The reduction of starch content occurred because the starch storage has been used during the night time.
During the night time, the plant cannot undergo photosynthesis, the only way the plant make food (starch) for themselves. However, during the night time, the plant still need to respire to generate energy for activities such as growth, repair, and to maintain life. This process uses starch.
Therefore, when the requirement for starch is higher than the production rate, the starch content after one night is reduced.
