Climate warming has the potential to deteriorate grain yield and quality of rice (Oryza sativa L.), offsetting the stimulative effects of elevating CO2. To know how the change in sink-source balances by reducing sink-size (RSS) may affect grain yield and quality of rice grown under various climate change scenarios, we conducted a temperature gradient chamber experiment with/without CO2 fumigation systems which were established in paddy field. Rice crops (cv. Ilmybyeo) were exposed to either ambient (396ppmV) or elevated CO2 of 673ppmV in three levels of air temperature [(Ta), local ambient Ta (24.8℃), 1.3℃ and 2.4℃ above ambient Ta] over whole seasons. Thus, the experiment was a 2×3 factorial design with three replicate plots of each CO2×Ta combination. At flowering, for two hills from each combination treatment total thirty (10 per each top, middle and basal parts of panicle) spikelets per panicle were removed with order of panicle appearance by scissors. This corresponded to a 25% reduction of total sink-size per hill. In ambient Ta and CO2 , grain yield decreased with RSS by 23.4%, approximately mirroring the reduced sink-size. With rising Ta, however, the yield reduction by RSS was significantly mitigated (-5.6% in 1.3℃ above ambient Ta), and the yield rather increased with RSS by 9.3% in 2.4℃ above ambient Ta. This was due primarily to the increased single grain mass with RSS. A similar response fashion of grain mass and yield with RSS to Ta was found in elevated CO2, but not CO2×Ta interaction. For brown rice, the fraction of normal rice was linearly reduced with rising Ta, ranging from 78.5～79.2% in local ambient Ta to 48.2～55.5% in 2.4℃ above ambient Ta over CO2 treatments. However, this deteriorative effect of rising Ta was significantly alleviated with RSS; the fractions of normal rice were a 81.9～84.1%, 75.9～77.2% and 64.0～66.3% in local ambient Ta, 1.3℃ and 2.4℃ above ambient Ta, respectively. The alleviative effect of RSS on rice quality was due mainly to the reduced immature rice, and was more conspicuous as Ta rises. These results suggest that current rice cultivars in Korea, at least cultivars tested in this experiment, will likely to be prone to source-limitation in the future projected warming with elevating CO2, and thereby will be needed a cultivar having either a greater source ability or a less sink size compared with current cultivars, in order to ensure a rice quality in the future warming conditions.

To know how interacting climate drivers may affect rice quality, we investigated physio-chemical properties of brown and milled rice. Rice crops (Oryza sativa L., cv. Ilmybyeo and Pyounganbyeo) were grown under either ambient [370ppmV (2008)/396ppmV (2009)] or elevated CO2 of 650ppmV (2008)/673ppmV (2009) in three levels of air temperature [(Ta), local ambient Ta [25.9℃ (2008)/24.8℃ (2009)], 1.3℃ and 2.4℃ above ambient Ta] over whole seasons, using six temperature gradient chambers established in paddy fields. Over 2 years, thus the experiments were a 2×3 factorial design with three replicate plots of each CO2×Ta combination. The fractions of normal brown rice were reduced with elevating CO2 by 8% (Ilmybyeo)～14% (Pyounganbyeo), and with rising Ta by 16% (+1.3 ℃)～27% (+2.4℃) in Ilmybyeo and by 27% (+1.3℃)～42% (+2.4℃) in Pyounganbyeo (p=0.015, 0.000, 0.059, 0.000 and 0.017 for CO2, Ta, CO2×Ta, cultivar and Ta×cultivar, respectively). With respect to immature rice, elevating CO2 increased milky-white rice, white-based rice and white-belly rice across cultivars. Warming also significantly increased all immature rice across cultivars, though no CO2×Ta interaction was observed. Over 2 years, the deteriorative effect of warming on brown rice quality was significantly greater in Pyounganbyeo than in Ilmybyeo. Across cultivars, protein contents of milled rice were decreased (c. 5～9%) with elevating CO2 but increased (c. 5%) with warming, though no CO2×Ta interaction was found (p=0.119). Elevating CO2 significantly increased whiteness of milled rice over cultivars but not amylose contents and gloss value of cooked rice, while warming had a strong affect these properties all related rice quality. Overall, our results suggest that warming and elevating CO2, in each alone or in combination, may have the potential to deteriorate physio-chemical properties of rice related to quality.