Question

Destarching of the leaves of a potted plant can occur only at night

Answer 1

Destarching of plant can be done in day too.
Only if you keep it in a dark cupboard for 24 to 48 hours. Actually to destarch a plant, what you need is darkness. There should be no light so that the plant cannot carry on photosynthesis, thus producing more glucose which are hence converted to starch..

Answer 2

Destarching of a leaves of a potted plant can occur only at night because during the day the plant make their food by process of photosynthesis but at night due to the absence of Sunlight the process of photosynthesis stops and the plant uses the starch as their food