Transcript Plant Growth

Plant Growth
Tropisms
• Plants grow in response to cues from their
environment. These responses are known
as tropisms.
• Geotropism
• Phototropism
• Thigmotropism
Geotropism
• Response to the force of gravity
• Roots turn towards the force of gravity
• Stems turn away from the force of gravity
Phototropism
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Response to light
Plants grow towards light
Response to daylight, respond to darkness
FYI: Darwin did some of the first
experiments investigating this tropism
Thigmotropism
• Response to touch
• Grow towards solid supports (ivy, beans,
vines)
• Adaptation to get more exposure to light,
escape crowding at ground level
Plant hormones
• Hormone: a substance produced in one part
of an organism that affects activities in other
parts of the organism (chemical
messengers)
• There are four major plant hormones:
auxin, cytokinin, gibberellin, ethylene
• Three of the four are classes of compounds
Auxin
• Stimulate cell growth and are produced in the
apical meristem (rapidly growing region at the tip
of a root or stem)
• This is the hormone that produces phototropism
• When light hits one side of a plant, auxin builds
up on the opposite side causing cells on that side
to grow more which bends the plant towards the
light.
• In stems: auxin stimulates growth to point the
plant upwards
• In roots: auxin inhibits growth causing roots to
grow downward
Cytokinin
• Affects the rate of cell growth and cell
division
• Ratio between auxin concentration and
cytokinin concentration determines growth
• Auxins inhibit growth in lateral branches
but cytokinin stimulates lateral growth
Gibberellin
• Stimulates cell division and elongation in
stems
• Cells in seed produce gibberellin to initiate
seed sprouting
Ethylene
• Stimulates ripening in fruit
• An auxin in fruit tissue stimulates the
production of ethylene, which in turn
stimulate ripening
• Commercial growers use ethylne to ripen
fruit that is harvested before it is ripe
Life Cycles
• Annuals: grow from seed to maturity in one
season
• Biennials: flower and produce seeds in the
second year
• Perennials: live for more than two years
Timing is everything for a plant
• Reproductive timing must be in sync with
all the others in the species
• Plants must store food for the winter in
temperate climates
• Plants must regulate water loss in
anticipation of the dry season
Plant Master Clock
• Phytochrome (a red pigment) senses day and night
• Abscisic acid is a hormone regulated by
phyochrome
– As nights grow longer, chlorophyll production stops
– Nutrients are drawn from leaves to the body of the plant
• Abscisic acid causes the leaves to drop
– Tips of branches grow thick waxy covers (bud scales)
over the tips to protect the meristem from the winter
weather
• Plant adds salts into the sap to keep the water in
the sap from freezing