Abstract. There is some evidence that rice cultivars respond differently to elevated CO 2 concentrations ([CO 2 ]), but [CO 2 ] Â cultivar interaction has never been tested under open-field conditions across different sites. Here, we report on trials conducted at free-air CO 2 enrichment (FACE) facilities at two sites in Japan, Shizukuishi (2007 and2008) and Tsukuba (2010). The average growing-season air temperature was more than 5 C warmer at Tsukuba than at Shizukuishi. For four cultivars tested at both sites, the [CO 2 ] Â cultivar interaction was significant for brown rice yield, but there was no significant interaction with site-year. Higher-yielding cultivars with a large sink size showed a greater [CO 2 ] response. The Tsukuba FACE experiment, which included eight cultivars, revealed a wider range of yield enhancement (3-36%) than the multi-site experiment. All of the tested yield components contributed to this enhancement, but there was a highly significant [CO 2 ] Â cultivar interaction for percentage of ripened spikelets. These results suggest that a large sink is a prerequisite for higher productivity under elevated [CO 2 ], but that improving carbon allocation by increasing grain setting may also be a practical way of increasing the yield response to elevated [CO 2 ].

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Projected global warming is expected to increase the occurrence of heat-induced spikelet sterility (HISS) of rice (Oryza sativa L.). Previous chamber experiments have shown that HISS can occur where temperature at flowering time exceeds the threshold temperature of around 35 . The occurrence of HISS is, however, difficult to predict because the thermal conditions of rice canopy can be different from the air temperature under field condition. To cope with this, we developed a simple micrometeorology model focusing canopy and panicle temperatures; IM PACT was applied to the meteorology dataset based on ANEMOS in order to analyse the Tp at Kanto and Tokai regions of extremely hot summer in 2007. There was a great gap in spatial distributions between the Tp and the daily maximum air temperature which is commonly used as a measure of HISS, because the difference of meteorology, especially relative humidity, among areas altered the panicle-air temperature difference. This strongly suggests that we must refer to the Tp instead of the air temperature in daily maximum, as a measure variable for HISS. The IM 2 PACT is a powerful tool to elucidate the Tp in the climate change impact study to bridge between the responses of crop susceptible to heat and the meteorological data.

Rising atmospheric CO2 concentrations will probably increase rice (Oryza sativa L.) yield but decrease grain nitrogen (GN) concentration. Grains attached to different positions in the panicles differ greatly in weight and quality, but their responses to elevated CO2 (e[CO2]) are poorly understood, which limits our understanding of the mechanisms of yield enhancement and quality degradation. Thus a free-air CO2 enrichment experiment was conducted to examine the effects of e[CO2] on grain mass (GM), grain carbon (GC), and GN accumulation in the spikelets attached to the upper primary rachis branch (superior spikelets; SS) and those attached to the lower secondary rachis (inferior spikelets; IS). e[CO2] stimulated the rice yield by 13% but decreased the N concentration in the panicle by 7% when averaged over two levels of N fertilizations (P < 0.01). The responses of SS and IS to e[CO2] were different particularly under higher N supply. For SS, e[CO2] decreased GN by 24% (P < 0.01) but did not affect GM. For IS, e[CO2] increased GM by 13% (P < 0.05) but GN was not affected. The reduction of GN due to e[CO2] started to appear at the beginning of grain filling. These results suggest that future [CO2] levels probably stimulate the grain growth of IS, most of which are not marketable due to limited size, at the expense of GN reduction in SS. Translocation of N from SS to IS may be a possible mechanism for reduction in GN of SS. This may degrade the grain quality of marketable rice under e[CO2].

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