Oxygen isotope enrichment in rice (Oryza sativa L.) grain organic matter captures signature of relative humidity.

The oxygen isotopic composition (δ18 O) of plant organic matter (OM) is primarily governed by the δ18 O of source water (δ18 OSW ) and climatic factor of relative humidity (RH). Among the cereals, the growth of rice plants is critically dependent on the water availability in the growth-environment. In the present study, we investigated the sensitivity of δ18 O in the bulk organic matter of rice grains to RH of their growth-environment. Our experimental setup consisted of both glasshouse and field experiments, where eight genotypes were grown at RH levels ranging from 67% to 87%. The δ18 O measured in bulk grain OM and source water was used to calculate the net oxygen isotopic enrichment (Δ18 OOM ). Regression analysis of Δ18 OOM with RH demonstrated a significant relationship (r2  = 0.96; p < 0.0001), thereby implying that the isotopic signature of evaporative conditions gets recorded in the rice grain OM. In addition, our study involved a separate experiment that monitored the degree of oxygen isotope enrichment in water samples extracted from different parts of the rice plant. For this purpose, we sampled four of the above eight genotypes along with three other rice genotypes that were grown in both open cultivation fields and glasshouse. Water present in the culms, leaves, and grains were extracted quantitatively. Isotopic analyses revealed progressive 18 O enrichment of the water in the culms and leaves and intermediate enrichment values of that in the grains. Based on the isotope data, we validated mechanistic models for prediction of δ18 O of the leaf water and that of the plant carbohydrates. The model predictions were in close agreement with the experimental observations. The study provides insights into the rice plant's oxygen isotope systematics that build the foundation for future applications of the stable isotope technique to study the interactions between rice and environment.