The red, far-red, and violet-blue regions of the visible light spectrum trigger structural development in plants. Sensory photoreceptors absorb light in these particular regions of the visible light spectrum because of the quality of light available in the daylight spectrum. In terrestrial habitats, light absorption by chlorophylls peaks in the blue and red regions of the spectrum.
As light filters through the canopy and the blue and red wavelengths are absorbed, the spectrum shifts to the far-red end, shifting the plant community to those plants better adapted to respond to far-red light. Blue-light receptors allow plants to gauge the direction and abundance of sunlight, which is rich in blue—green emissions.
Water absorbs red light, which makes the detection of blue light essential for algae and aquatic plants. Learning Objectives Compare the ways plants respond to light. Water absorbs red light, which makes the detection of blue light essential for algae and aquatic plants.
The phytochromes are a family of chromoproteins with a linear tetrapyrrole chromophore, similar to the ringed tetrapyrrole light-absorbing head group of chlorophyll. Phytochromes have two photo-interconvertible forms: P r and P fr. The minute difference between light defined as red or far-red is very important in this reaction.
Absorption of red or far-red light causes a massive change to the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. P fr is the physiologically active form of the protein; therefore, exposure to red light yields physiological activity.
Exposure to far-red light inhibits phytochrome activity. Together, the two forms represent the phytochrome system Figure 1. Figure 1. The biologically inactive form of phytochrome Pr is converted to the biologically active form Pfr under illumination with red light. Far-red light and darkness convert the molecule back to the inactive form. The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of environmental light.
The effect of red light is reversible by immediately shining far-red light on the sample, which converts the chromoprotein to the inactive P r form. Additionally, P fr can slowly revert to P r in the dark, or break down over time. In all instances, the physiological response induced by red light is reversed. The active form of phytochrome P fr can directly activate other molecules in the cytoplasm, or it can be trafficked to the nucleus, where it directly activates or represses specific gene expression.
Once the phytochrome system evolved, plants adapted it to serve a variety of needs. Unfiltered, full sunlight contains much more red light than far-red light. Because chlorophyll absorbs strongly in the red region of the visible spectrum, but not in the far-red region, any plant in the shade of another plant on the forest floor will be exposed to red-depleted, far-red-enriched light.
The preponderance of far-red light converts phytochrome in the shaded leaves to the P r inactive form, slowing growth. The nearest non-shaded or even less-shaded areas on the forest floor have more red light; leaves exposed to these areas sense the red light, which activates the P fr form and induces growth. In short, plant shoots use the phytochrome system to grow away from shade and towards light. Because competition for light is so fierce in a dense plant community, the evolutionary advantages of the phytochrome system are obvious.
In seeds, the phytochrome system is not used to determine direction and quality of light shaded versus unshaded. Instead, is it used merely to determine if there is any light at all.
This is especially important in species with very small seeds, such as lettuce. Because of their size, lettuce seeds have few food reserves. Their seedlings cannot grow for long before they run out of fuel. If they germinated even a centimeter under the soil surface, the seedling would never make it into the sunlight and would die. In the dark, phytochrome is in the P r inactive form and the seed will not germinate; it will only germinate if exposed to light at the surface of the soil.
Upon exposure to light, P r is converted to P fr and germination proceeds. Plants also use the phytochrome system to sense the change of season. Photoperiodism is a biological response to the timing and duration of day and night. It controls flowering, setting of winter buds, and vegetative growth. Detection of seasonal changes is crucial to plant survival.
Although temperature and light intensity influence plant growth, they are not reliable indicators of season because they may vary from one year to the next. The opposite happens in a stem. When a stem is placed horizontally , the bottom side contains more auxin and grows more - causing the stem to grow upwards against the force of gravity. Auxin and phototropism. Positive phototropism in plant stems. The tips have been removed.
No light reaches the tips. More light reaches one side of the tips. No auxin is produced. Equal concentration of auxin on both sides. Greater concentration of auxin on shaded side. The stems do not grow longer.
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