Journal Article

The role of tyloses in crown hydraulic failure of mature walnut trees afflicted by apoplexy disorder

Andrew J. McElrone, Joseph A. Grant and Daniel A. Kluepfel

in Tree Physiology

Volume 30, issue 6, pages 761-772
Published in print June 2010 | ISSN: 0829-318X
Published online May 2010 | e-ISSN: 1758-4469 | DOI:
The role of tyloses in crown hydraulic failure of mature walnut trees afflicted by apoplexy disorder

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In the Central Valley of California, mature walnut trees afflicted with apoplexy disorder exhibit rapid and complete crown defoliation within a few weeks of symptom initiation. Symptoms are typically found throughout the entire crown and are initially expressed as wilting and chlorosis followed by scorching of leaves. Since the cause of apoplexy disorder is unknown, we set out to elucidate the water relations physiology underlying this condition. Stem water potential (Ψs) of healthy, asymptomatic trees remained high throughout the 2007 growing season, while that of apoplexy-afflicted trees decreased significantly with the onset of symptoms for both healthy-appearing and symptomatic portions of these trees. Ψss were significantly reduced by at least 0.7 MPa in the lower, middle and upper portions of the symptomatic canopies compared with those from asymptomatic trees. Heat pulse velocities measured in the main trunk at three radial depths consistently decreased prior to the onset of symptoms and eventually reached zero with complete crown defoliation. Comparison of sap flow with predicted water use based on walnut evapotranspiration suggests that stomata of symptomatic trees were closing at higher evaporative demand prior to symptom formation. Specific hydraulic conductivity (Ks) of symptomatic stems was significantly lower than that of asymptomatic stems, 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 that hydraulic failure was isolated to the crown of these grafted trees. Light and scanning electron microscopy of stem and trunk sapwood revealed extensive tylose development in vessels throughout the crown of symptomatic trees. Analysis of the formation of tyloses on multiple dates revealed rapid development of these vessel occlusions in conjunction with visual symptom formation and dramatic decreases in sap flow. In 2008, tylose development was associated with elevated ethylene production in the active sapwood of symptomatic trees. The cause of elevated ethylene associated with tylose production and symptom formation is yet to be determined.

Keywords: ethylene; Juglans hindsii rootstock; Juglans regia; leaf scorching; sap flow; vascular dysfunction; water relations

Journal Article.  7601 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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