Seasonal variation in the xylem sap composition of six Australian trees and shrubs

In recent years, xylem sap composition has been shown to affect xylem hydraulics. However, information on how much xylem sap composition can vary across seasons and specifically under drought stress is still limited. We measured xylem sap chemical composition ([Ca2+], [K+], [Na+], electrical conductivity EC and pH) and surface tension (gamma) of six Australian angiosperm trees and shrubs over 1 year, which comprised of exceptional dry and wet periods. Percentage losses of hydraulic conductivity and predawn leaf water potential were also monitored. In all species, measured parameters changed considerably over the annual time course. Ions and pH tended to decrease during winter months whereas gamma showed a slight increase. No clear correlation was found between sap and hydraulic parameters, except for pH that was higher when plants suffered higher drought stress levels. Results indicate xylem sap composition to be complex and dynamic, where most variation in its composition seems to be dictated by season, even under severe dry conditions. However, pH might play a role as signals of drought stress.|The composition of xylem sap has been shown to affect the water status of plants, but little is known on how much it can vary across seasons and specifically under drought stress. In this study, we measured xylem sap composition of six Australian angiosperm trees and shrubs over one year, which comprised exceptional dry and wet periods. The water status of these plants was also monitored. In all species, xylem sap composition considerably changed across seasons, and no clear correlation between xylem sap and plants water status was found. Results indicate xylem sap composition to be complex and dynamic, where most variation in its composition seems to be dictated by seasons, even under severe dry conditions. However, pH was the only parameter which was higher when plants suffered higher drought stress levels and might thus play a role as signals of drought stress.