Nitrogen fertilization and δ O of CO have no effect on O-enrichment of leaf water and cellulose in Cleistogenes squarrosa (C ) - is VPD the sole control?

The oxygen isotope composition of cellulose (δ O ) archives hydrological and physiological information. Here, we assess previously unexplored direct and interactive effects of the δ O of CO (δ O ), nitrogen (N) fertilizer supply and vapour pressure deficit (VPD) on δ O , O-enrichment of leaf water (Δ O ) and cellulose (Δ O ) relative to source water, and p p , the proportion of oxygen in cellulose that exchanged with unenriched water at the site of cellulose synthesis, in a C grass (Cleistogenes squarrosa). δ O and N supply, and their interactions with VPD, had no effect on δ O , Δ O , Δ O and p p . Δ O and Δ O increased with VPD, while p p decreased. That VPD-effect on p p was supported by sensitivity tests to variation of Δ O and the equilibrium fractionation factor between carbonyl oxygen and water. N supply altered growth and morphological features, but not O relations; conversely, VPD had no effect on growth or morphology, but controlled O relations. The work implies that reconstructions of VPD from Δ O would overestimate amplitudes of VPD variation, at least in this species, if the VPD-effect on p p is ignored. Progress in understanding the relationship between Δ O and Δ O will require separate investigations of p and p and of their responses to environmental conditions.