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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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  • 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.
    • A plant’s morphology and physiology are constantly tuned to its variable surroundings.
Upon receiving a stimulus, a receptor initiates a specific series of biochemical steps, a signal transduction pathway.
  • Ultimately, a signal-transduction pathway leads to the regulation of one or more cellular activities.

How do plants control their growth in response to environmental stimuli?

  • Most plants do this by way of chemical messengers known as hormones.
  • A hormone is a chemical that is produced in one part of an organism and transferred to another part to affect the activities of that part of the plant.

Hormone-producing cells

Movement of hormone

Target cells

Hormone-producing cells

research on how plants grow toward light led to the discovery of plant hormones
Research on how plants grow toward light led to the discovery of plant hormones
  • Studies of grass seedlings, particularly oats.
  • In the late 19th century, Charles Darwin and his son observed that a grass seedling bent toward light only if the tip of the coleoptile was present.
    • This response stopped if the tip was removed or covered
  • Later, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance.
In 1926, F.W. Went extracted the chemical messenger for phototropism, naming it auxin.




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.

  • When the auxin concentrations decrease in the autumn, the ripened fruit will fall. The plants will begin to lose their leaves.


  • Auxin also alters gene expression rapidly, causing cells in the region of elongation to produce new proteins within minutes.
    • Some of these proteins are short-lived transcription factors that repress or activate the expression of other genes.


3. Enhance apical dominance

  • Produced in the growing tip of a plant and transported downward (polar transport)
  • Terminal bud suppresses lateral growth


  • In growing shoots auxin is transported unidirectionally, from the apex down to the shoot.
    • Auxin enters a cell at its apical end as a small neutral molecule, travels through the cell as an anion, and exits the basal end via specific carrier proteins.
    • Outside the cell, auxin becomes neutral again, diffuses across the wall, and enters the apex of the next cell.
    • Auxin movement is facilitated by chemiosmotic gradients established by proton pumps in the cell membrane.


  • Auxin is transported through the plant body by polar transport.
  • Requires specific carrier proteins built into the cell membrane.
  • ATP provides the energy for a proton pump that pumps protons out of the cytoplasm, creating a pH gradient necessary for the transport of auxin.

Can be used as a rooting powder


5. Treating a detached leaf or stem with rooting powder containing auxin often causes adventitious roots to form near the cut surface.


4. Auxin is also involved in the branching of roots

6. Auxin also affects secondary growth by inducing cell division in the vascular cambium and by influencing the growth of secondary xylem.

7. Developing seeds synthesize auxin, which promotes the growth of fruit.

  • Stimulate cytokinesis and cell division
    • Experiment with plant embryos
  • Work with auxin to promote growth and cell division
  • Work against auxin in relation to apical dominance
  • Delay senescence (aging) by inhibiting protein breakdown
    • Florists spray flowers to keep them fresh.
  • Produced in roots and travel upward in the plant
  • Discovered in 1931
  • 1941- coconut milk
  • 1961-first isolated cytokinin from corn (Zeatin)
  • Now found in almost all higher plants
  • Highest in meristematic areas.
  • Made in roots
Cytokinins interact with auxins to stimulate cell division and differentiation.
    • In the absence of cytokinins, a piece of parenchyma tissue grows large, but the cells do not divide.
    • In the presence of cytokinins and auxins, the cells divide.
    • If the ratio of cytokinins and auxins is balanced, then the mass of growing cells, called a callus, remains undifferentiated.
    • If cytokinin levels are raised, shoot buds form from the callus.
    • If auxin levels are raised, roots form.


  • Growth hormones that cause plants to grow taller.
  • They also increase the rate of seed germination and bud development.
  • There are certain tissues in the seeds that release large amounts of gibberellins to signal that it is time to sprout.
  • Production occurs mainly in the roots and young leaves
The effects of gibberellins in enhancing stem elongation are evident when certain dwarf varieties of plants are treated with gibberellins.
    • if applied to normal plants, there is often no response.

Abscisic acid

  • It inhibits plant growth during times of stress, such as cold temperatures or drought.
  • Closes stomata during times of stress
  • Promotes seed dormancy.
    • Overcome by the leaching of ABA in water
  • First thought to control abscission.
  • Gas
  • Promotes fruit ripening
  • Involved in flower production
  • Influences leaf abscission a

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



  • Response of stems and roots to the force of gravity.
  • It is important when seeds are sprouting.
  • Both Auxin and gibberellins are involved.
  • If stem is horizontal, auxin concentrates on the underside causing elongation of cells.

Gravitropism Clip




  • Ability of a plant to respond to light.
  • Auxin moves down stem on dark side causing elongation on cells.
    • Unequal distribution of auxin


  • The response of a plant to touch.
  • Climbing plants, ivy, and vines use thigmotropism in order to find their way up or around a solid object for support.
nastic movements
Nastic Movements
  • Nastic movements are rapid, reversible responses to non-directional stimuli (eg. Temperature, Humidity, Light irradiance). Nastic movement is caused by turgor pressure change due to movement of water in cells as opposed to tropic movement which is actual growth and therefore irreversible
Mimosa plant

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.

  • Light is an especially important factor in the lives of plants.
    • photosynthesis
    • light also cues many key events in plant growth and development.
    • light reception is also important in allowing plants to measure the passage of days and seasons.
  • Photoperiodism is the response of plants to changes in the photoperiod.
    • Photoperiod- relative length of daylight and night
Action spectra reveal that red and blue light are the most important colors regulating a plant’s photomorphogenesis.
    • These observations led researchers to two major classes of light receptors:
      • a heterogeneous group of blue-light photoreceptors
      • a family of photoreceptors called phytochromes that absorb mostly red light.
1 blue light photoreceptors are a heterogeneous group of pigments
1. Blue-light photoreceptors are a heterogeneous group of pigments
  • The action spectra of many plant processes demonstrate that blue light is most effective in initiating a diversity of responses.
2 phytochrome
2- Phytochrome
  • Phytochrome, a protein modified with light absorbing chromophore.

2 forms:

  • Pr (p660)and Pfr (p730)
  • They are photoresiversible
    • When one is exposed to the other wavelength, it will convert to the other.
    • This conversion helps the plant keep track of time.

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



This interconversion between isomers acts as a switching mechanism that controls various light-induced events in the life of the plant.

The Pfr form triggers many of the plant’s developmental responses to light.

Exposure to far-red light inhibits the germination response.

During the day Pr (p660)and Pfr (P730) are in equilibrium.

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.

In addition to phytochrome, another chemical called cryptochrome has been found to be responsible for initiating flowering as a result of exposure to blue light.
If there are short periods of dark during the day

no change

Flashes of red lightduring the night

resets the clock.

In the 1940s, researchers discovered that it is actually night length, not day length, that controls flowering and other responses to photoperiod.
Short-day plant will only flower when the light period shorterthan a critical length to flower.

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.

Short-day plants are actually long-night plants, requiring a minimum length of uninterrupted darkness.

Cocklebur is actually unresponsive to day length, but it requires at least 8 hours of continuous darkness to flower.

Red light is the most effective color in interrupting the nighttime portion of the photoperiod.

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.

Fig. 39.23

A higher proportion of FR light allows plants to detect when they are shaded.

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.