Plant Growth Responses Chapter 26

Tropisms

Plant growth toward or away from a stimulus is called a tropism

The organ of the plant (stem, petiole, etc) curves because one end grows faster than the other end

Phototropism

• Phototropism is movement of plants towards a light source

– Positive phototropism: toward the light

– Negative phototropism: away from the light

• When the light is received by a cell, the hormone auxin is transported to cells on the shady side of the plant that are not receiving light

– In the presence of auxin, cells elongate faster.

– This forces the plant to curve away from the auxin and towards the light

• What we don’t know is how transduction (the signal from the stimulus to the response) occurs.

• In other words, we don’t know what triggers auxin to move.

Gravitropism

Gravitropism is growth according to gravity

Positive tropism: with gravity

Negative tropism: against gravity

Auxin, again, moves to the lower or higher part of the cell depending on whether the plant is growing positively (roots) or negatively (stems)

Thigmotropism

• Growth due to contact with solid objects is called thigmotropism

• The plant grows straight until it touches an object

• Cells in contact with the object grow less while the cells on the opposite side elongate, creating a coil

– Some tendrils coil so fast they are capable of a full circle in 10 minutes

• Responses to contact can be immediate or delayed

• Responses are powered with light, ATP, or both

• Thigmotropism can be artificially stimulated in plants as well by touching or stroking the plant

https://www.youtube.com/watch?v=H9MV5CgPgIQ

Nastic Movements

• A nastic movement is a response to touch, shaking, or thermal stimulation

• Plants with nastic movements (mimosa, Venus flytraps), have some sort of nerve impulse system involving K+ ions

• Whatever the communication is, we’re still not sure.

• Communication must exist between different parts of the plants because a single stimulation (needle-prick in one leaf, for example) causes a response in all other parts of the plant

http://www.youtube.com/watch?v=PRo4rg07_gg&feature=autoplay&list=PLCD554EFB6B7041B9&lf=results_main&playnext=2

https://www.youtube.com/watch?v=Zq3UuHlPLQU&feature=results_main&playnext=1&list=PLCD554EFB6B7041B9

Auxin

• Auxins are hormones that prevent growth of axillary buds or roots.

– When a terminal bud or root is removed, the auxin then allows for branching to occur

• As discussed, auxins are also involved in phototropism and gravitropism

– Auxins bind to receptors and activate the ATP-proton pump of cells

– As H+ ions are pumped out the cell wall becomes acidic, breaking cellulose fibrils

– As the cellulose breaks, water enters the cell and turgor pressure increases causing the cell to expand

– The cells are then repaired with the new elongated cells fully erect and the plant fully curved

Gibberellins

• Gibberellins are growth promoting hormones that end seed dormancy and trigger sprouting

• Seeds have a large starchy endosperm which must be broken down to provide energy for initial growth of the plant

– Remember: the seed may not have access to light so the first rounds of growth cannot be done with photosynthesis

– The starch breakdown provides glucose for cellular respiration

• Scientists have observed amylase enzymes will break down the starch into glucose only in the presence of gibberellins

– What the connection is between presence of gibberellins and appearance of amylase, we’re not sure of yet

Cytokinins and Senescence

• Cytokinins are a class of plant hormones that promote cell division

• Cytokinins activate enzymes to form and attach sections of cell walls (a requirement for mitosis)

• Cytokinins also control the speed of senescence (the process of aging)

• During senescence, large molecules within dying cells are broken down and transported to other parts of plants

– Lower leaves being broken down to build leaves higher up on the stem, for example

Abscisic Acid (ABA)

• ABA is the “stress” hormone of plants because it involves seed and bud dormancy and forces stomata to close

– Dormancy is the process of a plant readying itself for adverse conditions by stopping growth

• ABA moves into vegetative buds in autumn, forcing leaves to fall and converting the buds into winter buds.

– Winter buds are covered by thick, hardened scales for protection during hibernation

• Reducing ABA promotes seeds to germinate and buds to begin leaf production

Ethylene

• Ethylene is the hormone that triggers abscission, or the dropping of leaves, fruits and flowers from a plant

• Abscission is caused by breaking down of cellulose, and this enzyme (cellulase) is triggered by ethylene

• Ethylene is a unique hormone that can travel through the air and induce abscission in plants over long distances

– If one fruit in your fridge or counter top begins to ripen and/or go bad, it triggers the release of ethylene, which initiates ripening or over-ripening in the other fruits as well.

– Hence the phrase, “one bad apple spoils the whole barrel”

Photoperiodism

• A physiological response prompted by changes in the length of day or night is called photoperiodism

• Angiosperms begin growing flowers based on the length of time their leaves receive sunlight in a single stretch of time (moonlight is not enough to affect these hormones)

• Each plant has a critical length of time

– The critical length is the maximum or minimum amount of time a plant can be exposed to light before blooming

• Short-day plants: flowers grow when the day length is SHORTER than the plant’s critical length (chrysanthemum, poinsettia)

• Long-day plants: flowers grow when the day length is LONGER than the plant’s critical length (wheat, spinach)

• Day-neutral plants: flowers grown independently of day length (tomato, cucumber)

Phytochrome and Plant Flowering

• How do plants detect the amount of light in a day?

• Plants have a pigment called phytochrome which exists in two forms

– Pr (phytochrome red). Absorbs red light and is converted to Pfr.

– Pfr (phytochrome far-red). Absorbs far-red light and is converted to Pr.

• Direct sunlight contains more red light than far-red light. Therefore, plants will contain more Pfr in the day.

• At shade and sunset, there is more far-red light, so plants will contain more Pr.

• Plants are thus able to detect the presence and amount of Pr and Pfr and deduce the length of a day.

Phytochrome and Plant Flowering

• Phytochrome is also involved in seed germination and plant flowering signals

– Telling seeds when enough sunlight is present for germination

– Some seeds are inhibited by light and others aided. Therefore, being familiar with each plant is important for gardening.

• The presence of phytochrome is also a part of stimulating activation of other proteins and pigments in chloroplasts

– Only when the plant detects enough sunlight is present will mesophyll activation occur and photosynthesis can begin.

Extra Credit Question

This question is worth an extra 5% on your essay exam

You may check your answers with me ahead of time for a yes or no response as many times as you like.

Welwitschia plants grow in the Namib desert, near the African coast on the Atlantic ocean. Only 1 cm of rain falls each year, yet each plant can survive for 1000’s of years and grows as tall as a human. Explain how.