Question

why plants generally with in the afternoon and regain their freshness in the next morning

Answer 1

the lifespan of a plant is a day so that is the reason

Answer 2

the main reason for this is more rate of water loss through transpiration that the roots can absorb from the soil.in the afternoon the rate of transpiration through the leaves is a great deal more than the water absorbed from the soil. as water is essential for mechanical stiffness of the plant, the plants wilt (not whither) during the afternoon and regain their freshness again the next morning.

hope this helps