Journal of Arboriculture 31(6): November 2005 277 DISCUSSION Although the influence of PBZ treatment on the rate of wound closure varied among the species investigated, PBZ treatment reduced the rate of closure of both bark and pruning wounds in five of the nine species investigated. The exceptions were American sycamore, white ash, white pine, and yellow poplar, in which wound closure was not affected by PBZ treatment. Even excessive dose rates of PBZ applied to American sycamore and yellow poplar did not influence the rate of wound closure (Table 2). We are not aware of published data showing different responses of shoot growth when soil drench or soil injection application methods are compared. Hence, the data for yellow poplar at different dose rates using both soil injection and soil drench treatment methods were considered comparable. Like yellow poplar, American sycamore is noted for its low sensitivity to growth retardants in terms of shoot growth suppression (Sperry and Chaney 1999) and that low sensitivity appears to exist with regard to growth of callus tissue around wounds in these trees treated with PBZ. Discoloration of the wood beneath bark wounds was minimal (Figures 2 and 3) and was not influenced by treatment with PBZ. It is known that the amount and intensity of discolored wood that develops after wounding is dependent on the size and depth of the wound within the xylem. Drill wounds that bore into the xylem have apprecia- bly more discoloration than surface wounds (Blanchette 1992). The bark wounds in this study were made in April when the cambium was active and the bark and phloem tissue removed with the cork borer separated easily from the succulent cambial cells with little or no damage to the xylem. Armstrong et al. (1981) also found that wood discoloration associated with bark wounds on black walnut was less when made in the spring rather than in the fall. The differences among species in the rate of wound closure in response to PBZ treatment found in this study are consistent with species sensitivities to PBZ reflected in the dose rate charts provided by distributors of commercial formulations of PBZ that recommend six rate categories, A through F (1.25 to 4 g PBZ [0.04 to 0.14 oz] per inch dbh). White ash, white pine, yellow poplar, and American sy- camore are recognized as the least sensitive and are in category F for the highest dose rate. Sweetgum is quite sensitive to PBZ and is in category B. Both red and white oak are intermediate and are in category D. The dose rate applied to all trees in this study was that for category D or F, depending on the diameter of the tree treated. PBZ inhibits the biosynthesis of gibberellins produced via the terpenoid pathway (Rademacher 2000). The most frequently reported and best-known consequence of gibberellin synthesis inhibition is reduced cell elongation and shortened shoot internodes. However, growth retar- dants also are known to suppress cell division both in intact plants and cell suspension cultures (Grossmann 1990). Diameter growth, resulting from cambial activity including cell division, enlargement, and differentiation, is regulated by gibberellins and other plant hormones (Kozlowksi 1971) and has been shown to be reduced in trees by PBZ treat- ment (Bai et al. 2004). Besides gibberellins, phytosterols are synthesized via the terpenoid pathway, and their production is also inhibited by PBZ (Rademacher 2000; Chaney 2003). Because sterols are structural components of membranes, inhibition of their synthesis accounts for the fungistatic properties of PBZ as well as reduction in cell division (Grossmann 1990) and, possibly, the reduced growth of callus tissue around wounds of some tree species as shown in this study. Although the results of this study are the first to our knowledge to show an effect of PBZ on wound sealing in trees, PBZ and similar growth retardants have been shown to reduce growth of callus tissue of nonwoody species such as tobacco (Nicotiana spp., Wisconsin 38-W38TO) (Premachandra et al. 1996), rice (Orysa sativa) (Grossmann 1988), and celery (Apium graveolens) (Haughan et al. 1988) on agar media and in cell solution cultures. Reduction of the rate of closure of both bark and pruning wounds by PBZ in some species persisted at least 3 years, consistent with previous reports for the duration of shoot growth reduction (Chaney 2003). It is known that trees of different species and even different genotypes within a species respond differently to wounding (Shigo et al. 1977; Garrett et al. 1979; Armstrong et al. 1981). The data presented here also show that tree species have different wound responses following PBZ treatment. Indeed, the pattern of PBZ effect on wound closure among species was exactly the same as the varied range of PBZ effects on cambial growth in the same species in another study (Bai et al. 2004). PBZ applied to the soil is slowly dissolved in water and moves principally upward in the xylem tissue. It tends to bind reversibly to soil and/or to wood as it passes into the xylem and up the vascular system of trees. Sometimes, there is a delay between the time of soil treatment with PBZ and the evidence of growth suppression in a tree crown. The length of this delay is determined by a combined effect of root distribution, transpiration rate, transport drag imposed by adsorption of chemical to the walls of the vascular cells, propensity for chemical in the transpiration stream to be diverted into leaves, and the need to consume existing available endogenous gibberellins before PBZ-induced symptoms become apparent (Lever 1986). PBZ applied in spring can move into trees and affect the biosynthesis and amount of active gibberellins in that growing season, slowing the closure of bark wound and pruning wounds in the year of treatment for some species. However, for other species, the inhibiting effect was not observed in the year of ©2005 International Society of Arboriculture
November 2005
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