The plumpness of grain crops and the sweetness of melons and fruits are related to the photosynthesis and respiration of plants. When there is light during the day, plants can carry out photosynthesis, respiration and transpiration, while at night, when there is no light, plants can only carry out respiration and transpiration, which are related to temperature. In a certain range, the higher the temperature during the day, the stronger the photosynthesis of plants and the more organic matter they produce. The lower the temperature at night, the weaker the respiration of plants.

So choose B.