Turnover time of the non‐structural carbohydrate pool influences δ18O of leaf cellulose

Certainty regarding the degree to which organic molecules exchange oxygen with local water during plant cellulose synthesis (p_ex) is necessary for cellulose oxygen isotope (δ¹⁸O_cell)‐based applications in environmental and ecological studies. However, the currently accepted notion that p_ex is a constant of ca. 0.42 appears inconsistent with biochemical theory, which predicts that marked variation may be present in p_ex, in relation to variation in the turnover time (τ) of the carbohydrate pool available for cellulose synthesis. The above prediction was tested in the present study with the analysis of data collected from leaves of Ricinus communis grown in controlled environmental conditions that varied in light intensity and vapour pressure deficit. The results revealed the existence of considerable variation in both p_ex and τ across plants in the various growth environments. Moreover, despite uncertainties in estimates of the proportion of source water in the synthesis water (p_x) and of the biochemical fractionation factor (ε_o), our experiment yielded strong evidence that p_ex exhibits a significant, positive relationship with τ, consistent with biochemical theory. The observed variation in p_ex in association with τ has important implications for the interpretation of δ¹⁸O_cell data in environmental/ecological studies.