Understanding the Mechanisms Associated with Elevated Temperature and Increased Arsenic in Rice

Abstract:

There is growing appreciation that the threat of climate change to global food security must consider food nutritional quality. A key component of nutritional quality is the concentration of toxins that foods contain. For rice —a staple food crop for roughly half the world’s population — arsenic is a ubiquitous carcinogenic contaminant. At present, rice is a major global contributor to dietary arsenic exposure and therefore represents an important threat to food quality and human health. Exacerbating this existing threat is the recent finding that elevated temperature can increase arsenic in rice plant tissue. Further investigation into the temperature-fueled changes in the plant-soil system responsible for the increased arsenic concentrations is critical to inform mitigation efforts in a warmer future.

We grew potted rice plants (Oryza sativa cv. M206) in climate-controlled growth chambers using Californian paddy soil with low baseline arsenic concentrations. We cultivated plants under different elevated temperature conditions relevant to the IPCC temperature forecast for Northern California, an important rice growing region within the state. Elevated temperature increased arsenic concentrations in porewater, root iron plaque, and plant tissue, including edible rice grains. Raising the growing temperature by 5°C increased inorganic arsenic levels from very low to well beyond the safety thresholds used for infants, illustrating that rice regions previously considered safe may become of greater concern in a warmer future. Total arsenic content (arsenic concentration × biomass) also increased with temperature, while allocation patterns were comparable between treatments. Elevated temperature increased both transpirational rate and transpired water volume, though a simple mass balance calculation indicated that increased transpiration was not the dominant factor leading to increased arsenic content in plant tissue. Rather, temperature-fueled mobilization of arsenic from soil explained a larger portion of the increased arsenic content in plant tissue. This calculation suggests that mitigation measures should focus on reducing arsenic bioavailability to rice plants (ex. silicon amendments or alternate wetting and drying).