What light plants use

Photoreceptors in plants interact with light wavelengths to reflect colour that we perceive. These photoreceptors are divided into two main groups: sensors that absorb light primarily in the red and blue regions of the spectrum and transduce responses accordingly. The specialisation of photoreceptors is closely related to the type of information plants obtain from daylight and how they use it.

Chlorophyll absorbs photons, shifting the light that passes through plant canopies to lower wavelengths, making this section of the spectrum crucial for sensing light quality. Blue light receptors, known as 'cryptochromes', serve as an indicator of sunlight intensity, as blue light is scattered more easily and creates distinct gradients of light intensity across plant tissues. In aquatic environments, blue light becomes even more important because water naturally absorbs red light, leaving blue light as the dominant wavelength. Some cryptochromes can also be activated by UV-A radiation.

Red light photoreceptors, or 'phytochromes', allow plants to sense not only the quality of light but also its presence, intensity, and duration. Phytochromes are critical for phototropism and detecting the direction of light. They are synthesised in an inactive form (Pr) that has a maximum absorption at 660 nm. This receptor helps regulate many growth processes, including sensing competition for light. Carotenoids, another group of light sensors, are responsible for detecting light intensity and are found in specific plant cells. Carotenoid photoreceptors have been identified in seed plants through genetic and biochemical studies.

In shaded conditions, far-red light becomes more prevalent, shifting the balance of phytochromes from the active Pfr form to the inactive Pr form. This shift signals the presence of competition, and plants use this information to adjust their growth, often accelerating it to outgrow nearby plants and access more light. The balance between Pr and Pfr forms also regulates the cycling of phytochromes in response to different light intensities. This process can control germination, with high light levels promoting germination, while the absence of the Pfr signal in shade inhibits it. Mature plants in shaded environments respond by elongating their stems and conserving resources, allowing them to grow taller until they can overcome the shade.

In 1957, Emerson measured the efficiency of photosynthesis using monochromatic light and discovered that quantum yield, the number of O₂ molecules produced per light quanta, decreases sharply in the far-red region of the spectrum (around 680 nm). This phenomenon is known as the 'Red Drop'. Emerson further noted that when shorter red light wavelengths were combined with far-red light, photosynthetic efficiency greatly increased. This became known as the 'Emerson Effect'. Peavy and Gibbs (1975) confirmed this by showing that photosynthetic yield increased by 25% when two lights were provided simultaneously. They demonstrated this by isolating chloroplasts from Spinacia oleracea (spinach) and measuring photosynthetic rates.

The equation for the Hill Reaction, which demonstrates the energy potential of light in photosynthesis, is as follows:

2H20 (4 Photons, Mn+(Water splitting enzyme)) → 4H+ + 4e- + 02

Below is a table outlining the different pigmentation involved in light absorption.

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