Final answer:
Red light converts phytochrome red (Pr) to its active form, phytochrome far-red (Pfr), which plays a key role in plant physiology by regulating growth and development.
Step-by-step explanation:
Red light converts phytochrome red (Pr) to phytochrome far-red (Pfr). This process is a crucial part of the phytochrome system, which acts as a biological light switch in plants, controlling various physiological responses. Pr absorbs red light at about 667 nm wavelength and gets converted into its active form, Pfr. Conversely, Pfr can absorb far-red light (~730 nm) and revert back to Pr. The conversion to Pfr allows the phytochrome to directly activate other molecules in the cytoplasm or alter gene expression in the nucleus. The active form, Pfr, is crucial for processes such as germination and is an example of how plants adapt to their light environment to optimize growth.