Effects of Temperature and Day-Length on Heading Habit of Recently Developed Korean Rice Cultivars

To understand the heading habit of recently developed 20 Korean rice cultivars, rice plants in a phytotron were exposed to different temperature: $22.5^{\circ}C(day\;27^{\circ}C/night\;18^{\circ}C),\;27.5^{\circ}C(day\;32^{\circ}C/night\;23^{\circ}C)$, and day-length conditions: 10, 12, 13, 14, 15 hours. Four rice cultivars (Sobibyeo, Juanbyeo, Ilpumbyeo and Shindongjinbyeo) showed relatively short Basic Vegetative Phase (BVP) of 17 to 18 days, while Dasanbyeo showed the longest (35 days) BVP, compared to other remaining 15 tested cultivars which exhibited 24 to 31 days of BVP. In this experiment, it was tried out to separate the eliminable vegetative phase into photosensitivity and thermo-sensitivity with two different pathways. Many tested cultivars, however, exhibited quite different responses under low temperature and / or long day-length conditions. Especially, Surabyeo and Juanbyeo were the most difficult cultivars to separate into photo- or thermo-sensitivity in that the eliminable vegetative phase of these two cultivars increased greatly only under low temperature $(22.5^{\circ}C)$ and long day-length (15 hr.) conditions. Regarding the heading response to temperatures, tested cultivars could be categorized into 2 groups. In 1st group of rice cultivars, the eliminable vegetative phase decreased almost equally as the temperature changed from $20.0^{\circ}C\rightarrow22.5^{\circ}C\rightarrow25.0^{\circ}C\rightarrow27.5^{\circ}C$. In contrast, the 2nd group of rice cultivars exhibited eliminable vegetative phase slowly decreasing when the temperature changed from $22.5^{\circ}C\rightarrow25.0^{\circ}C\rightarrow27.5^{\circ}C$, but rapidly decreasing when the mean temperature changed from $20.0^{\circ}C\;to\;22.5^{\circ}C$. All the cultivars belonged to 2nd group, the heading date would be very delayed if cool summer comes.