Isotopic composition of transpiration and rates of change in leaf water isotopologue storage in response to environmental variables

Abstract

During daylight hours, the isotope composition of leaf water generally approximates steady-state leaf water isotope enrichment model predictions. However, until very recently there was little direct confirmation that isotopic steady-state (ISS) transpiration in fact exists. Using isotope ratio infrared spectroscopy (IRIS) and leaf gas exchange systems we evaluated the isotope composition of transpiration and the rate of change in leaf water isotopologue storage (isostorage) when leaves were exposed to variable environments. In doing so, we developed a method for controlling the absolute humidity entering the gas exchange cuvette for a wide range of concentrations without changing the isotope composition of water vapour. The measurement system allowed estimation of 18O enrichment both at the evaporation site and for bulk leaf water, in the steady state and the non-steady state.We show that non-steady-state effects dominate the transpiration isoflux even when leaves are at physiological steady state. Our results suggest that a variable environment likely prevents ISS transpiration from being achieved and that this effect may be exacerbated by lengthy leaf water turnover times due to high leaf water contents.

Online CO2 and H2O oxygen isotope fractionation allows estimation of mesophyll conductance in C4 plants, and reveals that mesophyll conductance decreases as leaves age in both C4 and C3 plants

“…Leaves remained in the chamber for 30-60 min, and measurements over the final 20 min were averaged. The inlets to the photosynthesis systems were connected to a vaporization module to provide a CO 2 -free air stream with constant vapour concentration and d 18 O, as described by Simonin et al (2013). The mass flow controllers within the vaporization module were set to deliver a minimum of 2.5 l min À1 flow, to provide sufficient flow to supply both photosynthesis systems and the water vapour laser.…”

“…The stable oxygen isotope composition of water vapour was measured using a Cavity Ring-Down Spectrometer (L1102-i; Picarro Inc., Sunnyvale, CA, USA), as described by Simonin et al (2013). The L1102-i was calibrated using two standard water samples (d 18 O À14.93 and À0.95&, relative to VSMOW, measured using a stable isotope mass spectrometer at The Australian National University, Canberra, Australia) that spanned the range in d…”

“…Again, this technique has not been applied since their initial paper. Recent developments in optical isotope analysis of CO 2 and H 2 O coupled with leaf gas exchange (Barbour et al, 2007;Tazoe et al, 2009;Simonin et al, 2013) open the way to applying 18 O techniques to quantify g m in C 4 leaves. Here we describe an online, real-time measurement system for g m in both C 3 and C 4 plants by coupling traditional gas exchange with laser absorption spectrometers capable of measuring the stable carbon and oxygen isotope composition of CO 2 , and the stable oxygen isotope composition of transpired water vapour.…”

High‐resolution isotope measurements resolve rapid ecohydrological dynamics at the soil–plant interface

“…Only when transpiration is in isotopic steady state is its isotopic composition equal to that of xylem water, which integrates all water sources taken up by a plant. Recent studies applying laser spectroscopy showed that the measured isotopic composition of transpiration often deviated from isotopic steady state and that, especially in a highly variable environment, long leaf water turnover times might prevent isotopic steady-state transpiration (Simonin et al, 2013;Dubbert et al, 2014;Song et al, 2015). To test for potential deviations from isotopic steady state, we performed a preliminary experiment with the same experimental set-up as described above.…”

“…The short leaf water residence times, calculated according to Farquhar & Cernusak (2005) and amounting on average to c. 25 min for oak and c. 20 min for beech between 09:00 h and 19:00 h, might have favored isotopic steady state in transpiration under these conditions. We acknowledge that step changes in environmental conditions as applied by Simonin et al (2013) and long residence times of leaf water of > 5 h as determined by Dubbert et al (2014) might be more problematic with our approach. In general, closed static chamber systems might inherently cause errors in isotope measurements as a result of nonsteady operating conditions (Ohlsson et al, 2005;Yepez et al, 2005).…”