Nitrogen fertilization and 18O of CO2 have no effect on 18O-enrichment of leaf water and cellulose in Cleistogenes squarrosa (C4) - is VPD the sole control?

The oxygen isotope composition of cellulose (O-18(Cel)) archives hydrological and physiological information. Here, we assess previously unexplored direct and interactive effects of the O-18 of CO2 (O-18(CO2)), nitrogen (N) fertilizer supply and vapour pressure deficit (VPD) on O-18(Cel), O-18-enrichment of leaf water (O-18(LW)) and cellulose (O-18(Cel)) relative to source water, and p(ex)p(x), the proportion of oxygen in cellulose that exchanged with unenriched water at the site of cellulose synthesis, in a C-4 grass (Cleistogenes squarrosa). O-18(CO2) and N supply, and their interactions with VPD, had no effect on O-18(Cel), O-18(LW), O-18(Cel) and p(ex)p(x). O-18(Cel) and O-18(LW) increased with VPD, while p(ex)p(x) decreased. That VPD-effect on p(ex)p(x) was supported by sensitivity tests to variation of O-18(LW) and the equilibrium fractionation factor between carbonyl oxygen and water. N supply altered growth and morphological features, but not O-18 relations; conversely, VPD had no effect on growth or morphology, but controlled O-18 relations. The work implies that reconstructions of VPD from O-18(Cel) would overestimate amplitudes of VPD variation, at least in this species, if the VPD-effect on p(ex)p(x) is ignored. Progress in understanding the relationship between O-18(LW) and O-18(Cel) will require separate investigations of p(ex) and p(x) and of their responses to environmental conditions.