Phytochrome is a pigment found in plants and bacteria that senses light and controls photoperiods, germination, flowering, and chloroplast synthesis. It has two forms, Pr and Pfr, and absorbs red and far red light. Phytochrome helps develop photoperiodism and ensures optimal use of light for photosynthesis. Its discovery began with the observation of plant responses to day and night, leading to the identification of a single pigment responsible for photoperiod, which was named phytochrome in 1959.
Phytochrome is a pigment found in most plants and some bacteria that is used to monitor the color of light. Plants can use this pigment to determine photoperiods, when to germinate seeds, when to flower, and when to produce chloroplasts, a key chemical used in photosynthesis. Photosynthesis is a process by which plants convert sunlight into food. Phytochrome can also be instrumental in controlling leaf shape and size, seed length, the number of leaves that form, and the optimal seed length to make the most of the light at hand.
A pigment is a substance that alters the color of an object by reflecting some light waves and selectively absorbing others. For example, imagine that red, yellow, and blue rays are shining on a ball. If the ball reflects blue and absorbs all other light waves, the ball will appear blue to an observer. Phytochrome is a special pigment with two forms, Pr and Pfr, which absorb red light and far red light respectively, emitting a green to blue hue. Red light and far red light are relatively low energy and frequency light sources, compared to other light waves in the electromagnetic spectrum.
Phytochrome is a photoreceptor or protein that senses light on an organism and elicits a response. It has both a protein component and a chromophore component, the piece responsible for absorbing red light. The molecule starts receiving red light in the Pr form, which causes the phytochrome to undergo a chemical change to become Pfr. This Pfr state of phytochrome is the active state, or the state that initiates response processes in the plant, and prefers to absorb far red light.
In flowering plants, this light-sensing method helps develop photoperiodism, or responses to day and night. Plants can also use phytochrome to change the shape and size of leaves and to initiate chloroplast synthesis. This ensures that photosynthesis can make optimal use of the light at hand. It’s also important to monitor the light so the seeds can grow successfully, without drying out or getting too little sun.
The discovery of phytochrome began with the observation of photoperiodism in plants. Scientists began to notice that plants responded differently to day and night; some plants impaired processes for longer days, some favored flowering during shorter days, and some stopped flowering if exposed to light for even a few minutes at night. In the 1930s, at the Beltsville Agricultural Research Center, botanist Sterling Hendricks, physiologist Marion Parker and chemist Harry Borthwick teamed up to investigate this phenomenon.
In 1948, spectrographic tests indicated that a single pigment was responsible for the photoperiod. In 1952, tests revealed that germination was halted when a plant was exposed to far red light and restarted when exposed to red light. In 1959, the team performed conclusive tests on turnip seeds and named the pigment phytochrome.
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