PLANT DEV. RESPONSES TO SIGNALS
PLANT DEVELOPMENTAL RESPONSES TO EXTERNAL AND INTERNAL SIGNALS |
SENSING ENVIRONMENTAL STIMULI |
Among the environmental factors that are perceived and transduced and initiate adaptive responses are the following. |
LIGHT |
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GRAVITY |
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TOUCH |
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TEMPERATURE |
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WATER |
PLANT MOVEMENTS
Tropic Movements |
Nastic Movements |
Turgor Movements |
Tropic movements
Tropism |
is the directional growth response to environmental stimuli such as light, gravity, and touch. |
Phototropism |
Induced by light e.g. bending of stems towards light. |
Geotropism |
Induced by gravity e.g. growth of roots towards gravity |
Thigmotropism |
Movement caused by contact e.g., twining stem and tendril and the drooping of leaves of sensitive plant by touch |
Hydrotropism |
Induced by water i.e., growth of roots towards source of water. |
Nastic Movement
The nastic (nastein : bending) movements |
are the growth movements resulting due to difference in the rate of growth on opposite sides of an organ |
When the lower side grows more rapidly than upper side, it is called as |
hyponasty |
When upper side of an organ grows faster than the lower side, the movement is called |
epinasty |
Turgor Movements
Movements are due to change in the volume of water inside the cell. |
Turgor pressure- |
Pressure exerted by fluid in a cell that presses the cell membrane against the cell wall |
Turgid- |
When more water is present in the cell it is fully expanded and becomes rigid or hard. |
Flaccid- |
When less water is present inside the cell, it is not fully expanded and remains soft. |
Turgor movements in the sensitive plant (Mimosa pudica). |
A trigger hair must be touched more than once or two trigger hairs must be touched with in seconds of each other |
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Seed pods of some plants open on maturity, vigorously expelling their seed. |
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Photoperiodism
Photoperiodism |
is any response of a plant to the relative lengths of daylight and darkness. |
Short-day Plants (SDP) |
flower when the night length is equal to or greater than some critical period |
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Chrysanthemum, Cosmos, Dahlia, poinsettias are short- day plants. |
Long-day Plants (LDP) |
They produce flowers when exposed to a light period longer than a fixed day-length |
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Lettuce, spinach potatoes radish, are long-day. |
Intermediate-day plants (IDP) |
do not flower when day length is either too long or too short |
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Sugarcane, onion, and coleus are intermediate-day plants. |
Day-neutral Plants (DNP) |
In these plants flowering is not affected by length of light period |
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Cucumber, Tomato, Corn and Sunflower, are day-neutral plants. |
FLORIGEN AND PHYTOCHROME
ROLE OF FLORIGEN AND PHYTOCHROME IN FLOWERING |
Florigen |
is responsible for initiation of flowering in plants. |
Phytochrome |
known as light absorbing pigment and it makes the plants sensitive to light and participates in seed germination and flowering. |
PR |
absorbs red light |
Pfr |
absorbs far-red light |
Circadian Rhythms
Latin circum, |
“around,” and diurn, “daily”) |
Circadian Rhythms |
“daily”) help an organism respond to the time of day |
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an internal timer, or biological clock |
red light– |
absorbing phytochrome |
blue/ultra violet- |
A light– absorbing |
cryptochrome |
implicated in resetting the biological clock |
Vernalization
Vernalization |
is the process of accelerating the process of flowering by subjecting or exposing the plant to low temperature. |
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Some examples include carrots, beets, onions, winter wheat, cabbage, and turnips. In order to produce flowers and seeds, these plants have to go through the process of vernalization. |
RESPONSES TO HERBIVORES AND PATHOGENS
Each plant cells has an innate immune system to fight against local infection. When a molecule produced by a pathogen or herbivore binds to a receptor in a plant cell, it triggers a signal transduction pathway. |
Hypersensitive Response (HR) |
found in all higher plants and is characterized by a rapid cell death at the point of pathogen ingress. It is usually associated with pathogen resistance. |
Jasmonic Acid activates several plant defenses |
This lipid derived plant hormone, triggers the production of enzymes that confer an increases resistance against herbivorous (plant-eating) insects. |
Methyl Salicylate may induce systemic acquired resistance |
Systemic acquired resistance (SAR) is a mechanism of induced defense that confers long-lasting protection against a broad spectrum of microorganisms. |
Tobacco plants infected with tobacco mosaic virus release methyl salicylate into the air |
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