Question

Why a plant which is left in the dark for a long period of time will test negative for starch?

Answer 1

When a plant is left in dark for several hours, it does not get sunlight. Isn't it?
When a plant does not get sunlight, it cannot prepare food for itself , i.e, photosynthesis does not take place. When it does not prepare food in the form of starch, it will test negative for it.

hope it helps!