Plant Bioregulators Plant Growth Regulators

Application of selected plant bioregulators (PBRs) can inhibit growth in fruit trees (Miller, 1988). Auxins (NAA), ethylene-releas-ing compounds (ethephon), and GA biosynthesis inhibitors (paclo-butrazol, prohexadione-calcium) are the primary PBRs used to reduce growth. Daminozide (Alar), a powerful growth regulator used for several decades in apple production, was removed from the market in the late 1980s. Research indicates daminozide reduced the translocation of GAs or GA precursors to actively growing sites and may have promoted GA catabolism and conjugation.

Auxins applied to scaffold limbs of trees induce bud dormancy, preventing growth of water sprouts from latent buds. Ethyl ester forms of auxins applied at high rates are phytotoxic and desiccate shoots. Ethylene is known to reduce cell or stem elongation. Compounds such as ethephon have been shown to interfere with auxin biosynthesis and with polar auxin transport. These auxin-mediated effects are probably the mechanism by which ethephon reduces shoot growth.

The most common group of PBRs used to control growth and induce dwarfing in fruit trees are those that interfere with GA biosynthesis. These retardants can be divided into three groups: quater nary ammonium compounds (e.g., chlormequat chloride), compounds with a nitrogen-containing heterocycle (e.g., flurprimidol, paclobutrazol, uniconazole), and acylcyclohexanediones (e.g., prohexadione-calcium). Each group interferes at a specific place in the GA biosynthesis pathway, inhibiting the endogenous formation of biologically active GAs. These active GAs (primarily GA1) are significant in the longitudinal growth of plants. Blocking production of the active GAs results in shortened internodes, stem thickening, and darker green foliage— characteristics of a dwarf tree.

The dwarfing process for tree fruit is a complex response involving genetic and/or physiological changes that result in a tree of smaller stature. Although the specific mechanisms that lead to dwarfing are varied, depending on the horticultural technique(s) employed, and often not well understood, the outcome is a tree that is easier to manage and likely to be more efficient than a similar nondwarf tree.

Related Topics: BREEDING AND MOLECULAR GENETICS; CARBOHYDRATE PARTITIONING AND PLANT GROWTH; HIGH-DENSITY ORCHARDS; PLANT GROWTH REGULATION; PLANT NUTRITION; ROOT-STOCK SELECTION; TRAINING AND PRUNING PRINCIPLES; WATER RELATIONS

SELECTED BIBLIOGRAPHY

Atkinson, C. and M. Else (2001). Understanding how rootstocks dwarf fruit trees.

Compact Fruit Tree 34:46-49. Beakbane, A. B. (1956). Possible mechanisms of rootstock effect. Ann. Appl. Biol. 44:517-521.

Faust, M. (1989). Physiology of temperate zone fruit trees. New York: John Wiley and Sons.

Miller, S. S. (1988). Use of plant bioregulators in apple and pear culture. Hort. Rev. 10:309-401.

Schmidt, H. and W. Gruppe (1988). Breeding dwarfing rootstocks for sweet cherries. HortScience 23:112-114. Scorza, R. (1988). Progress in the development of new peach tree growth habits.

Compact Fruit Tree 21:92-98. Simons, R. K. (1987). Compatibility and stock-scion interactions as related to dwarfing. In Rom, R.C. and R. F. Carlson (eds.), Rootstocks for fruit crops (pp. 79-106). New York: John Wiley and Sons. Tukey, H. B. (1964). Dwarfed fruit trees. New York: The Macmillan Co.

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