Δ13c of Bulk Organic Matter and Cellulose Reveal Post-Photosynthetic Fractionation During Ontogeny in C4 Grass Leaves

The C-13 isotope composition (delta C-13) 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 C-13 remains uncertain. We investigated the effects of post-photosynthetic fractionation on delta C-13 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 increasing leaf age class, the C-12/C-13 fractionation of leaf organic matter relative to the delta C-13 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 class, with a maximum value of 1.6 parts per thousand, 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 C-12/C-13 fractionation associated with carbon export of -0.5 parts per thousand to -1.0 parts 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 lower sensitivity to post-photosynthetic fractionation and the similar sensitivity in responses to environmental changes.