All forms of life on Earth require food to grow. Animals cannot produce their own food and obtain it either from plants or from other animals. Plants on the other hand can produce their own food. The presence of chlorophyll in plants allows them to undergo photosynthesis which is used to convert carbon dioxide and water into carbohydrates using energy from the Sun.
The carbohydrates that plants manufacture is also their source of food. One of the important reasons many forms of complex organisms can survive is that plants produce a lot more carbohydrate than they require as food for themselves.
If a plant were to be kept in the dark, it could obtain energy to be able to carry out photosynthesis. In spite of this, the consumption of carbohydrate for its own life processes goes on. For this plants use the carbohydrates stored in the form of starch in their body. This leads to a reduction in the starch stored in plants. Leaves store a very small amount of starch in them. This would make them the first part to be affected by food production through photosynthesis coming to a stop.