Prediction of the effects of elevated CO2 and temperature on irrigated rice yields in the low-country dry zone of Sri Lanka using a process-based simulation model

Abstract

Atmospheric CO2 concentration has been increasing at a rate of 1.2-1.4 ppm per year and is expected to he doubled before the end of next century. As CO2 is the main substrate for photosynthesis in all plants, an increase in the ambient CO2, level would have a direct effect on hiomass production and yields of all agricultural crops. Increased concentrations of CO2 and other 'greenhouse' gases lead to greater absorption of infra-red wavelengths in the outgoing radiation causing global warming. The estimated increases of ambient temperatures with a doubling of ambient CO2., range from 1°C to 3.5°C, As all plant physiological processes are dependent on temperature, a rise in temperature would also have an impact on yield formation processes and consequently on crop yields. The present study was undertaken to predict the yield response of rice to elevated CO2 and temperature using a process-based, mechanistic simulation model. The model quantifies the effects of CO2 and temperature on crop duration, radiation interception, photosynthesis, respiration and partitioning of assimilates to grains. The model was validated by running it for the existing atmospheric conditions at Maha-Illuppallama representing the low-country dry zone of Sri Lanka. The validated model was used to predict yield variations in response to a doubling of atmospheric CO2 (i.e. from 350 to 700 ppm). The model predicted that rice yield perunit area would increase by 50% in response to the above change in CO2. In contrast, rice yield would decrease by 39-47% due to a temperature increase of 4°C. Simulation of the combined effects of elevated CO2 and temperature showed that the positive effects of elevated CO2 are almost totally negated by elevated temperatures with only slight increases (upto 17%) of yield from the current levels. The recommended remedial measures are increasing breeding efforts for producing crop varieties with greater tolerance to high temperature and drought, introduction of rice-based multiple cropping systems, increasing irrigation efficiency and promoting research to increase understanding of plant functioning under future climates.