Doctoral defence: Annika Joy Meitern “Impact of potassium ion content of xylem sap and of light conditions on the hydraulic properties of trees”

On 29 March at 15:00 Annika Joy Meitern will defend her doctoral thesis “Impact of potassium ion content of xylem sap and of light conditions on the hydraulic properties of trees” for obtaining the degree of Doctor of Philosophy (in Plant Ecology and Ecophysiology).

Supervisor:
Prof. Arne Sellin, University of Tartu

Opponent:
Prof. Elisabetta Oddo, University of Palermo (Italy)

Summary
Water transfer from roots to leaves is essential for plants to keep up photosynthesis, grow and survive. Water moves from roots to shoots through water-conducting tissue called xylem. The water transport capacity of the xylem is characterized by hydraulic conductance. The water inside plants always contains dissolved substances; therefore, it is called a xylem sap. The main driving force for long-distance water transport in the xylem is transpiration, but hydraulic conductance can also be adjusted via changes in xylem sap ionic content on a short time scale. The objectives of the thesis were to investigate circadian patterns of hydraulic conductance and physico-chemical properties of xylem sap; to evaluate the impact of phloem girdling on branch hydraulic conductance and xylem sap properties; to evaluate the impact of light quality and intensity on leaf and shoot hydraulic properties. The study objects were fast-growing broadleaved trees like hybrid aspen and silver birch. The work was carried out in the mixed forest of Järvselja Training and Experimental Forestry District and the Free Air Humidity Manipulation experimental site (FAHM) located at Rõka. In the thesis, I proved that water transport capacity is modulated by xylem sap ionic content – increased K+ concentration of xylem sap enhances trees hydraulic conductance, indicating the involvement of potassium ions in the regulation of trees hydraulic efficiency. Both hydraulic and physico-chemical properties of xylem sap in trees follow coordinated circadian patterns driven by environmental conditions. Phloem transport disruption by phloem girdling at branch base affects hydraulic properties and gas exchange but not xylem sap properties in hybrid aspen. Both light availability and quality and xylem sap ionic content significantly affect trees hydraulic properties. They enable plants to optimize their water use and ensure positive water balance under changing environmental conditions.