The researchers studied how different levels of irrigation affected the growth and yield of winter wheat. They tested six irrigation treatments, including rainfed and various levels of supplemental irrigation based on soil moisture measurements. They found that irrigating the 0-40 cm soil layer at jointing and anthesis led to the highest grain yield and water use efficiency. This treatment also had the highest photosynthetic rate and transpiration rate, indicating better plant growth. Overall, irrigating this specific soil layer resulted in the best balance of water use and crop yield for winter wheat.