While previous studies have examined the growth and yield response of rice to continued increases in CO2 concentration and potential increases in air temperature, little work has focused on the long‐term response of tropical paddy rice (i.e. the bulk of world rice production) in situ, or genotypic differences among cultivars in response to increasing CO2 and/or temperature. At the International Rice Research Institute, rice (cv IR72) was grown from germination until maturity for 4 field seasons, the 1994 and 1995 wet and the 1995 and 1996 dry seasons at three different CO2 concentrations (ambient, ambient + 200 and ambient + 300 μL L–1 CO2) and two air temperatures (ambient and ambient + 4 °C) using open‐top field chambers placed within a paddy site. Overall, enhanced levels of CO2 alone resulted in significant increases in total biomass at maturity and increased seed yield with the relative degree of enhancement consistent over growing seasons across both temperatures. Enhanced levels of temperature alone resulted in decreases or no change in total biomass and decreased seed yield at maturity across both CO2 levels. In general, simultaneous increases in air temperature as well as CO2 concentration offset the stimulation of biomass and grain yield compared to the effect of CO2 concentration alone. For either the 1995 wet and 1996 dry seasons, additional cultivars (N‐22, NPT1 and NPT2) were grown in conjunction with IR72 at the same CO2 and temperature treatments. Among the cultivars tested, N‐22 showed the greatest relative response of both yield and biomass to increasing CO2, while NPT2 showed no response and IR72 was intermediate. For all cultivars, however, the combination of increasing CO2 concentration and air temperature resulted in reduced grain yield and declining harvest index compared to increased CO2 alone. Data from these experiments indicate that (a) rice growth and yield can respond positively under tropical paddy conditions to elevated CO2, but that simultaneous exposure to elevated temperature may negate the CO2 response to grain yield; and, (b) sufficient intraspecific variation exists among cultivars for future selection of rice cultivars which may, potentially, convert greater amounts of CO2 into harvestable yield.

Recent anthropogenic emissions of key atmospheric trace gases (e.g. CO2 and CH4) which absorb infra‐red radiation may lead to an increase in mean surface temperatures and potential changes in climate. Although sources of each gas have been evaluated independently, little attention has focused on potential interactions between gases which could influence emission rates. In the current experiment, the effect of enhanced CO2 (300 μL L–1 above ambient) and/or air temperature (4 °C above ambient) on methane generation and emission were determined for the irrigated tropical paddy rice system over 3 consecutive field seasons (1995 wet and dry seasons 1996 dry season). For all three seasons, elevated CO2 concentration resulted in a significant increase in dissolved soil methane relative to the ambient control. Consistent with the observed increases in soil methane, measurements of methane flux per unit surface area during the 1995 wet and 1996 dry seasons also showed a significant increase at elevated carbon dioxide concentration relative to the ambient CO2 condition (+49 and 60% for each season, respectively). Growth of rice at both increasing CO2 concentration and air temperature did not result in additional stimulation of either dissolved or emitted methane compared to growth at elevated CO2 alone. The observed increase in methane emissions were associated with a large, consistent, CO2‐induced stimulation of root growth. Results from this experiment suggest that as atmospheric CO2 concentration increases, methane emissions from tropical paddy rice could increase above current projections.

Rice (Oryza sativa L.), a semiaquatic species, is best adapted to culture in a submerged soil. Rice grown in rain fed environments may suffer significant yield losses due to drought. At the International Rice Research Institute, Philippines, thousands of genotypes are screened annually for drought resistance in upland fields during the dry season. We monitored the leaf water potential of 17 diverse genotypes to investigate the physiological basis for genotypic differences obtained through visual scoring of plant symptoms. The genotypes were grown in small plots, established by sprinkler irrigation. Twenty‐two ays after emergence, irrigation was stopped and a stress period initiated. Thirty‐nine days later, differentiation of genotypes was considered optimal by visual scoring and intensive diurnal sampling of leaf water potential began. Significant differences were detected among the genotypes for maintenance of relatively high leaf water potential. Two visual scoring techniques, one based on leaf rolling and the other on leaf tip drying, were highly correlated with maintenance of leaf water potential. Visual scoring based on leaf rolling behavior appeared to be influenced by diurnal adjustment of the pressure potential. The integral for the day (—bar.day) was quite effective in separation of cultivars for maintenance of high leaf water potential throughout the day and highly correlated with ranking by visual scoring.

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The aim of this paper was to characterize the southwest monsoon onset over Myanmar based on the model. The Regional Climate Model (RegCM3) was run for a period of 10 years (