Factors Contributing to Increased Arsenic in Rice Under Warmer Temperatures

Recent studies indicate that warmer temperatures can increase concentrations of arsenic in rice. Arsenic is a toxin and carcinogen commonly found in rice paddy soil. This finding presents a looming public health crisis as climate change is expected to increase average temperatures in rice growing regions around the world. Rice is presently a major source of dietary arsenic around the world, and we can only expect this threat to worsen in a warming future. Understanding the mechanisms surrounding the temperature-fueled increase in rice arsenic is crucial to prepare for, and potentially mitigate, increases in rice arsenic. To explore these mechanisms, we performed two separate experiments using rice plants (Oryza sativa cv. M206) in pots filled with low arsenic Californian paddy soil in climate-controlled growth chambers. In the first experiment, we applied a uniform temperature from germination to full-maturity. We utilized 4 temperature treatments spanning a range of IPCC forecasts for Northern California, an important location for rice cultivation in the United States. We analyzed arsenic concentrations in various rice plant tissue types and root plaque. We utilized a simple mass-balance calculation to characterize the importance of temperature-fueled mobilization and transpiration, and compare these values to observed arsenic accumulation. This calculation highlights the importance of temperature fueled mobilization of arsenic from soil into porewater. In the second experiment, we exposed rice plants to a 20-day heat spike ~3.5°C above baseline temperatures during either vegetative or ripening stages of development. We find a more muted response to shorter temperature spikes compared to a longer temperature exposure, in various tissues including in edible rice grains. A detectable arsenic increase was present in some vegetative tissues following a short heat spike event. These results indicate that isolated, sporadic heat waves may not dramatically increase arsenic concentrations in low soil arsenic systems. Further work is needed to understand if high soil arsenic systems respond differently.