Photoreceptors in plants

Photoreceptors in plants are categorised into two main groups: those that absorb light primarily in the red and blue regions of the spectrum. These receptors are specialised to handle different types of light information available during daylight.

Chlorophyll, a key pigment in plants, absorbs photons and shifts the transmitted light to lower wavelengths. This makes the detection of light quality in this portion of the spectrum crucial for plant health. Plants use blue light receptors known as cryptochromes to gauge sunlight. These receptors are particularly sensitive to the scattering of light across short tissue ranges. In aquatic environments, blue light is even more significant due to the natural absorption of red light by water. Certain cryptochromes can also be activated by UV-A radiation. Red light is detected by another type of photoreceptor called phytochrome. Phytochromes help plants sense light quality, presence, intensity, duration, phototropism, and direction. They are synthesized in an inactive form that has a peak absorption at 660 nm. Another photoreceptor, carotenoid, detects light intensity in specific cells and has been identified in seed plants through genetic and biochemical research.

In shaded environments, far-red light becomes more prevalent, causing a shift in the balance between the Pr and Pfr forms of phytochrome toward Pr. This shift helps plants detect competition and adjust their growth to maintain light levels. When a plant senses a competitor, it can accelerate its growth to better compete for light. Phytochromes cycle between their forms in response to varying photon frequencies. The cycling rates are influenced by light intensity and are adapted to different photon levels. This cycling process helps regulate plant responses to light conditions. In high light levels, phytochrome inactivation can inhibit seed germination. In shaded conditions, the reduction of the Pfr signal prevents stem elongation and promotes the development of increased plant stature while conserving resources until the shade is overcome.