Sap flow sensors can be used to study the hydraulic function of healthy versus diseased trees. Sap flow sensors can detect a decrease in hydraulic function associated with disease prior to the onset of outward, physical symptoms. Sap flow sensors, therefore, provide a tool for researchers to better understand the physiology of disease in trees.
McElrone et al (2010) used sap flow sensors, along with other plant water relations instrumentation, to better understand “walnut apoplexy disorder”. This disorder is characterised by the sudden wilting and collapse of mature walnut trees during the height of the growing season. Walnut apoplexy disorder can significantly impact commercial walnut operations, however the causes of the disease are unknown.
McElrone et al hypothesised that, since the disorder resembles rapid onset of drought-induced symptoms, hydraulic failure causes walnut apoplexy due to tyloses.
Tyloses are formed as outgrowths of parenchyma cells and can “occlude”, or block, xylem function. Tyloses are a wound response that is meant to protect the plant from further damage or stress. Tyloses are known to be triggered by stressors such as flooding, pathogen infection and mechanical damage. Tylosis formation is also a natural part of plant aging, associated with heartwood formation and leaf senescence.
McElrone et al confirmed that walnut apoplexy disorder was associated with rapid tylosis formation throughout all parts of the canopy including petiole, branches and trunk.
Sap flow sensors were able to detect a decline in hydraulic function prior to the onset of obvious visual symptoms. Sap flow measurements were more sensitive detection tool than other plant water relations measurements such as leaf water potential. Sap flow sensors were able to detect the onset of symptoms up to 2 weeks prior to visual symptoms.
McElrone et al explained how sap flow sensors were able to detect walnut apoplexy disorder prior to other instruments and visual inspection. In the walnut tree, tyloses formed rapidly which occluded xylem. This caused a decrease in hydraulic conductivity and sap flow. However, leaf water potential was maintained at a “healthy level” due to partial stomatal closure. Two weeks later, visual symptoms appeared, and this was reflected in leaf water potential measurements.
Incidentally, McElrone et al still could not explain the initial cause, or onset, of walnut apoplexy disorder. They did hypothesis that it was due to excessive irrigation leading to flooding around roots – a known cause of tylosis formation.
The study of McElrone et al did demonstrate that sap flow sensors can be successfully deployed in research of tree disease, providing invaluable insight into physiological dynamics.
McElrone et al (2010). The role of tyloses in crown hydraulic failure of mature walnut trees afflicted by apoplexy disorder. Tree Physiology, 30, 761-772. Weblink.