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Formation of radial resin ducts and their associated secretory cells in loblolly pine (Pinus taeda L.) 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 frontalis Zimmermann, and associated microorganisms, since the resin-producing system is a primary defense against these agents.

Several low molecular weight phenolic metabolites of Ceratocystis minor (Hedgcock) Hunt, a blue-stain fungus of loblolly pine, were isolated from cultures grown on 2.5% malt-extract liquid medium. The most abundant compounds were 6,8-dihydroxy-3-hydroxymethyl isocoumarin and 6,8-dihydroxy-3-methyl isocoumarin. Two unknown phenolic compounds were also isolated. Solutions of these fungal metabolites stimulated transpiration in loblolly pine seedlings when administered through the cut stem. Rates of water loss were doubled within 24 h after treatment with isocoumarin solutions. Administration of raw or sterile fungal culture (in liquid medium) on the other hand produced a small but significant inhibition of water loss shortly after treatment. Reduction in transpiration after treatment with fungal culture probably is the result of partial tracheid blockage by as yet unidentified fungal products. Effects of metabolites of C. minor on the water relations of loblolly pine are discussed in terms of a possible mutualistic association between C. minor and the southern pine beetle, Dendroctonus frontalis Zimmermann.