Online xylem water isotope monitoring and soil water content profiling reveal spatial root water uptake dynamics in sunflower

Abstract. Knowledge about plant water stress regulation mechanisms (e.g., plant stem capacitance) from in-situ observation is crucial for the study and modeling of plant root water uptake (RWU). We present a proof of concept and a first application of a simple method for online, minimally invasive monitoring of the water stable isotopic composition of sap xylem water of Helianthus annuus (sunflower) by inserting a sampling tube connected to a laser spectrometer in the plant stem. After careful calibration of our method, we applied it successfully to individual sunflower plants grown in soil columns. We followed the dynamics in stem water isotopic composition in response to changing light intensity and to depth-specific, isotopically labeled water pulses. We further establish that these isotopic dynamics matched changes in RWU profiles monitored simultaneously, independently, and non-destructively by the Soil Water Profiler. We finally highlight from modeling exercises the significance of the role of plant stem capacitance: water exchanges between xylem and stem non-conducting tissues were estimated to amount to about one sixth of RWU of Helianthus annuus, showing that the stem itself can be expected to be a quickly accessible reservoir of water for transpiration, very similar to what is found in trees.