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· The 18O composition of plant cellulose is often used to reconstruct past climate and plant function. However, uncertainty remains regarding the estimation of the leaf sucrose 18O signal and its subsequent attenuation by 18O exchange with source water during cellulose synthesis.· We grew Lolium perenne at three CO2 concentrations (200, 400 or 800 mmol mol-1) and two relative humidity (RH) levels (50% or 75%), and determined 18O enrichment of leaf sucrose (Δ18OSucrose), bulk leaf water (Δ18OLW), leaf cellulose (Δ18OCellulose) and water at the site of cellulose synthesis (Δ18OCelSynW). · Δ18OCellulose correlated with Δ18OSucrose (R2=0.87) but not with Δ18OLW (R2=0.04), due to a variable 18O discrepancy (range 2.0-9.0‰) between sucrose synthesis water (Δ18OSucSynW, estimated from Δ18OSucrose) and bulk leaf water. The discrepancy resulted mainly from an RH effect. The proportion of oxygen in cellulose that exchanged with medium water during cellulose formation (pex), was near-constant when referenced to Δ18OSucSynW (pex-SucSynW = 0.52±0.02 SE), but varied when related to bulk leaf water (pex-LW = –0.01 to 0.46). · We conclude that previously reported RH-dependent variations of pex-LW in grasses are related to a discrepancy between Δ18OSucSynW and Δ18OLW that may result from spatial heterogeneity in 18O gradients of leaf water and photosynthetic sucrose synthesis.

We explore here our mechanistic understanding of the environmental and physiological processes that determine the oxygen isotope composition of cellulose (δ18Ocellulose). A new allocation-and-growth model was designed and added to the 18O-enabled soil-vegetation-atmosphere transfer model MuSICA to predict seasonal (April–October) and multi-annual (2007‑2012) variation of δ18Ocellulose and 18O-enrichment of leaf cellulose (Δ18Ocellulose) in a drought-prone, temperate grassland ecosystem. Modelled δ18Ocellulose agreed best with observations when integrated over c. 400 growing degree-days, similar to the average leaf lifespan observed at the site. Over the integration time, air temperature ranged from 7 to 22°C and midday relative humidity from 47 to 73%. Model agreement with observations of δ18Ocellulose (R2 = 0.57) and Δ18Ocellulose (R2= 0.74), and their negative relationship with canopy conductance, were improved significantly when both the biochemical 18O-fractionation between water and substrate for cellulose synthesis (ϵbio, range 26‑30‰) was temperature-sensitive, as previously reported for aquatic plants and heterotrophically grown wheat seedlings, and the proportion of oxygen in cellulose reflecting leaf water 18O-enrichment (1 ‑ pexpx, range 0.23‑0.63) was dependent on air relative humidity, as observed in independent controlled experiments with grasses. These recently published results (Hirl et al. 2020, The New Phytologist, doi: 10.1111/nph.17111) demonstrate that disentangling the physiological and climatic information in δ18Ocellulose requires quantitative knowledge of direct climatic effects on pexpx and ϵbio.

We explore here our mechanistic understanding of the environmental and physiological processes that determine the oxygen isotope composition of leaf cellulose (δ$^{18}$O$_{cellulose}$) in a drought‐prone, temperate grassland ecosystem. A new allocation‐and‐growth model was designed and added to an $^{18}$O‐enabled soil–vegetation–atmosphere transfer model (MuSICA) to predict seasonal (April–October) and multi‐annual (2007–2012) variation of δ$^{18}$O$_{cellulose}$ and $^{18}$O‐enrichment of leaf cellulose (Δ$^{18}$O$_{cellulose}$) based on the Barbour–Farquhar model. Modelled δ$^{18}$O$_{cellulose}$ agreed best with observations when integrated over c. 400 growing‐degree‐days, similar to the average leaf lifespan observed at the site. Over the integration time, air temperature ranged from 7 to 22°C and midday relative humidity from 47 to 73%. Model agreement with observations of δ$^{18}$O$_{cellulose}$ (R$^{2}$ = 0.57) and Δ$^{18}$O$_{cellulose}$ (R$^{2}$ = 0.74), and their negative relationship with canopy conductance, was improved significantly when both the biochemical $^{18}$O‐fractionation between water and substrate for cellulose synthesis (ε$_{bio}$, range 26–30‰) was temperature‐sensitive, as previously reported for aquatic plants and heterotrophically grown wheat seedlings, and the proportion of oxygen in cellulose reflecting leaf water $^{18}$O‐enrichment (1 – p$_{ex}$p$_{x}$, range 0.23–0.63) was dependent on air relative humidity, as observed in independent controlled experiments with grasses. Understanding physiological information in δ$^{18}$O$_{cellulose}$ requires quantitative knowledge of climatic effects on p$_{ex}$p$_{x}$ and ε$_{bio}$.