Many plant species open their flowers in the morning and then close them for the evening. Other species only have their flowers open at night. The term for this trait is nictonasty. The answer may be in the photograph of the California poppy directly below.
It is most likely to protect the pollen.
California poppy flowers open to reveal pollen that is small and delicate. Notice how the lower petals cradle the pollen that has fallen from the anthers. The theory is that some plant species have evolved to ensure their flowers are open when the insects that pollinate them are active.
Flowers that close at night include poppies, tulips, crocus, and hibiscus.
A strong case for pollen protection as the reason for nictonasty can be made when one considers many plant species that depend on animals active at night for their pollination close during the day and open at dusk. If a flower is pollinated by bats or moths there is no point in being open when these animals are not active.
Charles Darwin had another idea. He theorized nictonasty was to protect the flowers from freezing. Charles Darwin was fascinated by movement in the opening and closing of plants in response to light. In 1877, Darwin wrote about Oxalis and Euphorbias in his sleep movement research.
How plants open and close their flowers
There are a few different methods plants use. Some plants, such as kalanchoe, actually grow new cells to facilitate petal movement. New cells are produced on the inside of the flower to open it and on the outside of the flower to close it. Other plants grow their lower petals faster than their upper petals, thus forcing the flower to close. This second method is most likely driven by cooler temperatures at night, suggesting to me a response to temperature as opposed to light.
In the case of poppies and most plants that exhibit nictonasty, they do it by pumping water out of the flowers. It is a clever case of wilting, actually. We’ve all seen plants wilt and it is normally a sign we need to water them. Or it may be because of root rot and the plant simply can’t deliver water to the stems and leaves.
What is particularly remarkable about plants that use this method to close their flowers is the only part of the plant affected is the flowers themselves.