Plant responses to environment

Question 1

what stimuli can plants detect

Answer 1

chemicals
gravity
light
moisture
temperature
infections
CO2 and O2 concentrations
parasite infestation
physical disruption and touch

Question 2

what are plant hormones

Answer 2

chemicals occurring in low concentrations which regulate plant growth, development and differentiation

Question 3

what makes plants and animals different in terms of hormone production

Answer 3

plants lack glands , instead each cell is capable of producing hormones

Question 4

what are auxins and name the most common one

Answer 4

one of the most important groups of hormones in plants (are light sensitive)
- Indoleacetic acid (IAA)

Question 5

Briefly explain the gradient along the main axis of the plant caused by auxins

Answer 5

1. auxins constantly are made by cells in the tip of a shoot (in the apical bud)
2. very small amounts are produced there
3. the auxins diffuse downwards towards the roots
4. the gradient can be described as the highest concentration of auxins being at the tip and the lowest in the root.

Question 6

List the 3 functions of auxins

Answer 6

Auxins…

1. promote growth
2. cause apical dominance
3. play a part in tropic movements

Question 7

Explain auxins role in promoting growth

Answer 7

they stimulate cell division (mitosis) and cell elongation = growth

[the further stem tissues are from the shoot = more sensitive to auxins therefor wont need as much for cell elongation] if auxin concentration too high it gets stopped

Question 8

what is apical dominance

Answer 8

when higher auxin concentrations in the apical bud inhibits the growth of the higher lateral buds. This is because development of the lateral buds would result in competition for light and nutrients with the apical bud.

Question 9

Explain how auxins cause apical dominance

Answer 9

Lower down the plant the auxin concentration lessen so that lower lateral buds produce shoots which grow in lateral branches ( causing a pyramid shape)

if the apical bud is removed the lateral buds will no longer be inhibited by auxins and a bushier plant will develop( known as pruning)

Question 10

What part do auxins play in tropic movements

Answer 10

external stimuli ( phototropism and geotropism) bring about tropic movements. This is because Auxins cause cells to elongate on only one side of the stem/root = uneven plant growth.

Question 11

What are tropic movements

Answer 11

when a plant detects where conditions are best and responds by altering their growth in order to “move” to the more favourable condition.
- this all depends on the direction of the stimulus and results in differential growth

Question 12

What are tropisms

Answer 12

growth movement of part of a plant in response to environmental stimulus ( can be a negative or positive growth)
phototropism : growth response to light
geotropism : growth movement of part of plant in response to gravitational pull of the earth

Question 13

How do shoots respond to light

Answer 13

Auxins move away from a light source (light sensitive)

1. light comes from one direction
2. auxin from the growing tip move away from the light
3. results in a higher concentration of auxin in cells on shady side
4. increased concentration causes the cells to elongate more than those on the bright side
5. this causes the shoot to bend towards the light

Question 14

How do shoots and roots respond to gravity

Answer 14

auxins accumulate on the lower side of the shoot and root, in the
shoot:
the high concentration causes more growth to take place on lower side = upward curvature
main stems are negatively geotropic
root:
growth on the lower side is inhibited due to the increased concentration of auxin, cells on the top of the root where concentration in less will elongate and grow downwards
primary roots are positively geotropic

Question 15

Name 2 other plant hormones

Answer 15

Gibberellins - Gibberellic acid
Abscisic acid (ABA)

Question 16

name two functions of gibberellic acid

Answer 16

1. cause elongation of main stem, by causing internodes to lengthen
2. stimulate seed germination, seed absorbs water and embryo produces GA which causes stored starch in the seed to be broken down into glucose…eventually diffusing into the embryo for growth (germination)

Question 17

What makes Gibberellins and Auxins different

Answer 17

gibberellins have nothing to do with apical dominance or tropic movement

Question 18

What is the role of Abscisic acid

Answer 18

it is a growth inhibitor and plays a role in helping plants to adapt to adverse conditions by:

1. causing stomata to close when soil water insufficient to keep up with transpiration
2. promoting dormancy in seeds and buds when conditions unfavourable . also converts apical bud into dormant state to protect from mechanical damage + drying out
3. promotes abscission of leaves and fruits when env. conditions become unfavourable. (abscission-cutting off)

Question 19

How can hormones be used for weed control

Answer 19

weed killer with high concentrations of auxins

1. kills broad leaf weeds by speeding p growth so fast = weakens and dies
2. no harm to monocotyledons ( narrow leaf plants) e.g. grasses and grains

Question 20

Give examples of defensive chemicals

Answer 20

1. digestive enzyme inhibitors e.g. tannins in fynbos
2. bitter-tasting chemicals e.g. alkaloids (nicotine)
3. toxins that kill e.g. pyrethrins from pyrethrum flowers which attack the NS
4. stinging hairs
5. essential oils e.g. peppermint
6. allergy causing chemicals
7. antibacterial chemicals e.g. garlic to counteract pathogenic bacteria

Question 21

Give examples of physical defenses in plants

anti-herbivore mechanisms

Answer 21

1. thorns
2. pathogenic bacteria and fungi, found on thorns resulting in anthrax/tetanus
3. warning colouration of thorns e.g. white thorns of Acacia karoo