1. Plant tropisms and hormonal control
Chapter 14

2. Aims:
> describe the hormonal system of plants & list the 5 main groups
> environmental stimuli & plant response
> phototropism, geotropism & thigmotropism = movements
> how light effects seedlings
> rel’ship btw photoperiod & flowering in plants
> turgor (water pressure) leads to movement in plants
> describe bud dormancy, seed dormancy & vernalisation

3. Plants
> fixed position (therefore have to make do with the environment)
> have to become TOLERANT of changes
> communication between cells is controlled by HORMONES

4. What is a Hormone?
A c_____ produced by cells in the plant that can be transported to other cells in the plant.
Hormones help with the TIMING of responses (eg. when to flower or produce new buds)
Hormones TARGET particular cells for them to RESPOND. Less specific in plants than in animals.

5. Tropisms A DIRECTIONAL growth response of a plant to a stimulus.
Growth is either towards or away from a stimulus

6. The 5 things that PLANT hormones are responsible 4:
1) Phototropism – growth in response to light. Photo = light.
2) Geotropism (Growth in response to Gravity)
3) Apical dominance – stopping lateral branches from growing – gardeners would rely on this hormone if they wanted their plant to grow TALL without side branches
4) Ripening of fruit
5) Abscission – shedding of leaves and flowers (particularly
with deciduous trees – in autumn)

7. Shedding of leaves – Deciduous trees!

8. 5 types of HORMONES to affect the changes of the plant
> 1) Auxins: produced in the meristem (tip of plant), concentrates on the opposite side of the plant to the light source & when in excess it INHIBITS buds on the sides
> 2) Gibberellins – whole plant growth
> 3) Cytokinins – cell division
> 4) Inhibitors – one type = abscisic acid (ABA) – helps plants to adapt to their enviro. STOPS growth. Concentration increases in stressed conditions. Therefore conserve resources.
> 5) Ethylene – for ripening fruits and flowers. It’s a gas.

9. Hormone Where produced Effective site Action Visible effect
Auxins
Shoot tip (meristem)
Growing region of shoot
Cells elongated under turgor pressure
Tip bends towards light; apical dominance; used as herbicides; stimulate cuttings to root
Gibberellin
Fruits, seeds growing buds, elongated stems
Roots, shoots, seeds; also found in fungi
Growth of cells
Shoot elongation, germination of seeds, flowering fruit enlargement
Cytokinins
Roots and developing fruits
Branch and leaf buds; moves through xylem and phloem
Stimulates cell division, cell enlargement and tissue differentiation
Growth of lateral branches; used to promote life of vegetables in storage
Abscisic acid
chloroplasts
Gene expression in nuclei
Growth inhibition
Seed dormancy; vernalisation; drought-tolerance
Ethylene
Ripening fruits; flowers, seeds, leaves and roots
Cellular metabolism
Fruit ripening; leaf drop
Fruit ripening; leaf and fruit drop; used commercially to ripen bananas and pineapples

10. Environmental Stimuli
What is a stimuli?
2 types: chemical & physical
PHOTOTROPISM – seed growth towards light – mostly BLUE light. What wavelength?
GEOTROPISM – shoots grow away from gravity (negatively geotropic)
THIGMOTROPISM – touch & contact with a surface (eg. vines twirl around objects).
Create a table that summarises the similarities and differences between these different types if tropisms.

11. Photoperiod
Length of the NIGHT affects growth = PHOTOPERIOD. Short day & long day plants. Therefore can only flower in certain environments with the RIGHT night length.
Note: Day length does NOT affect growth

12. Turgor Turgor = water pressure.
Stomata -> respond to the pressure by opening or closing.
Respond to the levels of CO2, light, humidity (water in the air)

13. Dormancy
Seeds remain dormant until conditions are RIGHT (dormant during winter).
Vernalisation = exposure to cold conditions to break seed dormancy

14. Questions:
How do we use plant responses to our advantage in agriculture?
What is the advantage of being anchored to the ground if you are alive?
Answers:
Via location, soil moisture, sunlight
Constant nutrient access