Conclusions and Future Research Needs

The ability to reliably predict effects of anticipated global climate change on rice production should provide an indispensable basis for forming a worldwide consensus to reduce greenhouse gas emission. If global climate change becomes unavoidable, reliable rice simulation models would greatly aid in the development of rice production technologies. Future food security, especially for rice-farming societies consisting of small-hectarage subsistence farmers, would be highly desirable. Over the past two decades, many research and modelling efforts have been conducted to determine the effects of [CO2], temperature and other environmental factors on rice growth and yield formation processes. Major findings from these efforts include the following:

• Long-term doubling of [CO2] promoted canopy net photosynthetic rate of rice by 30-40% and reduced specific respiration rate.

• Both increasing air temperature and CO2 enrichment accelerated rice developmental rates and reduced the number of days to heading.

• Carbon dioxide enrichment resulted in minor effects on N uptake, which appeared to be associated with relatively minor effects of [CO2] on leaf area growth of rice.

• Averaged over several different studies, a doubling of [CO2] increased rice biomass production under field conditions by about 24% over a relatively wide range of air temperatures. However, this relative enhancement was strongly influenced by soil N availability. With sufficient N fertilizer application, relative enhancement of biomass production under CO2 enrichment may reach 30% or more.

• Although a synergistic effect of [CO2] and temperature on leaf-level photosynthesis has been reported for rice, such an effect on canopy photosynthesis and biomass production is likely to be slight under field conditions.

• With a doubling of [CO2], rice WUE increased by 40-50% at optimum growth temperatures. This is mainly due to increased biomass production and partly to a reduction in transpiration. However, this effect of [CO2] on WUE sharply declined with increases in air temperature above the optimum.

• A doubling of [CO2] substantially increased the yield capacity of rice through enhanced tiller and spikelet production per unit ground area.

• The optimum daily mean temperature for rice yield was between 23 and 26°C. As air temperature deviated from this optimum, yield declined to near zero at temperatures below 15—18°C or above 36-40°C, depending on the cultivar. This effect is due mainly to spikelet sterility induced by either low or high temperatures. Rice yield increased by about 30% with CO2 enrichment at optimum temperatures and declined with increases in temperature mainly because of high-temperature-induced spikelet sterility. Furthermore, it appears that CO2 enrichment increased susceptibility to high-temperature-induced spikelet sterility.

• Rice genotypes differ in their ability to tolerate or avoid high-temperature-induced spikelet sterility at flowering and in stomatal responses to the interactive effects of [CO2] and temperature.

• Rice responses to the effects and interactions of [CO2] and temperature in terms of biomass production, WUE and yield differ considerably between pot- and field-grown plants, due mainly to difference in light interception. Therefore, special care is necessary for extrapolating experimental results from rice grown in pots to field conditions for global climate change studies.

Although a considerable amount of information has been accumulated concerning rice responses to [CO2] and temperature, more experimental studies are needed to quantify the effects and interactions of [CO2] and temperature on biomass production, stomatal conductance, WUE, spikelet sterility and final yield. Also, more detailed studies are needed to define the mechanism(s) governing the adverse effects of enriched [CO2] on high-temperature-induced spikelet sterility. The genetic differences among rice cultivars in response to [CO2] and temperature, especially in the areas of crop water use and high-temperature-induced spikelet sterility, need to be determined. This information is required to identify rice genotypes that are better adapted to potential future global climate changes.

The integration of information into rice models is indispensable for impact assessments of global climate change on regional rice production. Simulation studies based on existing rice models suggest that, while future climate change has the potential to increase irrigated rice yield in northern and tropical Asia, yield reductions are also likely in some inland areas of subtropical Asia, especially in rice produced in the dry season.

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