GROWTH DYNAMICS AND GENOTYPIC VARIATION IN TROPICAL, FIELD-GROWN PADDY RICE (ORYZA-SATIVA L.) IN RESPONSE TO INCREASING CARBON-DIOXIDE AND TEMPERATURE

While previous studies have examined the growth and yield response of rice to continued increases in CO2 concentration and potential increas es in air temperature, little work has focused on the long-term respon se of tropical paddy rice (i.e. the bulk of world rice production) in situ, or genotypic differences among cultivars in response to increasi ng CO2 and/or temperature. At the International Rice Research Institut e, rice (cv IR72) was grown from germination until maturity for 4 fiel d seasons, the 1994 and 1995 wet and the 1995 and 1996 dry seasons at three different CO2 concentrations (ambient, ambient + 200 and ambient + 300 mu L L-1 CO2) and two air temperatures (ambient and ambient + 4 degrees C) using open-top field chambers placed within a paddy site. Overall, enhanced levels of CO2 alone resulted in significant increase s in total biomass at maturity and increased seed yield with the relat ive degree of enhancement consistent over growing seasons across both temperatures. Enhanced levels of temperature alone resulted in decreas es or no change in total biomass and decreased seed yield at maturity across both CO2 levels. In general, simultaneous increases in air temp erature as well as CO2 concentration offset the stimulation of biomass and grain yield compared to the effect of CO2 concentration alone. Fo r 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 a nd temperature treatments. Among the cultivars tested, N-22 showed the greatest relative response of both yield and biomass to increasing CO 2, while NPT2 showed no response and IR72 was intermediate. For all cu ltivars, 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 ind icate that (a) rice growth and yield can respond positively under trop ical paddy conditions to elevated CO2, but that simultaneous exposure to elevated temperature may negate the CO2 response to grain yield; an d, (b) sufficient intraspecific variation exists among cultivars for f uture selection of rice cultivars which may, potentially, convert grea ter amounts of CO2 into harvestable yield.