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

The O-18 enrichment (Delta O-18) of leaf water affects the Delta O-18 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 Delta O-18 of bulk leaf water (Delta O-18(LW)) and leaf sucrose (Delta O-18(Sucrose)). We grew Lolium perenne (a C-3 grass) in mesocosm-scale, replicated experiments with daytime relative humidity (50% or 75%) and CO2 level (200, 400 or 800 mu mol mol(-1)) as factors, and determined Delta O-18(LW), Delta O-18(Sucrose) and morphophysiological leaf parameters, including transpiration (E-leaf), stomatal conductance (g(s)) and mesophyll conductance to CO2 (g(m)). The Delta O-18 of photosynthetic medium water (& UDelta;O-18(SSW)) was estimated from Delta O-18(Sucrose) and the equilibrium fractionation between water and carbonyl groups (epsilon(bio)). Delta O-18(SSW) was well predicted by theoretical estimates of leaf water at the evaporative site Delta O-18(e)) with adjustments that correlated with gas exchange parameters (g(s) or total conductance to CO2). Isotopic mass balance and published work indicated that nonphotosynthetic tissue water was a large fraction (similar to 0.53) of bulk leaf water. Delta O-18(LW) was a poor proxy for Delta O-18(Sucrose), mainly due to opposite Delta O-18 responses of nonphotosynthetic tissue water (Delta O-18(non-SSW)) relative to Delta O-18(SSW), driven by atmospheric conditions.