Plant Responses to The Environment
Introduction
● Plants respond to stimuli like water, sunlight, gravity, chemicals + touch
● They use chemicals (hormones - plant growth regulators) which affects it’s growth by
stimulating plant cells to divide, enlarge, elongate + stop growing
Key Terminology:
Hormone ● Chemical messenger produced in 1 part of plant + has effect on
another part of stem
Tropism ● Growth/turning movement of plant/part of a plant in response to
external stimuli
Phototropism ● Growth of part of plant in response to stimulus of light
Geotropism ● Growth of plant in response to gravity
Unilateral light ● Light coming from 1 side/direction
Herbicide ● Substance that’s toxic to plants + destroy unwanted vegetation
(weeds)
Apical bud ● Growing point of stem located at tip of stem
Apical dominance ● When auxins produced at tip of stem inhibit the growth of branches
closer to tip of stem
Auxins
● They’re produced in growing tips of stems + roots and in apical buds = main hormones that
cause plants to grow
● They stimulate/prevent (inhibit) growth
● Main functions:
○ Apical dominance
○ Growth regulation (responsible for phototropism + geotropism)
● Other functions:
○ Cell division (mitosis)
○ Formation of adventitious roots in cuttings
○ Development of flowers + fruits
○ Abscission of leaves + ripe fruits (cause leaves + fruit to fall off)
Apical Dominance:
● When stem grows up, auxins = produced at tip of stem, the auxins then move down + inhibit
the growth of lateral branches (apical dominance) - the auxins promote it
, ● Results of apical dominance:
○ Lateral branches closer to tip of stem = inhibited by auxin, resulting in them being
shorter than lateral branches lower down the stem
○ Results in tapered growth of stem (wider at bottom but narrower at top)
What happens when tip is removed?
● Auxins are removed so lateral branches begin growing
○ It’s useful for fruit industry where plants are kept short with more lateral branches so
fruit is closer to the ground + easier to pick
● Remember: removal of tip of stem stops apical dominance
Growth Regulation (tropisms):
● The response to an external stimulus = tropism
● When plant moves away from stimulus = negative tropism
● When plant moves towards stimulus = positive tropism
● 2 ways that plants responds to stimuli:
○ Phototropism - response to light
○ Geotropism - response to gravity
Phototropism
● Growth of parts of plant towards light stimulus when stem of plant = exposed to unilateral
light, the stem bends towards light
Role of Auxins in Phototropism:
Direction of Light Stimulus Effect of Light on Auxins Observations
● Shoot A - sunlight ● Auxins produced at tip ● stem/shoot A grows
directly overhead of stem/shoot straight upwards
● Auxins move down towards light
evenly
● This distributions
brings equal growth on
sides of stem
Introduction
● Plants respond to stimuli like water, sunlight, gravity, chemicals + touch
● They use chemicals (hormones - plant growth regulators) which affects it’s growth by
stimulating plant cells to divide, enlarge, elongate + stop growing
Key Terminology:
Hormone ● Chemical messenger produced in 1 part of plant + has effect on
another part of stem
Tropism ● Growth/turning movement of plant/part of a plant in response to
external stimuli
Phototropism ● Growth of part of plant in response to stimulus of light
Geotropism ● Growth of plant in response to gravity
Unilateral light ● Light coming from 1 side/direction
Herbicide ● Substance that’s toxic to plants + destroy unwanted vegetation
(weeds)
Apical bud ● Growing point of stem located at tip of stem
Apical dominance ● When auxins produced at tip of stem inhibit the growth of branches
closer to tip of stem
Auxins
● They’re produced in growing tips of stems + roots and in apical buds = main hormones that
cause plants to grow
● They stimulate/prevent (inhibit) growth
● Main functions:
○ Apical dominance
○ Growth regulation (responsible for phototropism + geotropism)
● Other functions:
○ Cell division (mitosis)
○ Formation of adventitious roots in cuttings
○ Development of flowers + fruits
○ Abscission of leaves + ripe fruits (cause leaves + fruit to fall off)
Apical Dominance:
● When stem grows up, auxins = produced at tip of stem, the auxins then move down + inhibit
the growth of lateral branches (apical dominance) - the auxins promote it
, ● Results of apical dominance:
○ Lateral branches closer to tip of stem = inhibited by auxin, resulting in them being
shorter than lateral branches lower down the stem
○ Results in tapered growth of stem (wider at bottom but narrower at top)
What happens when tip is removed?
● Auxins are removed so lateral branches begin growing
○ It’s useful for fruit industry where plants are kept short with more lateral branches so
fruit is closer to the ground + easier to pick
● Remember: removal of tip of stem stops apical dominance
Growth Regulation (tropisms):
● The response to an external stimulus = tropism
● When plant moves away from stimulus = negative tropism
● When plant moves towards stimulus = positive tropism
● 2 ways that plants responds to stimuli:
○ Phototropism - response to light
○ Geotropism - response to gravity
Phototropism
● Growth of parts of plant towards light stimulus when stem of plant = exposed to unilateral
light, the stem bends towards light
Role of Auxins in Phototropism:
Direction of Light Stimulus Effect of Light on Auxins Observations
● Shoot A - sunlight ● Auxins produced at tip ● stem/shoot A grows
directly overhead of stem/shoot straight upwards
● Auxins move down towards light
evenly
● This distributions
brings equal growth on
sides of stem
