INCOMPLETE ABSCISSION OF NEEDLE CLUSTERS AND RESIN RELEASE FROM ARTIFICIALLY WATER-STRESSED LOBLOLLY PINE (PINUS TAEDA): A COMPONENT FOR PLANT-ANIMAL INTERACTIONS

A selected Pinus taeda L. tree was rapidly and permanently water-stressed by severing and sealing the bole and observed daily for seven wk and then weekly for six months. Insects were captured before and during treatment from the stressed and adjacent control trees. Tissue samples were extracted at selected intervals for comparison of the stressed and control trees. Drying of the crown of the stressed tree commenced immediately. By the tenth day the previous yrs' needle clusters were red and dropping from the tree while the current yr's needle clusters remained green and did not fall. By the eleventh day beads of resin, predominantly α- and β-pinene, formed and dripped from the abscission scars of the youngest needle clusters of the previous yr. At this time and for several days thereafter, insects were attracted to the stressed tree. Microscopy of abscission sites of the stressed and control trees shows that the sites where resin flowed had incomplete abscission layers, lysigenous collapse of adjacent cortical cells to form a cavity that coalesced the resin ducts associated with the needle cluster, and had mechanically ruptured the tissue connections so that resin normally contained within the duct system was released to the exterior of the tree. Volatilized α-pinene is attractive to the insects associated with pines. Failure of abscission in the stressed tree leading to abnormal resin release is thus proposed as a means by which insects are attracted to stressed trees.