Title: 2023 PLANT HORMONES
URL Source: blob://pdf/0eac70f1-2680-43b1-9609-56c359ac3dda
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## PLANT HORMONES
> pg 207
> Control
Growth & Development Definitions
Growth = increase in number of cells due to cell division, followed by an increase in size of cells
Development = changing of shape (cell differentiation) that leads to a change in function (cell specialization) Plant Hormones
Chemicals that regulate plant growth, development and cell/tissue differentiation
Occur in very small amounts
Control processes
eg. growth, flower formation, shedding leaves, development & ripening of fruit
Plants lack glands the cells produce hormones Three Hormones
- Auxins - Gibberellins - Abscisic acid( ABA) 1. Auxins
Made in tip of shoots (apical bud/apical meristem)
(Small amounts also in root tips )
Auxins diffuse downwards from the tip of the shoot towards roots Are light sensitive (see phototropism) 1. Auxins
Functions
1. Tropic movements A. Phototropism
B. Geotropism
2. Apical dominance
3. Promotes growth => cell elongation & mitosis too much auxin (high concentration) inhibits cell elongation/growth Tropisms
Tropism: is growth of part of a plant in response to a stimulus.
Plants grow towards sources of water and light, which they need to survive and grow.
Auxin is a plant hormone produced in the stem tips and roots, which controls the direction of growth. Phototropism Auxins move AWAY from light
> NB: Pic pg. 208
Auxins and phototropism
Auxins are produced at the shoot tips .
They make the cells here elongate .
Causes plants to grow towards a light source Clinostat
Apparatus which has a small motor and can rotate slowly Used in practical demos Phototropism is the growth movement of a plant in response to light stimulus. When the stem of a plant grows towards a unilateral (one-sided) light stimulus it is positively phototropic. Auxins move to the dark, shaded side and stimulates cell elongation in that part of the stem. Cell elongation occurs more rapidly on the shaded side because of the higher concentration of auxins. The stem bends towards the light stimulus. Auxins and Geotropism
Auxins in root tips have the opposite effect to those in shoot tips they stop elongation of cells.
Encourages positive geotropism
Helps roots to anchor the plant in place
Geotropism: Growth in response to gravity Geotropism is the downward growth movement of a root in response to a unilateral gravitational force. When a root is placed horizontally (onto its side), auxin is drawn to the lower half of the root by the force of gravity. A high auxin concentration inhibits (prevents) root growth. Cell elongation takes place faster in the upper half of the root so cells on the upper side grow faster, causing the root to turn downwards. The root is positively geotropic. Stems are negatively geotropic because they grow away from gravity. The high concentration of auxin on the ventral (bottom) end stimulates cell division and elongation in STEMS thus the stem would grow upwards VIDEO:
Geotropism https://www.youtube.com/watch?v=Ti6ZKZ5-Y9M Responses of different parts of the plant
Response Part of plant (stem tip or root tip)
Direction of growth Advantage
Positive phototropism
Negative phototropism
Positive geotropism
Negative geotropism Apical dominance
[High auxin] near apical bud inhibits growth of lateral buds
If the apical bud is removed, the plant will become bushier High auxin concentration
Low auxin concentration
Drawings depicting Coleus (Lamiaceae family)
Evidence for the role of auxin in apical dominance 2. Gibberellins
Functions
1. Stimulate growth of stems (lengthens internodes)
2. Stimulate seed germination and cell elon gation
NB: Not involved with apical dominance
Play no role in tropic movements 3. Abscisic acid
Helps plant to cope with adverse conditions
Functions
Growth inhibitor :
a) Causes stomata to close
b) Promotes abscission (cutting off)
c) Promotes dormancy in seeds and buds
Dormancy stops
seeds germinating
until conditions are ideal for growth. Weed Control using hormones
Selective weedkillers kill some plants but not others => contain [high auxin]
This can be useful for getting rid of dandelions in a lawn without killing the grass
The selective weedkiller contains growth hormone
that causes the broadleaf weeds to grow too quickly => weaken and die
Does not harm narrow leaf plants eg. Grain crops Plant Defence Mechanisms
Require some form of
protection from
herbivores
eg. caterpillars, birds, giraffes
2 forms of defence:
Chemical defences
Physical defences Chemical Defences
Parts of the plant are unpalatable or poisonous to
herbivores :
1. Digestive enzyme inhibitors eg. tannins
2. Bitter tasting chemicals eg. caffeine
3. Toxins eg. pyrethrins
4. Stinging hairs eg. Stinging nettles
5. Essential oils eg. peppermint
6. Allergy causing chemicals
7. Antibacterial chemicals eg. garlic Physical Defences
Eg thorns, spines etc
Thorns slow down feeding rate, may contain pathogenic bacteria & fungi, warning colouration VIDEO:
How plants defend themselves https://www.youtube.com/watch?v=Hja0SLs2kus Responses of different parts of the plant