Grass blades as tree rings: environmentally induced changes in the oxygen isotope ratio of cellulose along the length of grass blades

In this study, we tested the hypothesis that environmentally induced changes in the oxygen isotope ratio of leaf water are recorded in grass blade cellulose during leaf-blade expansion. Grasses were grown hydroponically in chambers that allowed for control of relative humidity while keeping isotopic inputs (namely source water) constant. In experiments where relative humidity was changed from 35% to 93% during grass blade expansion, a 10parts per thousand shift in cellulose delta(18)O was observed along single grass blades of Lolium multiflorum. However, statistically significant changes were not detectable with relative humidity of 93% to 70%. It is shown that grass blades, analogously to tree rings, record environmental change on an interseasonal basis. In light of this, care must be taken to compare leaves of the same developmental stage to avoid confusion of environmental effects with physiological effects in interpretations of leaf organic material delta(18)O. The results presented here confirm the ability of the current empirical models to predict the oxygen isotope ratio of cellulose in both grass blades that expanded in constant and variable growth conditions.