Spatial variation in photosynthetic CO2 carbon and oxygen isotope discrimination along leaves of the monocot triticale (Triticum x Secale) relates to mesophyll conductance and the Peclet effect

Carbon and oxygen isotope discrimination of CO2 during photosynthesis (Delta C-13(obs) and Delta O-18(obs)) were measured along a monocot leaf, triticale (Triticum x Secale). Both Delta C-13(obs) and Delta O-18(obs) increased towards the leaf tip. While this was expected for Delta O-18(obs), because of progressive enrichment of leaf water associated with the Peclet effect, the result was surprising for Delta C-13(obs). To explore parameters determining this pattern, we measured activities of key photosynthetic enzymes [ribulose bis-phosphate carboxylase-oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPC) and carbonic anhydrase) as well as maximum carboxylation and electron transport rates (V-cmax and J(max)) along the leaf. Patterns in leaf internal anatomy along the leaf were also quantified. Mesophyll conductance (g(m)) is known to have a strong influence on Delta C-13(obs), so we used three commonly used estimation methods to quantify variation in g(m) along the leaf. Variation in Delta C-13(obs) was correlated with g(m) and chloroplast surface area facing the intercellular air space, but unrelated to photosynthetic enzyme activity. The observed variation could cause errors at higher scales if the appropriate portion of a leaf is not chosen for leaf-level measurements and model parameterization. Our study shows that one-third of the way from the base of the leaf represents the most appropriate portion to enclose in the leaf chamber.