Answer:
They need light energy for photosynthesis
Step-by-step explanation:
Plants make their own food using a process known as photosynthesis.
Photosynthesis:
light energy+carbon dioxide+water--> glucose+oxygen
The glucose the plant produce becomes their food. If the did not have light energy, they would not be able to photosynthesize. Thus, they wouldn't be able to produce food, and the plants would die.