Acommercial florist exposes her chrysanthemums—which are short-day plants—to a few minutes of red light during their daily darkness period in order to delay flowering. why is this action effective?

Red light act as a inhibitor and hence causes delay in flowering

Explanation:

Through Periodism phenomenon, the regulation in exposure to daylight for a certain time period can delay or enhance the flowering of the "light duration"sensitive plant.

When "short day plants" are exposed to light between  the essential long period of nights or dark period, the stage of flowering gets delayed.

Hence,  chrysanthemums on being exposed to red light during their daily darkness period lead  delay in flowering as red light act as an inhibitor.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.