Plants perceive light through internal photoreceptors absorbing a specified wavelength signaling or transferring the energy to a plant process. In plants, the photoreceptors cryptochrome and phototropin absorb radiation in the blue spectrum and regulate internal signaling such as hypocotyl inhibition, flowering time, and phototropism. Additional receptors called phytochrome absorb radiation in the red and far-red spectra and influence many aspects of plant development such as germination, seedling etiolation, transition to flowering, shade avoidance, and tropisms. Phytochrome has the ability to interchange its conformation based on the quantity or quality of light it perceives and does so via photoconversion from phytochrome red to phytochrome far-red. Pr is the inactive form of phytrochrome, ready to perceive red light. In a high R:FR environment, Pr changes conformation to the active form of phytochrome Pfr. Once active, Pfr translocates to the cellular nucleus, binds to phytochrome interacting factors, and targets the PIFs to the proteasome for degradation. Exposed to a low R:FR environment, Pfr absorbs FR and changes conformation back to the inactive Pr. The inactive conformation will remain in the cytosol, allowing PIFs to target their binding site on the genome and induce expression. FR has long been considered a minimal input in photosynthesis. In the early 1970’s, PhD physicist and soil crop professor Dr. Keith J. McCree lobbied for a standard definition of photosynthetically active radiation which did not include FR. More recently, scientists have provided evidence that a broader spectrum called photo-biologically active radiation is more applicable terminology. This range of wavelengths not only includes FR, but also UV-A and UV-B. The Emerson Effect established that the rate of photosynthesis in red and green algae was higher when exposed to R and FR than the sum of the two individually. This research laid the ground work for the elucidation of the dual photosystems in plants. Photosystem I and photosystem II work synergistically; through photochemical processes PSII transports electrons to PSI. Any imbalance between R and FR leads to unequal excitation between PSI and PSII, thereby reducing the efficiency of photochemistry.
