δ¹³C of bulk organic matter and cellulose reveal post-photosynthetic fractionation during ontogeny in C₄ grass leaves

The ¹³C 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 of Cleistogenes squarrosa, a perennial C₄ grass, in controlled experiments with different levels of vapour pressure deficit and nitrogen supply. With increasing leaf age class, the ¹²C/¹³C fractionation of leaf organic matter relative to the δ¹³C of atmosphere CO₂ (Δ_DM) increased while that of cellulose (Δ_cel) was almost constant. The divergence between Δ_DM and Δ_cel increased with leaf age class, with a maximum value of 1.60/100, indicating the accumulation of 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 –0.50/100 to –1.00/100. 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 lower sensitivity to post-photosynthetic fractionation and the similar sensitivity in responses to environmental changes.