Journal of Arboriculture 31(6): November 2005 273 WOUND CLOSURE IN TREES AFFECTED BY PACLOBUTRAZOL By Shuju Bai1 , William R. Chaney2 , and Yadong Qi3 Abstract. Experiments were conducted to investigate the effects of paclobutrazol on closure of wounds made through the bark or resulting from pruning branches in nine species of trees. The species studied were American sycamore (Platanus occidentalis L.), black walnut (Juglans nigra L.), European black alder (Alnus glutinosa L.), red oak (Quercus rubra L.), sweetgum (Liquidambar styraciflua L.), white ash (Fraxinus americana L.), white oak (Q. alba L.), white pine (Pinus strobus L.), and yellow poplar (Liriodendron tulipifera L.). Paclobutrazol was applied using either the soil drench or soil injection method. Circular wounds (2 cm [0.8 in.] diameter) were made in the bark on the main stem, and branches were pruned from treated and untreated trees at the time of paclobutrazol application in the spring. The rate of wound closure was determined one, two, and three growing seasons after treatment. The rate of bark wound closure was reduced in paclobutrazol-treated black walnut, European black alder, red oak, sweetgum, and white oak. There was no difference in the rate of closure of wounds in the bark of treated and untreated American sycamore, white ash, white pine, or yellow poplar. The rate of pruning wound closure was reduced in black walnut, European black alder, red oak, sweetgum, and white oak, but was not influenced by paclobutrazol treatment in white ash, white pine, or yellow poplar. The inhibitory effect of paclobutrazol on closure of both bark and pruning wounds in some species persisted at least 3 years. Key Words. Bark wound; growth retardant; paclobutrazol; pruning wound; tree growth regulator; wound closure. The growth regulator paclobutrazol (PBZ), used by arborists to reduce shoot growth, has been shown to have additional positive effects on trees and shrubs, including improved resistance to drought stress, darker green leaves, protection against some fungal and bacterial pathogens, and enhanced development of fibrous roots (Chaney et al. 1996; Fletcher et al. 2000; Rademacher 2000; Chaney 2003). Cambial growth as well as shoot growth has also been shown to be reduced in some tree species (Bai et al. 2004). Wounds are considered potentially detrimental to trees because they expose tissues to invasion by insects and to disease-causing and decay organisms. Infection of the phloem or xylem by primary and secondary invaders may result in wood discoloration and decay (Shigo 1989). Yet branch pruning is a sound and often essential arboricultural practice. When branch removal is necessary, rapid closure of pruning wounds is considered desirable for both aes- thetic and protective reasons because few pathogens or parasitic microorganisms can penetrate this barrier (Shigo 1984). Trees have an inherent capacity to seal wounds resulting from injury or pruning. Undifferentiated callus tissue originates from cells of the cambium around the outer periphery of the wound, gradually overgrowing the surface of the wound. Greater development of callus from the side of wounds than at the top and bottom sometimes results in an elliptical shape as the wound sealing process progresses (Neely 1970; Biggs 1992). New cambium develops adjacent to and usually in continuity with the existing undisturbed cambium (Zimmerman and Brown 1971; Biggs 1992; Blanchette 1992). This growth extending over a wound results in the strongest barrier in the wound-sealing process and is referred to as Wall 4 of the compartmentalization of decay in trees (CODIT) model (Shigo 1989). Our study was designed to investigate the effect of PBZ on the rate of closure of wounds in the bark and wounds made when branches were pruned. The research was conducted in response to concerns that, because PBZ reduces cambial growth as well as shoot growth, it might also reduce the rate of wound closure, thus prolonging the exposure of trees to decay and disease organisms. MATERIALS AND METHODS Experimental trees were located at Martell Experimental Forest Farm near the Purdue University campus in Tippecanoe County, Indiana, U.S. Two experiments were designed using combinations of nine tree species. Experiment I involved eight species ranging from 4 to 8 cm (1.6 to 3.2 in.) basal diameter and treated in April 1996 using the soil injection method: black walnut (Juglans nigra L.), European black alder (Alnus glutinosa L.), red oak (Quercus rubra L.), sweetgum (Liquidambar styraciflua L.), white ash (Fraxinus americana L.), white oak (Quercus alba L.), white pine (Pinus strobus L.), and yellow poplar (Liriodendron tulipifera L.). Experiment II investigated two species, American sy- camore (Platanus occidentalis L.) (18 to 22 cm [7.1 to 8.7 in.] average basal diameter) and yellow poplar (Liriodendron tulipifera L.) (14 to16 cm [5.5 to 6.3 in.] average basal diam- eter), treated in April 1998 using the soil drench method. Twelve (Experiment I) or eighteen (Experiment II) plantation- grown trees (2 × 2 m [6.6 × 6.6 ft] spacing) of each species used in the respective experiments were selected based on their similar size and condition. Adjacent trees were not selected for the studies to avoid root interaction and the ©2005 International Society of Arboriculture
November 2005
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