18 O enrichment of sucrose and photosynthetic and nonphotosynthetic 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 as to whether leaf water compartmentation between photosynthetic and nonphotosynthetic 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 (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 morphophysiological 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 nonphotosynthetic 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 nonphotosynthetic tissue water (Δ 18 O non-SSW ) relative to Δ 18 O SSW , driven by atmospheric conditions.