Paddy Rice Responses to Free-Air CO₂ Enrichment

抄録

　We conducted free-air CO₂ enrichment of rice growing in farmers’ paddies in Shizukuishi, Iwate, Japan. The three year experiment (1998 to 2000) showed that the CO₂ enrichment of 220-295 ppmV above ambient CO₂ concentration [CO₂] increased the rice yield by 7, 14, and 15 % with nitrogen (N) fertilizer applied at low, stand-ard, and high levels, respectively. With little changes in harvest index, the yield increase was nearly proportional to the increase in plant biomass. This may lead one to hypothesize a simple source-driven mechanism: the yield gain by the CO₂ enrichment is caused by the photosynthetic enhancement, which is limited by low N availability. Nonetheless, a detailed analysis of the yield components, plant biomass and N accumulation has led us to a sink-mediated mechanism for the interaction between N and grain yield in response to elevated [CO₂]. They are linked together by the close relationship between the N accumulation through to panicle initiation (PI) and the number of grains at harvest. The elevated CO₂ stimulated N uptake through to PI with standard and high N levels, but not with low N, and resulted in a [CO₂] x N interaction for the number of grains, which dominated the grain yield response to elevated [CO₂].