Estimating intra-seasonal photosynthetic discrimination and water use efficiency using δ 13C of leaf sucrose in Scots pine.

Sucrose has a unique role in recording environmental and physiological signals during photosynthesis in its carbon isotope composition (δ 13C) and transporting the signal to tree rings. Yet, instead of sucrose, total organic matter (TOM) or water-soluble carbohydrates (WSC) are typically analyzed in studies that follow δ 13C signals within trees. To study how the choice of organic material may bias our interpretation of the δ 13C records, we used mature field-grown Scots pine (Pinus sylvestris) to compare for the first time δ 13C of different leaf carbon pools with δ 13C of assimilates estimated by a chamber-Picarro system (δ 13CA_Picarro) and a photosynthetic discrimination model (δ 13CA_model). Compared to sucrose, the other tested carbon pools, such as TOM and WSC, poorly recorded the seasonal trends or absolute values of δ 13CA_Picarro and δ 13CA_model. Consequently, in comparison to the other carbon pools, sucrose δ 13C was superior for reconstructing changes in intrinsic water use efficiency (iWUE), agreeing in both absolute values and intra-seasonal variations with iWUE estimated from gas exchange. Thus, deriving iWUE and environmental signals from δ 13C of bulk organic matter can lead to misinterpretation and our findings underscore the advantage of using sucrose δ 13C to understand plant physiological responses in depth.