Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 7, 2010
Publication Date: May 6, 2010
Citation: Mcelrone, A.J., Grant, J.A., Kluepfel, D.A. 2010. THE ROLE OF TYLOSES IN CROWN HYDRAULIC FAILURE OF MATURE WALNUT TREES AFFLICTED WITH APOPLEXY DISORDER. Tree Physiology. 30:761-772. Technical Abstract: In the Central Valley of California, mature walnut trees afflicted with apoplexy disorder exhibit rapid and complete canopy defoliation within a few weeks of symptom initiation. Symptoms are typically found throughout the entire canopy and are initially expressed as wilting and chlorosis followed by scorching of leaves. The cause of apoplexy disorder is unknown, so we set out to elucidate the water relations physiology underlying this condition. Stem water potential ('s) of asymptomatic control trees remained high throughout the 2007 growing season while those of apoplexy afflicted trees decreased significantly with the onset of symptoms. 's’s were significantly reduced by at least 2.4 times in the lower, middle, and upper portions of the symptomatic canopies compared to those from asymptomatic, control trees. Specific hydraulic conductivity (Ks) of symptomatic stems was dramatically lower than that of control branches, and no detectable Ks could be measured on several of the symptomatic stem samples. However, shallow root Ks did not significantly differ between symptomatic and asymptomatic trees suggesting hydraulic failure was isolated to the canopy of these grafted trees. Heat pulse velocities measured in the main trunk at three radial depths consistently plummeted just prior to the onset of symptoms and eventually reached zero with complete canopy defoliation. Light and scanning electron microscopy of stem and trunk sapwood revealed extensive tylose development in vessels throughout the canopy of symptomatic trees. In 2008, the tylose development was linked to elevated ethylene production in the active sapwood of symptomatic trees. The cause of elevated ethylene that led to tylose production and subsequent apoplexy symptoms is yet to be determined.