Plant responses

Plants need to be able to respond to the environment to avoid stresses.

These can be either biotic (being eaten) or abiotic (drought)

Plants can respond to these stresses in different ways:

• Chemical production
• Water Stress
• Tropisms
• Nastic responses

Plants are more likely to survive if they can deter their predators (herbivores, including insects) from eating them. They can produce toxic hormones to prevent this:

• Alkaloids- These have a bitter taste, noxious smells or are poisonous. E.g. nicotine in tobacco plants.

• Tannins - Make leaves bitter and hard to digest.

• Pheromones - Can trigger other plants to make tannins/ alkaloids or can attract the attention of predators.

• Plants close their stomata to reduce water loss by transpiration.

• Some plants can also produce a type of antifreeze in extremely cold conditions.

• The growth of plants in a particular direction in response to the external stimuli.
• Growing towards the stimulus is a positive response.
• Growing away from the stimulus is a negative response.

Types of tropism

• Phototropism - light
• Geotropism - gravity
• Hydrotropism - water
• Chemotropism - chemicals
• Thigmotropism - touch

• These are non-directional responses to external stimuli.

• E.g. Mimosa pudica folds its leaves in response to touch.

• Some of these responses shown by plants are caused by growth hormones.
• These can accelerate or slow down the plant's growth.
• They are made in the growing regions of the plant (shoot tips, leaves, etc) and move to the area of the plant they are needed.

This can be done by:

• Active transport
• Diffusion
• Mass flow in the sap in the phloem or in the water in the xylem.

• At these target cells the hormone binds to a receptor protein in the plasma membrane with a complementary shape.

Plant hormones are responsible for:

• Cell division
• Cell elongation
• Senescence
• Abscission
• Cell differentiation
• Seed germination
• Stomata closure

Examples of hormones include:

• Auxins, including indole-3-acetic acid
• Gibberellins
• Cytokinins
• Abscinic acid
• Ethene

Senescence - Ageing/ colour changing in the leaf.

It starts when the day length shortens 

Cytokinin production is reduced as the leaf stops acting as a sink for nutrients. This causes the nutrient level in leaf to decrease, leading to the colour change.

The process is regulated by the amount of cytokinin

The process is stopped when it moves into abscission.

Abscission -the falling of a leaf / fruit.

This process is started in winter.

• Leaf senescence causes auxin production at the tip of the leaf to decrease.
• This causes cells in the abscission zone to increase ethene production.
• This stimulates production of cellulase.
• Cell walls in abscission zone are digested, so abscission occurs.

The process is regulated by the amount of auxin.

The process is stopped when the leaf falls off.

Germination -Sprouting of the seed to grow into a new plant.

• Starch is hydrolysed into glucose, which is used in respiration by the plant.
• This produces ATP, which the plant can use to grow.

Gibberellins is the hormone involved.

The plant needs water and warmth for the process to occur.

It starts by absorbing water.

It starts because it initiates the production of amylase.

Stomatal closure -The closing of stomata to reduce water loss by evaporation.

It needs to be dry or windy for the process to occur.

• It starts because the water potential decreases.
• So root makes abseic acid which binds to receptors.
• Guard cell becomes less turgid and stomata closes.

• All plant growth happens in the meristem tissues - these tissues are made of immature cells that are still able to divide.

• In other tissues the cell walls are too developed to allow much division or expansion.

• Apical meristem- at the tip of the roots and shoots and cause them to get longer.

• Lateral bud meristems- Found on the side buds and can cause side shoots to grow.

• Lateral meristem- Form a cylinder around the outside of the roots and shoots enabling them to become thicker.

• Auxin is produced by the apical bud (tip of the plant).

• While the apical bud is intact lateral buds will not develop.

• This prevents side shoots competing for light.

• As energy is not being used to grow lateral shoots the apical shoot can grow very tall very quickly.

• Abscinic acid inhibits bud growth.
• It is thought that the high level of auxin in the bud keeps the abscinic acid high in the lateral buds.
• When the tip os removed the auxin levels drops, allowing lateral buds to grow.

Cytokinins

• Cytokinins promote bud growth- if cytokinins are applied directly to the lateral buds they wil begin to grow, over riding apical dominance.

• Remove the apical bud and see if lateral buds grow and replace the top with a gel containing auxin - lateral buds will remain inhibited.

• Apply an auxin inhibitor just below the tip - lateral buds will grow.

• In shoots auxin promotes cell elongation
• In roots auxin inhibits cell elongation.
• Light breaks down auxin so auxin builds up on the shaded side of the shoot/ root.

In the shoots:

• The cells inside furthest from the light grow faster, that side gets longer and the shoot bends towards the light.

In the roots

• The cells side furthest from the light grows slower, remains shorter and bends the root away from the light.

Darwin

• Tip removed- see if tip caused response.
• Control ( bent)
• Tip covered in opaque cap 
• Base covered in opaque wrap ( bent)
• Tip covered in transparent cap (bent)

Boysen-Jensen

Tips of two shoots were removed and replaced with different blocks

• Mica (impermeable)
• Gelatin (permeable) (bent)

• Gibberllins are the hormones responsible for stem growth, seed germination, side shoot formation and flowering.

• Gibberllins are inhibited by the presence of abscenic acid.

• The gibberlins cause stem growth by stimulating cell elongation.

• Apply gibberellic acid to different plants, including dwarf varieties.

• This resulted in the dwarf varieties growing as tall as non dwarf varieties.

• In shoots auxin promotes cell elongation
• In roots inhibit cell elongation.
• Gravity pulls the auxn down so auxin builds up on the bottom side of the shoots/roots.

In the shoots

• The cells on the bottom grow faster, that side gets longer and the shoot bends up away from gravity.

In the roots

• The cells on the bottom grow slower, that side remains shorter and the root bends downwards towards gravity. 

• Grow plants in an area of low/ no gravity.

• Grow plants in an area where gravity is cancelled out - klinostat.