Sunflower Stems Hide a Secret Water Reservoir, New Isotope Probe Reveals

For decades, scientists trying to work out where a plant gets its water have relied on a deceptively simple idea: sample the water in the stem, compare its isotopic fingerprint with that of the soil at different depths, and you can reconstruct which soil layers the roots are drinking from. The trouble is that this logic assumes the stem is nothing more than a passive pipe. A new study published in Hydrology and Earth System Sciences shows that, at least for sunflower, the stem is far from a passive pipe — it is an active reservoir that exchanges roughly one sixth of all the water the roots take up with the surrounding tissues, buffering and delaying the isotopic signal that researchers have been interpreting for years.

The study, led by Youri Rothfuss of Forschungszentrum Jülich together with colleagues in Germany, Belgium and at the University of Bonn, presents a proof of concept for continuously monitoring the stable isotope composition of water inside the stem of a living, herbaceous plant — something previously achieved only for trees. The team drilled a small hole, 3.5 millimeters wide and 5 millimeters deep, into the stem of sunflower plants a couple of centimeters above the soil surface, sealed a thin PTFE sampling tube into the borehole, and connected it to a cavity ring-down laser spectrometer. Water vapor diffusing from the stem tissues was drawn through the tube at 28 milliliters per minute and analyzed for its hydrogen and oxygen isotope ratios every single second, giving researchers a real-time window into the plant’s internal plumbing.

Before any field-relevant conclusions could be drawn, the method had to be calibrated. In a first experiment, nine sunflower plants of the compact “Yellow Spray” cultivar were grown hydroponically for three months and then transferred into water baths with three distinctly different isotope compositions: one strongly depleted, one matching local tap water, and one strongly enriched. Once the stem vapor readings reached a stable plateau — which took between 20 and 45 minutes — the researchers could establish linear calibration functions linking the vapor measured in situ to the liquid water actually circulating in the xylem. The fits were remarkably tight: more than 99.9 percent of the variability in the hydroponic solutions was explained, with root mean square errors of just 1.8 permil for hydrogen and 0.8 permil for oxygen. Notably, in a separate soil-grown test, the response time of the isotope signal dropped to a mere 20 to 30 seconds — essentially the response time of the laser spectrometer itself.

With the calibration in hand, the team moved to the centerpiece of the study: two sunflowers grown individually in soil columns inside a climate-controlled chamber, where temperature, humidity and light were held under strict control. To independently verify what the isotope probe was seeing, the researchers deployed the Soil Water Profiler, or SWaP, a capacitive sensor developed at Jülich that slides up and down along the soil column on a precision axis, measuring volumetric water content at centimeter resolution with an accuracy of up to 0.003 cubic centimeters of water per cubic centimeter of soil. Because changes in light intensity rapidly alter transpiration and root water uptake while vertical water redistribution in the soil proceeds more slowly, the local rate of water content decline can be read as a direct measure of how much water the roots are extracting from each depth.

The experimental design was elegant in its simplicity. The researchers injected pulses of isotopically labeled water into specific depths of the soil — enriched water near the surface, depleted water at 30 centimeters depth through a porous aeration stone — and then simply watched. When enriched water was added to the top layer of the first plant’s column, the contribution of the upper soil layer to total root water uptake surged while uptake from the middle layer dropped, and within minutes the isotope composition of the stem water began to rise. When depleted water was later injected deep into the profile, the pattern reversed: uptake from the middle layer jumped from 0.06 to 0.95 milliliters per 10 minutes while surface uptake collapsed, and the stem isotope signal turned over in lockstep. The two independent techniques — isotope monitoring and soil water profiling — told the same story, validating the new approach against an established one.

