Effect of accumulated temperature before overwintering on wheat seedling growth status in north winter wheat area of China.

To explore the impacts of global climate change on the suitable sowing date for winter wheat in north winter wheat area of China, we carried out a wheat sowing date experiment during growing seasons of 2019-2021 at the Beijing Experimental Base of the Institute of Crop Sciences, CAAS. Two winter wheat cultivars with different tillering powers were selected as experimental materials. Four different sowing dates were set: September 25th (J), October 5th (S0), October 15th (S1) and October 25th (S2), to examine the responses of population quality, individual characters, and stem and tiller physiology to the accumulated temperature difference before overwintering. The results showed that with the delay of sowing date, the accumulated temperature before winter and their difference between the adjacent sowing dates decreased gradually. The accumulative temperature at the sowing J and S0 both exceeded 550 ℃, which met the basic condition for the formation of strong wheat seedlings before winter. The average accumulated temperature at sowing S1 and S2 was 148.0 and 282.4 ℃ lower than that of S0, which was not conducive to the establishment of strong wheat seedlings before winter. The average accumulated temperature decreased by 204.0, 148.0 and 134.4 ℃, when the sowing date was delayed by 10 days under the four different sowing dates, respectively. The days from sowing to emergence were affected by the average daily temperature. The days from sowing to emergence gradually increased with the delay of sowing date when the daily average temperature was lower than 15 ℃, while the days from sowing to emergence were constant when the daily average temperature was higher than 15 ℃. The total stem number, leaf area index, dry matter weight, nitrogen accumulation and tiller number per plant of wheat also decreased with the decreases of pre-winter accumulated temperature. The soluble sugar content and nitrate reductase activity at the seedling increased first and then decreased with the decreases of accumulated temperature before winter, while the soluble protein content and glutamine synthetase activity to accumulated temperature performed differently among varieties. According to the population quality and individual traits of wheat before winter, among the four different sowing dates, the total stem number and tiller number per plant of wheat before sowing on October 5 were the closest to the standard of strong seedlings before winter in north winter wheat area. The accumulated temperature before winter is conducive to the formation of strong seedlings. When the daily average temperature is 15-17 ℃, it is the best sowing time for winter wheat in Beijing.