Identification of Photothermal Responses in Soybean by Integrating Photope-riod Treatments with Planting-Date Experiments

Soybean [*Glycine max* (L.) Merr.] is a short-day crop that favors temperate weather. There have been a lot of studies on the responses to photoperiod, temperature or photothermal comprehensive regimes in soybean. However, little work has been conducted in the interactive effects of photoperiod and temperature on the development of soybean. To fully understand the photothermal responses of soybean and to identify the varietal variations in these traits, we designed two photoperiodic treatments of 12 h (short day or SD) and 16 h (long day or LD) integrated with two thermal conditions, high temperature (HT) (summer seeding in Beijing) and low temperature (LT) (spring seeding) in the current study. In 2007, the responses to the photoperiod, temperature, and photoperiod-temperature combinations of 10 spring sowing soybean varieties (lines) from the Northeast and 18 summer sowing varieties (lines) from Yellow-Huai-Hai River Valleys of China were identified in 4 photothermal regimes (LD+LT, LD+HT, SD+LT, SD+HT). In 2008, 50 varieties (lines) were used to test the photoperiodic responses. The results showed that SD promoted the developmental rate of soybean regardless of the temperature conditions; HT shortened the number of the days from emergence to flowering no matter whether the photoperiod was long or short. There was significant interaction between temperature and photoperiod from emergence to flowering. With the increase of temperature, the promotive effect of SD on developmental rate of soybean was enhanced, and the HT hastening effect was strengthened by SD as well. The apparent differences in photoperiod response sensitivity (PRS), temperature response sensitivity (TRS) and photothermal comprehensive response sensitivity (PTCRS) between ecotypes were observed. The above three indices of spring sowing soybean varieties from the Northeast were all lower than those of summer sowing varieties from Yellow-Huai-Hai River Valleys. However, the differences between TRS values under the two photoperiod treatments and that between PRS values under the two temperature conditions in spring sowing soybean varieties from the Northeast were both larger than those in summer sowing varieties from the Valleys, and it indicated that there was higher photoperiod × temperature interaction in the spring sowing varieties. The relationship between photothermal responses of soybean varieties and their ecological adaptability was discussed, and it proposed that, in breeding program, emphasis should be paid not only on the identification of responses of soybean varieties to the individual photoperiod or temperature factor but also on the photothermal interaction.