Soil moisture affects how well rape plants can photosynthesize and use water. The higher up the leaf on the plant, the better it can photosynthesize. When there's not enough water, the plant's photosynthesis doesn't change much, but it uses water more efficiently. With enough water, the plant loses more water through transpiration, and its photosynthesis and water use efficiency decrease. Drought makes the plant weigh less, but it uses water better. In well-watered conditions, the upper and lower leaves lose more water than the middle ones.