But the most striking finding emerged when the measured isotope dynamics were compared with steady-state model calculations. A classical mixing model, which assumes that water extracted from the soil instantly and completely mixes inside the roots and stem, predicted instantaneous jumps in stem water isotope composition following each change in uptake pattern. The real plants did nothing of the sort. Even under high transpiration demand, the observed isotope values took roughly two hours to approach a new plateau, and after a full night of uptake the stem water had still not fully equilibrated with the labeled soil water. Something inside the plant was slowing the signal down — and the culprit was the stem itself.

Using a simple first-order replacement model, the researchers estimated that only about one sixth of the water flowing through the xylem exchanges with the water stored in the stem’s non-conducting tissues, the pith and cortex. With a total root water uptake of roughly 3 milliliters per hour and a sampled stem volume of about 2 milliliters, this exchange fraction implies that it takes around ten hours for the stem reservoir to be nearly fully replaced by newly absorbed water. In other words, when the sun opens its stomata and transpiration spikes faster than the roots can respond, the sunflower does not immediately draw the difference from the soil — it mobilizes water stored in its own stem, much like a camel drawing on its hump. When transpiration falls, the surplus is redistributed back into those tissues. The stem, in effect, functions as a rapidly accessible buffer, a behavior long recognized in trees but rarely quantified in herbaceous crops.

This finding carries real consequences for the wider scientific community. Isotope-based studies of plant water sources — spanning agriculture, ecology and hydrology — routinely treat stem water as a faithful snapshot of current root uptake. The new results show that this assumption breaks down under dynamic conditions, particularly when light intensity changes or when the soil dries out. The isotopic composition of water in aboveground tissues, the authors argue, should not be read as a static map of belowground reservoirs but as a dynamic signal shaped by the plant’s ever-changing water demand. For large woody species, whose stem reservoirs are enormous relative to their sap flow, the distortion could be even more pronounced — a caveat that recent work on tree sap and tissue water has already begun to flag.

The method itself is strikingly accessible. Beyond the laser spectrometer, it requires no exotic equipment: a drill, a thin tube, and sealing paste. The plants showed no visible signs of stress during the experiments, and magnetic resonance imaging of the stems after the trial confirmed the drilling left only localized damage. Independent pre-tests suggest continuous monitoring is feasible for at least 96 hours, although the authors caution that long-term applications — and possible plant defense responses that might seal the borehole — remain to be tested. The technique proved sensitive to the exact insertion depth and height of the tubing, working best when the tube penetrated only a few millimeters into the stem close to the root crown, and calibration functions will need to be re-established for different growth stages and likely for other species.

The researchers are explicit about the limits of their proof of concept: with no experimental replicates, the study was not designed to draw definitive conclusions about sunflower physiology in general. Yet the combination of continuous stem isotope monitoring and non-destructive soil water profiling opens a door that ecohydrologists have been pushing at for years. It promises a way to quantify plant hydraulic capacitance directly, to study the residence and transit times of water inside plants in the same way isotopes have long been used to trace water through watersheds, and to reconcile the isotope compositions of root uptake, xylem water and transpiration within a single coherent framework. As droughts intensify across the world’s croplands, understanding how plants bank water in their own bodies — and how quickly they can draw on that bank — may prove to be far more than an academic curiosity.

Subject of Research: In situ xylem water isotope monitoring and root water uptake dynamics in sunflower

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

Article References: Rothfuss, Y., Le Gall, S., Brüggemann, N., Jahan, S., Javaux, M., Klaus, J., Vereecken, H., & van Dusschoten, D. (2026). Online xylem water isotope monitoring and soil water content profiling reveal spatial root water uptake dynamics in sunflower. Hydrology and Earth System Sciences, 30(18), 6095-6114. https://doi.org/10.5194/hess-30-6095-2026

Image Credits: AI Generated

DOI: 10.5194/hess-30-6095-2026

Keywords: sunflower, xylem water, stable isotopes, root water uptake, plant hydraulics, stem capacitance, laser spectrometer, soil water content, Soil Water Profiler, drought stress, ecohydrology, Hydrology and Earth System Sciences