Impact assessment and adaptation planning to enable growth of paddy rice (Oryza sativa L.) under conditions of ongoing climate change are necessary in the Hokkaido region of northern Japan because of the intensive climate warming ( 2-3 in summer) projected to occur during the 21st century. Most previous studies have been based on extrapolation of results from past and present experimental data on unexperienced warming conditions under projected climatic change. These facts motivated us to use

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... assessment method known as an open-field day-length-extension (ODE) experiment. First, we confirmed the feasibility of an ODE experiment, which involves artificially extending day length with supplemental lighting. ODE treatment was performed in Morioka in 2009 (treatment ODE09M), where the summer temperature was 2.5 higher than the current average summer temperature in Sapporo, and the results were compared with those obtained in Sapporo, Hokkaido, in 2010 (treatment C10S), which had an extraordinarily hot summer (mean daily temperature 2.3 higher than normal). The difference in heading date between ODE09M and C10S was small, i.e., 4 days for the three cultivars Koshihikari, Hitomebore, and Akitakomachi. Second, by comparing previous temperature gradient chamber (TGC) results with those from the ODE experiment, we found that a TGC experiment system was likely to overestimate the effect of temperature on heading date by up to 7 days. Finally, we calculated paddy rice cultivation under projected climate change conditions in Sapporo in terms of three critical dates for stable rice cultivation: the early limit for transplanting, the early limit for heading, and the late limit for heading. The warmer climate condition at Morioka formed by the ODE experiment that is an analog of future climate (2.5 higher than the current average) at an northern site, Sapporo, would allow successful cultivation of Hitomebore and Akitakomachi, but not Koshihikari, in Sapporo.