What plant pigment is responsible for photoperiodism?

Phytochrome is the plant pigment primarily responsible for photoperiodism, which is the plant's ability to detect and respond to the lengths of day and night. This pigment exists in two forms: Pr, which absorbs red light, and Pfr, which absorbs far-red light. When exposed to light, Pr is converted to Pfr; this conversion triggers various physiological responses, including flowering in certain plants, regulating processes such as seed germination and growth based on the day length.

In contrast, while chlorophyll is essential for photosynthesis, it does not function in sensing light duration. Carotenoids play a protective role and are involved in photosynthetic processes but are not directly involved in photoperiodism. Anthocyanins primarily serve as pigments responsible for coloration and can provide some protective functions against UV light, but they do not influence photoperiodic responses. Thus, among the provided options, phytochrome is uniquely suited to mediate the crucial processes linked to photoperiodism in plants.