Winter extreme low temperature events have been occurring frequently both before and after the winter season. The freezing resistance temperature of wheat is far lower than the intensity of low temperatures during the mid-winter period. Therefore, it is necessary to further quantify and evaluate the impact of low-temperature periods and durations during the early winter and the green-up period on the freezing resistance of wheat, based on different evaluation indicators. Through conducting experiments in an artificial low-temperature control chamber, this study investigates the critical temperature thresholds for the impact of different low-temperature periods and durations on the tiller and yield of winter wheat, as well as the critical temperature thresholds for soil effective negative accumulated temperature. The results demonstrate that (1) the tiller mortality rate (RT) and yield reduction rate (RY) of winter wheat during the winter increase with the severity and duration of low temperatures, showing an S-shaped curve. The winter wheat mortality rate during the early winter is related to the soil effective negative accumulated temperature in an exponential function, while the mid-winter and green-up stages have a linear relationship. (2) The freezing threshold temperatures for the RT, RY and soil negative accumulated temperature (SENAT) in different low-temperature periods (early winter, mid-winter, and green-up periods) range from - 11.7 to -17.9 degrees C, -9.4 to -15.6 degrees C, and 15.9 to 131.7 degrees Ch (2.2 to 16.8 degrees Cd), respectively. (3) The freezing threshold temperatures for the RT and RY in different low-temperature durations (1 day, 2 days, and 3 days) range from - 2.8 to -17.9 degrees C and - 9.4 to -15.6 degrees C, respectively. The findings of this study provide technical support and scientific guidance for the global cultivation structure and variety layout of winter wheat under the background of climate warming, as well as for the prevention and reduction of freezing damage and yield losses.

Rainfall variability, waterlogging and frequent natural hazards are the major obstacles for cropping system intensification in heavy textured soils of the coastal areas of Bangladesh. While earlier monsoon rice harvesting by introducing short duration varieties created opportunities for cultivating low water demanding non-rice crops in the dry season, such crops failed in many instances because of heavy rainfall and waterlogging. To address such issue, we have analysed dry season (Nov-Apr) rainfall patterns of six meteorological stations of coastal Bangladesh for studying the feasibility of growing irrigated rice and non-rice crops that can be harvested by April. Very heavy rainfall (>20 mm) occurred in 18-23% of the studied years and heavy rainfall (>10 mm) in 42-43% of cases creating the risk of water stagnation and damage to non-rice crops. The return intervals between occurrences of heavy rainfall and very heavy rainfall in November to December were 1.3-1.4 years and 1.5-2.5 years, respectively. These rainfall events generally delay establishment of non-rice crops. Similarly, in March and April, the return periods for heavy and very heavy rainfall were 1.3-1.5 years and 1.6-2.1 years, respectively. These rainfall events had a detrimental impact on non-rice crops, especially during their ripening stages. Such rainfall events during field experiments at the study locations were found in three years out of four cropping seasons that reduced sunflower and maize yields by 50-64% and sweet gourd and watermelon yields by 55-84% compared to their absence. The probability of high yield of non-rice crops was <25% and the yield variability was very high (40-75%) compared to general rice yield variability (5-6%). Risk factor analysis also revealed that dry season rice is less risky compared to other non-rice crops. To enhance risk management, intensification of cropping systems can be achieved by promoting cultivation of dry season irrigated rice where there is sufficient stored water for irrigation and encouraging farmers to grow pre-monsoon rice.