278 Bai et al.: Wound Closure in Trees Affected by PBZ treatment or was more pronounced in the second year after treatment. The same variable pattern among species for shoot growth suppression to occur in the year of treatment or to be delayed until the second year after treatment is well documented (Chaney et al. 1996). Reduction in the rate of closure of both bark and pruning wounds by PBZ could be considered a negative side-effect of PBZ that would expose treated trees to decay organisms and insects for a longer period of time. However, wound surfaces in both PBZ-treated and untreated trees were exposed for at least a portion of the growing season and even for two to three growing seasons in some species. It has been found in peach trees that when an infection court is created for a wound pathogen, disease is most severe when the inoculum arrives at the infection court immediately. If the inoculum arrives later, disease frequency and severity decline with time until the wounded tissues express resistance comparable to that of noninfected bark (Biggs 1986, 1989). There is no indication that wound closure is directly correlated with the amount of discoloration and decay in the wood beneath wounds (Blanchette 1992). The growth of callus over a bark or pruning wound and the eventual restoration of the vascular cambium that is contiguous with that of the rest of the tree is only one of the steps in wound sealing. In the CODIT model proposed by Shigo (1989), growth of callus over the surface of wounds forms Wall 4, the final step in the wound sealing process. Injury to tree tissues immediately evokes a complex array of physiological and biochemical responses in cells adjacent to the wound (Blanchette 1992). These are separated into rapid responses, such as the release of volatile inhibitors, and slow responses, such as biosynthesis reactions and formation of structural barriers. The rapid responses occur within seconds or minutes after wounding, whereas the slow responses occur over a period of one to many hours. The formation of tissue impermeable to water and penetration by most microorganisms is the most common feature observed in the wound response (Bostock and Stermer 1989). This process of cicatrization, or scar formation (Esau 1977), involves the infusion of lignins, suberin, waxes, and/ or wound gums in the layers of cells immediately subtending the wound surface to form barrier or protective zones in the wood beneath the wound. Hence, slower callus growth over wound surfaces in PBZ-treated trees (Wall 4 formation) does not necessarily mean that PBZ-treated trees will be more susceptible to decay organisms than untreated trees. In fact, visual observation of cross and longitudinal sections through bark wounds of control and PBZ-treated trees revealed no differences in wood discoloration or decay in the eight tree species investigated in this study. The fungistatic property of PBZ (Chaney et al. 1996) also may enhance the decay resistance of a treated tree over that of an untreated tree. PBZ-treated trees also should require ©2005 International Society of Arboriculture less frequent pruning and fewer pruning cuts due to shoot growth reduction than untreated trees. Hence, any negative effects due to increased time of exposure of wounds may be counterbalanced by the positive effects of PBZ treatment. LITERATURE CITED Armstrong, J.E, A.L. Shigo, A.L., D.T. Funk, E.A. McGinnes, Jr., and D.E. Smith. 1981. A macroscopic and microscopic study of compartmentalization and wound closure after mechanical wounding of black walnut trees. Wood Fiber Sci. 13:275–291. Bai, S., W. Chaney, and Y. Qi. 2004. Response of cambial and shoot growth in trees treated with paclobutrazol. J. Arboric. 30:137–145. Biggs, A.R. 1986. Phellogen regeneration in injured peach tree bark. Ann. Bot. 57:463– 470. ———. 1989. Temporal changes in the infection court following wounding of peach bark are associated with cultivar variation in infection by Leucostoma persoonii. Phytopathology 79:627–630. ———. 1992. Anatomical and physiological responses of bark tissues to mechanical injury, pp. 13–40. In Blanchette, R.A., and A.R. Biggs (Eds.). Defense Mechanisms of Woody Plants Against Fungi. Springer- Verlag, Berlin, Germany. Blanchette, R.A. 1992. Anatomical responses of xylem to injury and invasion by fungi, pp. 76–95. In Blanchette, R.A., and A.R. Biggs (Eds.). Defense Mechanisms of Woody Plants Against Fungi. Springer-Verlag, Berlin, Germany. Bostock, R.M., and B.A. Stermer. 1989. Perspectives on wound healing in resistance to pathogens, pp. 343371. In Cook, R.J., G.A. Zentmyer, and E.B. Cowling (Eds.). Annual Review of Phytopathology, Volume 27. Annual Reviews, Palo Alto, CA. Chaney, W.R. 2003. Tree growth retardants: Arborists discovering new uses for an old tool. Tree Care Indust. 14(3):54–59. Chaney, W.R., G.S. Premachandra, and H.A. Holt. 1996. Physiological basis for benefits of tree growth regulators, pp. 8–18. In Proceedings of the Western Plant Growth Regulator Society, Sacramento, CA, 24–25 Jan. 1996. Esau, K.1977. Anatomy of Seed Plants (2nd ed.). Wiley, New York, NY. 550 pp. Fletcher, R.A., A. Gilley, N. Sankhla, and T.D. Davis. 2000. Triazoles as plant growth regulators and stress protectants. Hortic. Rev. 24:55138. Garrett, P.W., W.K. Randall, A.L. Shigo, and W.C. Shortle. 1979. Inheritance of compartmentalization of wounds in sweetgum and eastern cottonwood. U.S. Forest Service Research Paper NE-443. 4 pp. Grossmann, K. 1988. Plant cell suspensions for screening and studying the mode of action of plant growth retardants, pp. 89–136. In Maramorosch, K., and G. Sato
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