Abstract: Given the fact that air temperature decreases with altitude, a lowtemperature experiment was conducted on the north slope of Changbai Mountain to understand the responses of rice seed setting to low temperature in flowering stage. The experimental setup included seven altitudes (temperature), three durations, and three flowering periods. The results showed that seed setting was sensitive to low temperature in flowering stage. The empty abortion rate was higherunder conditions of lower temperature and longer duration. The relationship between rice emptyabortion rate and 2-day average temperature followed linear function, but the influence coefficient of temperature was small. The relationship between empty abortion rate and ≥3 days average temperature followed logarithmic function, and the influence coefficient was greater. There were significant differences for the influence of low temperature among different flowering periods, with the biggest influence presenting in the fullflowering period, followed by the middle and earlyflowering periods. The influence coefficient of temperature in fullflowering period was three times as large as that in the earlyflowering period. On average, under the two, three, and four days lowtemperature treatments, the empty abortion rate increased by 0.4, 1.5 and 3.1 percentage points, respectively, with 1 ℃ decrease of daily temperature. The empty abortion rate increased by 0.4, 1.9 and 3.5 percentage points, respectively, with 1 ℃ decrease of daytime temperature. The relationships between the empty abortion rate and daytime temperature or the highest temperature were significantly stronger than that between the empty abortion rate and night temperature or the lowest temperature. The combined effect of the lowtemperature duration and the strength of lowtemperature on the empty abortion rate followed a binary linear equation. For example, the empty abortion rate raised 4.5 percentage points with the extended one day of the duration and raised 1.7 percentage points with 1 ℃ decrease of the temperature. In northeast China, shortterm lowtemperature (≤2 days) in flowering stage caused slight chilling injury and three days or more lowtemperature could cause moderate and severe chilling injury. Therefore, the daytime temperature and maximum temperature would be more effective than the daily average temperature and minimum temperature as chilling injury index of rice in flowering stage.

Key words: translocation, ¹⁵N-urea foliar application, ¹³C-photosynthate, apple, distribution