Biochemical effects of salinity on oxygen isotope fractionation during cellulose synthesis

The current isotope tree ring model assumes that 42% of the sucrose oxygen exchanges with stem water during cellulose synthesis and that the oxygen isotope biochemical fractionation is c. 27‰. However, previous studies have indicated that this model can overestimate the cellulose oxygen isotope ratio of plants under salinity or water stress. Saline stress increases soluble carbohydrates and osmolytes, which can alter exchange and biochemical fractionation during cellulose synthesis. To test the effect of salinity as well as the synthesis of osmolytes on exchange and biochemical fractionation, we grew wild-type and a transgenic mannitol synthesizer Arabidopsis thaliana hydroponically with fresh and saline water. We then measured the oxygen isotope ratios of leaf water, stem water and stem cellulose to determine the effects on exchange and biochemical fractionation. Biochemical fractionation did not change, but oxygen isotope exchange was twice as high for plants grown in saline water relative to freshwater-treated plants (0.64 and 0.3, respectively). Mannitol (osmolyte) synthesis did not affect exchange or biochemical fractionation regardless of salinity. Increases in salinity increased oxygen isotope exchange during cellulose synthesis, which may explain the overestimation of cellulose δ¹⁸O values under saline conditions.