following the removal of vigorous, upright limbs. If synthetic auxin is applied immediately following pruning, its application mimics apical dominance, and water sprout growth can be suppressed.

To bring young, high-density orchards into rapid production, tree training has taken precedence over pruning. In many cases, rapid growth does not provide the whorl of scaffold limbs where desired.

Spring Application of Synthetic Auxin Fall Application of Synthetic Auxin

• Auxin-induced ethylene causes epinasty.

• Carbohydrate accumulation decreases.

• Fruit growth rate decreases.

• Seed abortion is stimulated.

• Fruitlet abscission occurs.

• Auxin-induced ethylene enhances ripening.

• Auxin maintains an intact abscission zone.

• Riper fruit remain attached to the tree.

• Auxin-induced ethylene causes epinasty.

• Carbohydrate accumulation decreases.

• Fruit growth rate decreases.

• Seed abortion is stimulated.

• Fruitlet abscission occurs.

• Auxin-induced ethylene enhances ripening.

• Auxin maintains an intact abscission zone.

• Riper fruit remain attached to the tree.

FIGURE P3.1. A comparison of the differences in crop load, fruit size, and physiological effects that occur when synthetic auxins are applied in springtime as chemical thinners, or in fall to control preharvest drop of apple (Source: Illustration courtesy of Kathleen W. Hunt, University of Maryland, College Park, MD.)

To overcome apical control, cytokinin-plus-gibberellin mixtures are applied topically, to stimulate lateral bud outgrowth. In this case, the cytokinin leads to bud outgrowth and the gibberellin causes the shoot to develop vigorously (Williams and Billingsley, 1970). In laboratory situations, high concentrations of these chemicals are typically applied in a lanolin paste that allows uptake to occur slowly. In the field, applications of PGRs are made by hand, painting a solution of PGR directly onto the buds and tree bark. Commercial materials are typically mixed into wound-healing formulations used in arboriculture or are made by the grower through mixing with household latex paint (Harris, Clark, and Matheny, 1999).

Fruit Improvement

Another use of cytokinin-plus-gibberellin mixtures is to promote "typiness" in apple fruit. Typiness is the increase of the length-to-

diameter ratio. It is thought that temperatures during the period of cell division affect typiness—cool temperatures favoring elongation and warm temperatures suppressing elongation. To mimic this effect, cytokinin-plus-gibberellin mixtures can be applied at the onset of bloom or during the bloom period.

Gibberellins are used by cherry and apple growers for various other fruit quality benefits. Since cherries are seeded, gibberellic acid treatment does not have a dramatic influence on size, although some effect occurs. The growth-promoting effects of gibberellin application lead to slightly firmer and brighter sweet cherry fruit. Apple producers use gibberellic acid to improve fruit finish, primarily in 'Golden Delicious'. This cultivar is prone to russet development that occurs in response to cool, wet, springtime weather, or as phytotoxity following pesticide applications. Early season applications of GA3 reduce symptoms of russeting, leading to smoother fruit finish and improved marketability. Another minor use is to prevent cracking of the 'Stayman' cultivar. 'Stayman' cracking occurs well before harvest, rendering fruit unsalable. Multiple applications of GA3 are made to improve skin elasticity, thereby reducing cracking later in fruit development.

The most widespread use of any PGR is the use of gibberellic acid in producing seedless table grapes. Grapes require seed fertilization to stimulate initial growth, but embryo abortion does not lead to abscission. In seedless cultivars, growers make multiple applications of gibberellin early in the growing season. The first application acts to elongate the rachis, and subsequent applications stimulate berry growth. To enhance these effects, vine girdling is used to reduce vegetative growth and partition photosynthate to the developing clusters.

Facilitation of Early or Mechanical Harvest

Ethylene-releasing compounds can be used in a variety of tree fruit crops during maturation and ripening. Ethephon application stimulates red color development, degradation of chlorophyll, flesh softening, and abscission. To enhance color development and advance harvest, ethephon application can be made close to the anticipated harvest date. Although this allows growers to pick early and capture higher prices, fruit have less shelf life and should not be stored for extended periods. If fruit are not picked within one to two weeks after application, fruit softening and drop can occur.

Tart cherries and processing apples that are mechanically harvested can also be treated with ethephon. Application to these crops is made primarily to allow mechanical harvesters to pick a greater proportion of the fruit in a single harvest. Ethephon has a slight effect on softening, but this is minor when compared to the ease of onceover mechanical harvest.

Major uses of PGRs to regulate cropping are chemical thinning, preharvest drop control, and the enhancement of fruit quality. PGRs also are used to reduce vegetative growth, enhance flowering, and affect fruit development. As understanding of plant growth and development improves, PGR usage becomes more precise and effective.

Related Topics: CARBOHYDRATE PARTITIONING AND PLANT GROWTH; FLOWER BUD FORMATION, POLLINATION, AND FRUIT SET; HIGH-DENSITY ORCHARDS; PLANT HORMONES; POSTHARVEST FRUIT PHYSIOLOGY; TRAINING AND PRUNING PRINCIPLES

SELECTED BIBLIOGRAPHY

Adams, D. O. and S. F. Yang (1979). Ethylene biosynthesis: Identification of 1-aminocyclopropane-1-carboxylic acid as an intermediate in the conversion of methionine to ethylene. Proc. Natl. Acad. Sci. 76:170-174. American Crop Protection Association (2001). Crop protection reference CPR

2001. New York: C & P Press. Dennis, F. G. (1973). Physiological control of fruit set and development with growth regulators. Acta Horticulturae 34:251-257. Harris, R. W., J. R. Clark, andN. P. Matheny (1999). Arboriculture, Third edition.

Upper Saddle River, NJ: Prentice-Hall. Walsh, C. S., H. J. Swartz, and L. J. Edgerton (1979). Ethylene evolution in apple following post-bloom thinning sprays. HortScience 14:704-706. Williams, M. W. and H. D. Billingsley (1970). Increasing the number and crotch angles of primary branches of apple trees with cytokinins and gibberellic acid. J. Amer. Soc. Hort. Sci. 95:649-651. Zimmerman, R. H. (1972). Juvenility and flowering in woody plants. HortScience 7:447-455.

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