Surface tension of xylem sap

The high surface tension () of water is of particular importance for the water transport within a plant. It is responsible for tensions occurring in the water transport network (xylem) and prevents the entry of air into the transport system (embolism) under high tension. Plants can modulate xylem sap by changes in sap composition as demonstrated in two timberline conifers, which exhibited pronounced seasonal variation in xylem sap and respective changes in the vulnerability to drought- induced xylem embolism. However, it is unknown if other species show similar seasonal or short- term changes in xylem sap . Main goals of the proposed project, which is based on an abroad (Australia) and a return phase (Austria), therefore are: (i) Analysis of the temporal (seasonal) and spatial variation (within a plant) in xylem sap of different tree species (angiosperms and conifers) growing in different ecosystems. (ii) Analysis of effects on xylem hydraulics (i.e. transport efficiency and safety). (iii) Analysis of effects on leaf/needle hydraulics. (iv) Analysis of effects on winter hydraulics. We expect highest temporal and spatial variation of xylem sap in species growing in environments with high stress intensities as well as pronounced differences between angiosperms and conifers. Furthermore, we assume that low increases the vulnerability to both drought and freeze-thaw induced xylem embolism. The project is based on numerous methods, including innovative noninvasive in vivo techniques, such as X-ray microtomography and a recently developed optical method for leaf hydraulics. Research activities will also include the development of a portable device for analysis based on the pendant drop technique and a portable device to extract xylem sap based on the vacuum extraction technique. Project outcomes will help to improve our understanding of tree hydraulics with respect to basic and applied ecophysiological aspects, which are important for forestry and agriculture disciplines, especially in the light of predicted climate change. The project enables the applicant to broaden his theoretical and methodical knowledge and to get in contact with several working groups. It will also intensify cooperation between involved institutions and enable methodical exchange. The project is based on the collaboration between the Hawkesbury Institute for the Environment, Western University of Sydney (hosting institution for the abroad phase, cooperation with Prof. Brendan Choat) and the Department of Botany, University of Innsbruck (return phase, cooperation with Prof. Stefan Mayr). In the return phase, further cooperation is planned with Elettra Light Source in Trieste, Italy (SYRMEP beamline, Dott. Giuliana Tromba) and the University of Trieste, Italy (Prof. Andrea Nardini).

Plants can modulate their xylem sap composition as a response to external stimuli, and thus influence both the hydraulic conductance of their water transport system and its vulnerability to drought-induced failure (i.e., xylem embolism). Information on the ability of plants to modulate their xylem sap composition over seasons might help understanding how trees can cope with the forecasted global increase in intense drought events, responsible for widespread forest damage. Main goals of the present Erwin Schrödinger fellowship, which was based on an abroad (Prof. Brendan Choat's lab, Western University of Sydney, Australia) and a return phase (Prof. Stefan Mayr's lab, Universität Innsbruck, Austria), were to analyze (i) seasonal and spatial variation in xylem sap composition, and related effects on xylem water transport; (ii) the impact of extreme drought on trees and their recovery after favorable conditions; and (iii) the vulnerability to drought-induced xylem embolism of conifers with a recently developed optical technique. In the abroad phase, we observed major seasonal changes in xylem sap of six Australian woody species. Results also showed mangroves species to be extremely efficient in absorbing rainwater via the leaves. Though, rainwater did not affect xylem sap composition whereas high tides increased the overall ionic concentration. In our study on extreme drought, we found most native tree species to exhibit high rates of xylem embolism and extensive canopy browning following the 2017-19 drought. However, after 8-10 months of favorable conditions, most surviving trees repaired xylem embolism, which corresponded to decreased canopy browning indicating improved tree health. This dataset provides evidence that drought-induced xylem embolism is a major contributor in canopy dieback and tree mortality during extreme drought events. In the return phase, the optical technique for analysis of vulnerability to drought-induced xylem embolism was successfully established, and used to test the vulnerability to drought-induced xylem embolism of three alpine conifers and compared with two well-known methods: acoustic emission and centrifuge technique. Results indicated optical technique to underestimate the vulnerability of conifers under study. Findings are particularly important for the scientific community as these methods have not been compared before. This project enabled the applicant to broaden his theoretical and methodical knowledge, strengthened the collaboration between the two labs involved and allowed collaboration with other well-known international researcher groups. Outcomes resulted in several scientific articles and conference presentations.