3. Definition of transpiration
Phytochrome: is a photoreceptor,
a pigment that plants, and some bacteria and
fungi, use to detect light. It is sensitive to light in
the red and far-red region of the visible spectrum.
Biochemically, phytochrome is a protein with
a bilin chromophore.
The Pigment That Controls Growth and Flowering In
Many Plants
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5. Role of phytochrome
1. Many flowering plants use it to regulate the time
of flowering based on the length of day and night
(photoperiodism) and to set circadian rhythms.
2. It also regulates other responses including
the germination of seeds(photoblasty), elongation of
seedlings, the size, shape and number of leaves, the
synthesis of chlorophyll, and the straightening of
the epicotyl or hypocotyl hook of dicot seedlings.
3. It is found in the leaves of most plants.
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6. Founding of phytochrome
Phytochrome has been found in most plants
including all higher plants;
very similar molecules have been found in
several bacteria.
A fragment of a bacterial phytochrome now has a
solved three-dimensional protein structure.
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9. characteristics of phytochrome-mediated
responses
The action spectrum of the light needed for these
responses shows a peak in the red at about 660 nm.
These responses can be reversed by an application of far-
red light (peak at 730 nm) soon after the red treatment.
Sensitive spectrophotometers can measure a decrease in
absorbance at 660 nm and in increase in absorbance at 730
nm when sensitive plant tissues are exposed to red (660
nm) light.
The change in absorbance is caused by the conversion of a
the photoreceptor from one structural form to
another. The red-absorbing form changes to the far-red
absorbing form when it absorbs red light (660 nm) and back
again when it absorb far-red light (730 nm)
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10. The phytochrome molecule is the photoreceptor for red
light responses. It exists in two forms, Pr and Pfr:
The Pr form:
• Absorbs at a peak of 666 nm
• Is the form synthesized in dark-grown seedlings.
• When Pr absorbs red light, it is converted to the Pfr form.
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11. The Pfr form:
• Absorbs at a peak of 730 nm
• The Pfr form is the active form that initiates biological
responses
• When Pfr absorbs far red light, it is converted to the Pr form
• Pfr can also spontaneously revert to the Pr form in the dark
over time = dark reversion; Pfr is also susceptible to
proteinases.
• Pfr absorbs some red light, so in red light, there is a balance of
85% Pfr and 15% Pr
• Pr absorbs very little far red light, so in far red light, there is a
balance of 97% Pr to 3% Pfr
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