Leaf δ¹³C reveals post-photosynthetic fractionation during ontogeny in a C₄grass

Abstract

The13C isotope composition (δ13C) of leaf dry matter is a useful tool for physiological and ecological studies. However, how post-photosynthetic fractionation associated with respiration and carbon export influences δ13C remains uncertain. We investigated the effects of post-photosynthetic fractionation on δ13C of mature leaves ofCleistogenes squarrosa, a perennial C4grass, in controlled experiments with different levels of vapour pressure deficit and nitrogen supply. With the increase of leaf age classes, the12C/13C fractionation of leaf organic matter relative to the δ13C of atmosphere CO2(ΔDM) increased while that of cellulose (Δcel) was almost constant. The divergence between ΔDMand Δcelincreased with leaf age classes with a maximum value of 1.6‰, indicating the accumulation post-photosynthetic fractionation. Applying a new mass balance model that accounts for respiration and export of photosynthates, we found an apparent12C/13C fractionation associated with carbon export of –0.5 to –1.0‰. Different ΔDMamong leaves, pseudostems, daughter tillers and roots indicate that post-photosynthetic fractionation happens at the whole-plant level. Compared with ΔDMof old leaves, ΔDMof young leaves and Δcelare more reliable proxies for predicting physiological parameters due to the smaller sensitivity to post-photosynthetic fractionation and the similar sensitivity in responses to environmental changes.BRIEF SUMMARY STATEMENTΔ13C of bulk organic matter increases with leaf age classes while Δ13C of cellulose remain constant, lending support to the use of Δ13C of cellulose as a more reliable proxy for predicting physiological parameters due to the smaller sensitivity to post-photosynthetic fractionation.