Question

why was plant kept in dark and then in sunlight

Answer 1

so that the plant gets destarched

Answer 2

The plant would contain starch in it. If the plant would be in the sunlight it would simply produce more starch and then it would store it in it's leaves or other parts. If the plant is kept in the dark room then simply the plant would stop producing the glucose and the it would use up all the starch in it and the plant would become starch less. Therefore the plant is kept in the dark room for the de-starching.