Altered rice nutritional quality resulting from environmental change: An investigation into the implications of temperature, phenology, and flooding on rice metal and metalloid accumulation

Abstract

Rice (Oryza sativa) is one of the world’s most important food crops, serving as a key dietary staple to over half of the world’s population. Rice is especially important in many low-income countries, where micronutrient deficiencies are often widespread and there is less access to alternative nutrient dense foods. At the same time, rice is also a main dietary source of arsenic, a toxin and carcinogen. These underlying dietary burdens may be exasperated by alerted climatic and hydrologic factors which influence soil chemistry and plant ecophysiology. In this study, I investigate how elevated temperature impacts rice arsenic uptake and how seasonal flooding influences mineral dynamics in the Tonle Sap floodplain of Cambodia. In Chapter 2, we established four temperature treatments, increasing 2.5°C between treatments, in climate-controlled growth chambers where we grew potted rice plants (Oryza sativa cv. M206). We observed that arsenic concentrations in porewater, root iron plaque, and plant tissue increased in response to elevated temperature. There was a positive linear relationship between temperature and rice grain arsenic. Rice plants grown at higher temperatures had more adsorbed arsenic per unit of iron plaque (measured as [As]/[Fe]), indicating temperature may impact arsenic sorption to root plaque. We present evidence that increased soil mobilization of arsenic was the driving factor responsible for increased arsenic uptake into rice grain. Our soils had low soil arsenic concentrations and our findings indicated that elevated temperatures may increase dietary arsenic exposure in rice systems that were previously considered low risk. In Chapter 3, we expand on the previous chapter by delving into the impact of timing of temperature exposure during specific developmental windows. We grew potted rice plants of the same variety in temperature-controlled growth chambers with different temperature regimes: (1) baseline temperature, (2) elevated temperature, (3) baseline temperature with a heat spike in vegetative stage (4) baseline temperature with a heat spike in the ripening stage. We observed that heat spikes in the ripening stage greatly increased arsenic mobilization from soil to porewater, but heat spikes in the vegetative stage did not. Plants which experienced a vegetative heat spike had increased arsenic concentrations in plant tissue immediately following the heat spike, but the magnitude of this increase was small and did not persist to maturity. Plants which experienced a heat spike during the ripening stage had a significant increase in arsenic concentrations in various plant tissues which persisted to maturity. We present several likely explanations for the discrepancy between the two timepoints, including differences in microbial mobilization, plant biomass, and root development. Our research shows that heat spikes later in the rice growing season present a larger threat for dietary arsenic exposure from rice. In Chapter 4, I widen the scope of my work to also include mineral nutrients, specifically zinc and iron, as well as toxic arsenic. In this two-year field study, we investigated the movement of these metals (+ metalloid) in the Tonle Sap floodplain, an important area for rice cultivation in Cambodia. We collected surface soil, flood sediment, and dry-season plant tissue from 12 rice fields along a flooding gradient in the Steung Saen Municipality of Kampong Thom Province. We found that sites with greater flooding had higher concentrations of total metals but a lower fraction of plant available metals. Flood sediment had consistently greater concentrations of all three metals, indicating that flood sediment may act as a natural source of both nutrients and toxins. Concentrations of these elements were variable in rice tissue, though we found seasonal trends in zinc concentrations. Finally, we found relatively high concentrations of arsenic in rice grains given that Kampong Thom is often overlooked in studies on rice arsenic in Cambodia. These observations as well as additional in-depth studies are needed to understand how rice agriculture and rice nutritional quality might change with the anticipated alterations in flooding due to dam construction.