Leaf δ13C reveals post-photosynthetic fractionation during ontogeny in a C4 grass

The 13C isotope composition ({delta}13C) 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 {delta}13C remains uncertain. We investigated the effects of post-photosynthetic fractionation on {delta}13C of mature leaves of Cleistogenes squarrosa, a perennial C4 grass, in controlled experiments with different levels of vapour pressure deficit and nitrogen supply. With the increase of leaf age classes, the 12C/13C fractionation of leaf organic matter relative to the {delta}13C of atmosphere CO2 ({Delta}DM) increased while that of cellulose ({Delta}cel) was almost constant. The divergence between {Delta}DM and {Delta}cel increased with leaf age classes with a maximum value of 1.6{per thousand}, indicating the accumulation post-photosynthetic fractionation. Applying a new mass balance model that accounts for respiration and export of photosynthates, we found an apparent 12C/13C fractionation associated with carbon export of -0.5 to -1.0{per thousand}. Different {Delta}DM among leaves, pseudostems, daughter tillers and roots indicate that post-photosynthetic fractionation happens at the whole-plant level. Compared with {Delta}DM of old leaves, {Delta}DM of young leaves and {Delta}cel are 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 STATEMENT{Delta}13C of bulk organic matter increases with leaf age classes while {Delta}13C of cellulose remain constant, lending support to the use of {Delta}13C of cellulose as a more reliable proxy for predicting physiological parameters due to the smaller sensitivity to post-photosynthetic fractionation.