Estimating intra-seasonal photosynthetic discrimination and water use efficiency using δ13C of leaf sucrose in Scots pine

Comparison of assimilate delta C-13 values estimated from different leaf carbon pools, Picarro measurements, and modelled data demonstrates the potential for misinterpretation arising from bulk organic matter delta C-13 analysis. Sucrose has a unique role in recording environmental and physiological signals during photosynthesis in its carbon isotope composition (delta C-13) and transport of the signal to tree rings. Yet, instead of sucrose, total organic matter (TOM) or water-soluble carbohydrates (WSC) are typically analysed in studies that follow delta C-13 signals within trees. To study how the choice of organic material may bias the interpretation of delta C-13 records, we used mature field-grown Scots pine (Pinus sylvestris) to compare for the first time delta C-13 of different leaf carbon pools with delta C-13 of assimilates estimated by a chamber-Picarro system (delta C-13(A_Picarro)), and a photosynthetic discrimination model (delta C-13(A_model)). Compared with sucrose, the other tested carbon pools, such as TOM and WSC, poorly recorded the seasonal trends or absolute values of delta C-13(A_Picarro) and delta C-13(A_model). Consequently, in comparison with the other carbon pools, sucrose delta C-13 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 delta C-13 of bulk organic matter can lead to misinterpretation. Our findings underscore the advantage of using sucrose delta C-13 to understand plant physiological responses in depth.