Formation of radial resin ducts and their associated secretory cells in loblolly pine (Pinus taedaL.) is influenced by the age and growth rate of the annual ring in which the ducts are formed. The spatial pattern of radial ducts on the tangential plane is nonrandom, exhibiting a regular or dispersed pattern. A significantly higher density of radial ducts was found in the inner, first-formed growth rings at all heights within the tree. Radial duct formation was found to be positively correlated with radial growth rate, when growth rate is expressed as increment of cross-sectional area growth. These findings may partially explain why older, slower growing trees tend to be more susceptible to attack by the southern pine beetle,Dendroctonus frontalisZimmermann, and associated microorganisms, since the resin-producing system is a primary defense against these agents.
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