Plant Responses. Introduction. At every stage in the life of a plant, sensitivity to the environment and coordination of responses are evident. One part of a plant can send signals to other parts. Plants can sense gravity and the direction of light.
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How do plants control their growth in response to environmental stimuli?
Movement of hormone
1.Auxins are responsible forregulatingphototropism in a plant by stimulating the elongation of cells.
2.High concentrations of auxin help promote the growth of fruit and minimize the falling off of fruit from the plant.
3. Enhance apical dominance
7. Developing seeds synthesize auxin, which promotes the growth of fruit.
Tropism—a plant’s response to their environment
Geotropism—a plant’s response to gravity
Phototropism—a plant’s response to light
Thigmotropism—a plant’s response to touch
This occurs when motor organs at the joints of leaves, become flaccid from a loss of potassium and subsequent loss of water by osmosis.
It takes about ten minutes for the cells to regain their turgor and restore the “unstimulated” form of the leaf.
This interconversion between isomers acts as a switching mechanism that controls various light-induced events in the life of the plant.
Lettuce seeds exposed to flashes of light
The Pfr form triggers many of the plant’s developmental responses to light.
Exposure to far-red light inhibits the germination response.
Pr accumulates at night
No sunlight to make the conversion
Pfr breaks down faster than Pr
At daybreak, Pr begins to be converted to Pfr
So, night length is responsible for resetting the clock.
Flashes of red lightduring the night
resets the clock.
Ex: chrysanthemums, poinsettias, and some soybean varieties.
Long-day plants will only flower when the light period is longerthan a critical number of hours.
Ex: include spinach, iris, and many cereals.
Day-neutral plants will flower when they reach a certain stage of maturity, regardless of day length.
Ex: tomatoes, rice, and dandelions.
Cocklebur is actually unresponsive to day length, but it requires at least 8 hours of continuous darkness to flower.
Action spectra and photoreversibility experiments show that phytochrome is the active pigment.
If a flash of red light during the dark period is followed immediately by a flash of far-red light, then the plant detects no interruption of night length, demonstrating red/far-red photoreversibility.
Plants adapted for growth in full sun will display greater stem elongation when they are transferred to shade. They also develop smaller leaves and less branching. This change is due to greater proportion of Pr to Pfr.