¹⁸O enrichment of sucrose and photosynthetic and nonphotosynthetic leaf water in a C₃ grass—atmospheric drivers and physiological relations

Abstract

The 18O enrichment (Δ18O) of leaf water affects the Δ18O of photosynthetic products such as sucrose, generating an isotopic archive of plant function and past climate. However, uncertainty remains as to whether leaf water compartmentation between photosynthetic and nonphotosynthetic tissue affects the relationship between Δ18O of bulk leaf water (Δ18OLW) and leaf sucrose (Δ18OSucrose). We grew Lolium perenne (a C3 grass) in mesocosm‐scale, replicated experiments with daytime relative humidity (50% or 75%) and CO2 level (200, 400 or 800 μmol mol−1) as factors, and determined Δ18OLW, Δ18OSucrose and morphophysiological leaf parameters, including transpiration (Eleaf), stomatal conductance (gs) and mesophyll conductance to CO2 (gm). The Δ18O of photosynthetic medium water (Δ18OSSW) was estimated from Δ18OSucrose and the equilibrium fractionation between water and carbonyl groups (εbio). Δ18OSSW was well predicted by theoretical estimates of leaf water at the evaporative site (Δ18Oe) with adjustments that correlated with gas exchange parameters (gs or total conductance to CO2). Isotopic mass balance and published work indicated that nonphotosynthetic tissue water was a large fraction (~0.53) of bulk leaf water. Δ18OLW was a poor proxy for Δ18OSucrose, mainly due to opposite Δ18O responses of nonphotosynthetic tissue water (Δ18Onon‐SSW) relative to Δ18OSSW, driven by atmospheric conditions.

Half of the 18O enrichment of leaf sucrose is conserved in leaf cellulose of a C3 grass across atmospheric humidity and CO2 levels

“…Baca Cabrera et al (2023) interpreted this mismatch by (1) the presence of a very large non-photosynthetic tissue water fraction in leaf blades (approx. 53% in Lolium perenne, based on the anatomical studies of Charles-Edwards et al, 1974 and Dengler et al, 1994) coupled with (2) an environmentally-driven radial Péclet effect in the non-photosynthetic tissue water fraction of the leaf blade, while (3) the photosynthetic tissue water fraction was relatively close to theoretical estimates of average 18 O enrichment of water at the evaporative sites (see above).In particular, at low RH, source water accounted for a very large proportion (likely >80%) of the water contained in the non-photosynthetic tissue(Baca Cabrera et al, 2023), which was mostly (~77%) comprised of epidermis water according to the anatomical data ofCharles-Edwards et al (1974) andDengler et al…”

“…Quite certainly, therefore, Δ 18 O SSW is a much better measure of the Δ 18 O of (assimilation-weighted) photosynthetic medium water than Δ 18 O LW , which has been the traditional proxy of the Δ 18 O of photosynthetic medium water in grasses (e.g., Helliker & Ehleringer, 2002a;Liu et al, 2016) Liu et al, 2016). Again, this underscores that further attempts for a better understanding of p ex-LW will require studies on the drivers and mechanisms underlying divergent Δ 18 O of photosynthetic and non-photosynthetic leaf tissue water (Baca Cabrera et al, 2023).…”

“…α-cellulose was extracted from 50 mg of dry sample material by following theBrendel et al (2000) protocol as modified (i.e., the water-modified Brendel method) byGaudinski et al (2005), including a treatment with a 17.5% NaOH solution. Samples obtained with this protocol consistently contained at most traces of nitrogen (<0.05% of dry mass).Water-soluble carbohydrates were extracted from 50 mg aliquots of dry material from the youngest fully-expanded leaf blade, and sucrose was separated from other compounds using a preparative HPLC technique similar to that described byGebbing & Schnyder (2001) (Baca Cabrera et al, 2023).…”

“…After every 15-25 samples, heavy and light laboratory water standards, spanning the range of δ 18 O values in the data set and previously calibrated against V-SMOW, V-GISP, and V-SLAP, were measured for SMOW-scaling and possible drift correction. Analytical uncertainty was <0.2‰.Cellulose and sucrose samples were measured by isotope ratio mass spectrometry, as in BacaCabrera et al ( , 2023. Each sample (sucrose or cellulose) was measured against a laboratory working standard carbon monoxide gas, previously calibrated against a secondary isotope standard (IAEA-601, accuracy of calibration ±0.25‰ standard deviation).…”

Effects of increasing atmospheric CO2 on leaf water δ18O values are small and are attenuated in grasses and amplified in dicotyledonous herbs and legumes when transferred to cellulose δ18O values

“…Two processes can explain the pCO 2 sensitivity of Δ 18 O SUG in the grass. First, it is possible that Δ 18 O values of sucrose synthesis water vary in response to pCO 2 differently than Δ 18 O LW (Cabrera et al, 2023). Alternatively, the observed effects in Δ 18 O SUG could have been caused by variability in ε bio .…”