18 O enrichment of sucrose and photosynthetic and non-photosynthetic leaf water in a C 3 grass -- atmospheric drivers and physiological relations

The 18 O enrichment (Δ 18 O) of leaf water affects the Δ 18 O of photosynthetic products such as sucrose, generating an isotopic archive of plant function and past climate. However, uncertainty remains regarding how leaf water compartmentation between photosynthetic and non-photosynthetic tissue affects the relationship between Δ 18 O of bulk leaf water (Δ 18 O LW ) and leaf sucrose (Δ 18 O Sucrose ). We grew Lolium perenne (a C 3 grass) in mesocosm-scale, replicated experiments with daytime relative humidity (RH 50 or 75%) and CO 2 level (200, 400 or 800 μmol mol -1 ) as factors, and determined Δ 18 O LW , Δ 18 O Sucrose and morpho-physiological leaf parameters, including transpiration ( E leaf ), stomatal conductance ( g s ) and mesophyll conductance to CO 2 ( g m ). The Δ 18 O of photosynthetic medium water (Δ 18 O SSW ) was estimated from Δ 18 O Sucrose and the equilibrium fractionation between water and carbonyl groups (ε bio ). Δ 18 O SSW was well predicted by theoretical estimates of leaf water at the evaporative site (Δ 18 O e ) with adjustments that correlated with gas exchange parameters ( g s or total conductance to CO 2 ). Isotopic mass balance and published work indicated that non-photosynthetic tissue water was a large fraction (~0.53) of bulk leaf water. Δ 18 O LW was a poor proxy for Δ 18 O Sucrose , mainly due to opposite Δ 18 O responses of non-photosynthetic tissue water (Δ 18 O non-SSW ) relative to Δ 18 O SSW , driven by atmospheric conditions.