Plant Photoreceptors

Light is made of various wavelenghts

Photoreceptors are divided into two groups: sensor that absorbs maxmia and trandsue responses to light in the red and blue region. Specialisation of photoreceptors relate to the type of information available during daylight and its application. Absorption of photons by chlorophyll shifts the light transmitted through canopies to lower wavelengths so this section of the spectrum is important for sensing light quality. Blue receptors or ‘cryptochrome’ is applied by plants as an indicator of sunlight because it is more subject to scattering gradients of light intensity across a short range of tissues.

In aquatic environments blue light takes on greater importance due to natural absorption of red light by H20. Certain cryptochrome can be induced by UV-A radiation. Red light photoreceptors or ‘phytochrome’ are employed by plants to sense light quality, presence, intensity and duration, phototropism and to sense the direction of light. Phytochrome is synthesised in the inactive form, which has a maximum absorption rate of 660nm. Carotenoid or the third sensor detects light intensity in specific cells. This photoreceptor has been identified in seed plants by a combination of genetic and biochemical research.

Far-red light is enriched in shade, light shifts the balance between the Pr and Pfr forms significantly towards Pr. This is the process of identifying competition, plants utilise these receptors to obtain advantages over their neighbours. Once a plant detects a competitor it can increase growth to gain the ability to sustain light levels.

In nature there will be various frequencies of photons that are absorbed by the two forms of phytochrome. This will result in individual molecules of phytochrome cycling between them. It has been found that the cycling rates can be influenced by light strength and that these cycling speeds can be converted to a signal regulated by adaptations to different photon intensities. The inactivation leads to the inhibition of seed germination in plants that usually thrive in high light levels. When mature plants respond to shade, the disappearance of Pfr signal which prevents stem elongation and stimulates development, leads to increased stature and limited allocation of resources to leaves until shade has been overcome.

Emerson, (1957) measured the efficiency of photosynthesis using a monochromatic light. He witnessed quantum yield (number of 02 evolved per light quanta) and two quanta are needed to transfer one electron. Therefore, eight quanta are required for the evolution of one 02 molecule. Quantum yield decreases sharply towards the far-red light part of the spectrum (680nm); this region is described as the ‘Red Drop’. Emerson further observed that if the red light of shorter wavelengths was superimposed with far-red the rate of quantum yield would greatly be enhanced; this is referred to as the ‘Emerson Effect’. Peavy and Gibbs, (1975) identified that photosynthesis yield was increased by 25% when two lights were supplied simultaneously. They confirmed this by isolating chloroplasts in Spinacia oleracea. The following equation known as The Hill Reaction demonstrates the energy potential:

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

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

Table listing major plant pigments used in photosynthesis

Table 1 Pigments distribution in plants and bacteria