Half of the O enrichment of leaf sucrose is conserved in leaf cellulose of a C grass across atmospheric humidity and CO levels.

The O enrichment (ΔO) of cellulose (ΔO) is recognized as a unique archive of past climate and plant function. However, there is still uncertainty regarding the proportion of oxygen in cellulose (p) that exchanges post-photosynthetically with medium water of cellulose synthesis. Particularly, recent research with C grasses demonstrated that the ΔO of leaf sucrose (ΔO, the parent substrate for cellulose synthesis) can be much higher than predicted from daytime ΔO of leaf water (ΔO), which could alter conclusions on photosynthetic versus post-photosynthetic effects on ΔO via p. Here, we assessed p in leaves of perennial ryegrass (Lolium perenne) grown at different atmospheric relative humidity (RH) and CO levels, by determinations of ΔO in leaves, ΔO (the ΔO of water in the leaf growth-and-differentiation zone) and both ΔO and ΔO (adjusted for ε, the biosynthetic fractionation between water and carbohydrates) as alternative proxies for the substrate for cellulose synthesis. ΔO was always close to irrigation water, and p was similar (0.53 ± 0.02 SE) across environments when determinations were based on ΔO. Conversely, p was erroneously and variably underestimated (range 0.02-0.44) when based on ΔO. The photosynthetic signal fraction in ΔO is much more constant than hitherto assumed, encouraging leaf physiological reconstructions.