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

ABSTRACT The 13 C isotope composition (δ 13 C) 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 δ 13 C remains uncertain. We investigated the effects of post-photosynthetic fractionation on δ 13 C of mature leaves of Cleistogenes squarrosa , a perennial C 4 grass, in controlled experiments with different levels of vapour pressure deficit and nitrogen supply. With the increase of leaf age classes, the 12 C/ 13 C fractionation of leaf organic matter relative to the δ 13 C of atmosphere CO 2 (Δ DM ) increased while that of cellulose (Δ cel ) was almost constant. The divergence between Δ DM and Δ cel increased 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 apparent 12 C/ 13 C fractionation associated with carbon export of –0.5 to –1.0‰. Different Δ DM among leaves, pseudostems, daughter tillers and roots indicate that post-photosynthetic fractionation happens at the whole-plant level. Compared with Δ DM of old leaves, Δ DM of young leaves and Δ 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 Δ 13 C of bulk organic matter increases with leaf age classes while Δ 13 C of cellulose remain constant, lending support to the use of Δ 13 C of cellulose as a more reliable proxy for predicting physiological parameters due to the smaller sensitivity to post-photosynthetic fractionation.