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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.
Ave. seedling length (mm)
0 100 50
5 100 27
10 80 13
15 90 7
20 90 10
25 100 5
30 40 5
50 30 2
75 10 1
100 0 0 0
20
40
60
80
100
120
0 5 10 15 20 25 30 50 75 100
Conc. of juice
% G
erm
.
0
10
20
30
40
50
60
0 5 10 15 20 25 30 50 75 100
Conc. of Juice
Av
era
ge
se
ed
ling
len
gth