Contents: Environment

Plant Movements

Phototropism

Tropism Mechanism

Other Tropisms

Nastic Responses

Hormones

Timing

Plant Rhythms

Flowering

Phytochrome

More Phytochrome

Plant Relationships

PLANT RESPONSES US 8934 Relate

aspects of plant responses

to environmental

factors

AND

AS 90716 Describe animal

Behaviour and Plant

Responses to Environmental

factors

ENVIRONMENT

Biotic Factors:Intraspecific

CompetitionCooperationAggressionReproduction

InterspecificCompetitionExploitationMutualismCommensalism

Abiotic Factors:LightWaterWindHumidityNutrientsGravityTemperature

The environment of an organism is made up of all of the factors that affect it. A habitat is a zone with a certain range of factors, such as a savannah.

An organism has adaptations that help it survive in its habitat. The better adapted it is, the more chance it has of successful reproduction – survival of the fittest.

PLANT RESPONSESPlants can move to respond to the environment. There are two types of movement…

TROPISMS are growth responses, and act in a direction relative to the stimulus (positive or negative).

NASTIC RESPONSES are fast, reversible movements and are non-directional.The stimuli for growth may be:

Light

Water

Gravity

Chemicals

Touch

Temperature

This is called a:

Photo

Hydro

Gravi/Geo

Chemo

Thigmo

Thermo

Nasty

or

Tropism

For instance: the opening and closing of flowers during the day is a type of…

Photonasty

Manual pg 178/9

PHOTOTROPISMA well-studied example of a tropism is Phototropism. Many experiments have been done using coleoptiles (growing shoots).

What is the adaptive advantage of this response?

Increased light… more photosynthesis.

Check out some time-lapse footage of TROPISMS.

POSSIBLE TROPISM MECHANISMAn explanation for what is happening inside the plant is that a hormone called auxin (or IAA – indoleacetic acid) controls the elongation of cells.

Auxin sent out from the tip – encourages

cell elongation.

Light breaks down auxin on the sunny side.

Cells on the shady side elongate more – plant

bends.

Manual pg 180

AUXINAuxin has a number of functions, and can affect different cells in different ways.

Auxin naturally moves down the plant due to gravity and so collects on the lowest surfaces of stems and roots.

In the stem it promotes cell elongation, leading to negative geotropism.

In the roots it inhibits elongation → positive geotropism.

As it comes from the tip, the concentration decreases down the plant. In high conc. It inhibits lateral buds, in lower conc. It promotes the buds. This leads to apical dominance.

Removing the tip of a plant makes it grow bushy.

OTHER TROPISMS

Name Mechanism Advantage

Hydrotropism

Thigmotropism

Chemotropism

What is the advantage of this?

Stem: Gets light, PS, reproduce more

Roots: Gain water, nutrients, stability.

Manual pg 181

Roots are positively hydrotropic

Collect more water

Some stems are positively thigmotropic and grow around other objects

Roots, either positive or negative

Increased light without energy output on structure

Gain or avoid chemicals

NASTIC RESPONSES

They provide much faster response to a stimulus than tropisms.

They are often controlled by water (turgor) pressure.

Some examples are:

The opening and closing of stomata

Sleep movements (leaf droop at night time)

The opening and closing of flowers during the day

Manual pg 182

Check out some time-lapse footage of NASTIC RESPONSES (esp. 1st three and last one - sunflower nutation)

HORMONESPlant hormones (unlike animals’) can produce a number of responses, many of which overlap, and interact with other hormones to promote or inhibit them, depending on the conditions.Some of the main aspects of each type…

Auxins (IAA) Cytokinins Gibberellins Abscissic acid (ABA)

Ethylene (Ethene)

Role

Promote cell elongation in stems, inhibit in roots. (And others)

Cell division, lateral bud growth. Inhibits leaf drop.

Stem elongation.

Promotes leaf fall abscission).Inhibits some other hormones, and germination, closes stomata in drought.

Promotes fruit ripening, abscission of fruit leaves and flowers.

Plac

e

Growing points (tips)

Root tips, unripe fruit

Growing points (tips)

All over. All over (aging / damaged areas)

Transport

From shoot and root tips to other places by active transport.

Through Xylem and Phloem

Through Xylem and Phloem

Through Xylem and Phloem

Diffusion

Normal growth Fast growth Protection Ripening

Manual pg 183/4

TIMINGBoth plants and animals need to be able to detect time in order to coordinate activities in appropriate conditions.

There are a number of environmental cues.

These include astronomically caused cycles, such as...

• The earth orbiting the sun (seasons) “circannual”

• The Moon orbiting the Earth (phases of the moon) “circalunar”

• The Earth spinning on its axis (day/night) “circadian”

Manual pg 187

PLANT RHYTHMS

Manual pg 195

A plant’s “Biological Clock” is controlled by either:endogenous factors (inside the plant – genetic)exogenous factors (outside the plant) – environmental

Some circadian rhythms are: flower opening and closing, leaf tilt, stomata opening and closing, and sun tracking.

Some seasonal rhythms are: acclimationdormancyvernalisationstratification

Most seasonal rhythms are in response to temperature, moisture or photoperiod.

Find definitions for these on page 196 of the manual.

FLOWERINGFlowering (as well as a number of other responses) is controlled by the phytochrome system.

LeafDetects light

Flower budBegins

developmentMessage sent with

hormone called Florigen (theoretically).

Phytochrome has 2 forms:

Pr is “phytochrome red”Pfr is “phytochrome far-red”

Pr Pfr

Fast with red light (from the sun)

Slow overnight

PHYTOCHROME SYSTEM

Pr Pfr

Fast with red light (from the sun)

Slow overnight

Pfr → Pr can also be achieved quickly (for experimental purposes) by using far-red light (hence it’s name).

Pfr promotes flower growth in long-day (summer) plants.

Pfr inhibits flower growth in short-day (winter) plants.

Day neutral plants tend to flower all of the time.

After a short night (summer) there will still be

lots of Pfr left.

After a long night (winter) there will

be lots of Pr.

Fast with far red light

Confused?

DON’T PANIC!

MORE ON PHYTOCHROMESome experiments on the phytochrome system:

Manual pg 197-8

Short day plant

Long day plant

PLANT RELATIONSHIPSPlants compete with other plants for resources, such as light, water, and minerals. Plants can reduce competition by producing chemicals that harm other species. This is called allelopathy.

Some plants also protect themselves from herbivory by using spines, waxy cuticles, hard seed coats, and divaricating habit.They may also produce chemicals that are toxic or unpalatable to animals. In response, some animals have produced a resistance to these chemicals (coevolution).

Plants may also undergo commensalism, mutualism or parasitism.

GERMINATIONWhy don't tomato (and other) seeds germinate when they are still in the fruit? Perhaps there is some sort of chemical inhibition going on that stops the seed growing until it has been eaten or the fruit has rotted down.

The plant practical on tomato seed germination.

GERMINATION PRAC. RESULTSConc. of juice

% Germ.

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