Plant Growth Overview

Growth in Animals



Animals grow throughout the whole

organism

Growth in Plants



Specific regions of growth:

meristems

 stem cells: perpetually embryonic tissue

 regenerate new cells

 apical shoot meristem

 growth in length

 primary growth

 apical root meristem

 growth in length

 primary growth

 lateral meristem

 growth in girth

AP Biology

Root structure & growth

Shoot growth

Young leaf primordium

Apical meristem

Older leaf primordium

Lateral bud primordium



Apical bud & primary

growth of shoot

 region of stem growth

 axillary buds

 “waiting in the wings”



height from tip

 primary growth

 apical meristem



diameter from sides

 secondary growth

 lateral meristems

 vascular cambium

 makes 2° phloem & 2° xylem

 cork cambium

 makes bark

Growth in woody plants

Secondary growth



Secondary growth

 growth in diameter

 thickens & strengthens older part of tree

 cork cambium makes bark

 growing ring around tree

 vascular cambium makes xylem & phloem

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Vascular cambium

last year’s xylem

early late phloem

bark





cork cambium

vascular cambium

xylem

Why are early & late growth

Woody stem

vascular cambium

xylem early

late

phloem _bark

How old is this tree?

1

2

Tree trunk anatomy

tree girdling

What does girdling do to a tree?

Plant hormones



auxin



gibberellins



abscisic acid



ethylene

Auxin (IAA)



Effects

 controls cell division

& differentiation

 phototropism

 growth towards light

 asymmetrical distribution of auxin

 cells on darker side elongate faster than cells on brighter side

Gibberellins



Family of hormones

 over 100 different gibberellins identified



Effects

 stem elongation

 fruit growth

 seed germination

Abscisic acid (ABA)



Effects

 slows growth

 seed dormancy

 high concentrations of abscisic acid

 germination only after ABA is inactivated or

leeched out

 survival value:

seed will germinate only under optimal conditions

Cytokinins



Effects

 promote cell division

 prevents senescence

Ethylene



Hormone gas released by plant cells



Effects

 fruit ripening

 leaf drop

 like in Autumn

 apoptosis

Fruit ripening



 hard, tart fruit protects

developing seed from herbivores

 ripe, sweet, soft fruit attracts

animals to disperse seed



 triggers ripening process

 breakdown of cell wall

 softening

 conversion of starch to sugar

 sweetening

 positive feedback system

 ethylene triggers ripening

Plant Responses

•

Tropism

– Plant growth toward or away from a

unidirectional stimulus

• Positive tropism is growth toward the stimulus

• Negative tropism is growth away from the stimulus

– Gravitropism - movement in response to

gravity

– Phototropism - movement in response to

light

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Plant Responses

•

Gravitropism

– When a plant is placed on its side, the

stem grows upward, opposite of the pull of gravity

– Stems with root caps grow downward

• Response depends on sensors called statoliths

– Auxin may be responsible for:

Gravitropism

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a.

b.

gra

v

it

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Plant Responses



Phototropism

 Positive phototropism of stems

 Occurs because cells on the shady side of the stem elongate due to the presence of auxin

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Plant Responses



Thigmotropism

 Unusual growth due to contact with

solid objects

 Coiling of tendrils

 Thigmomorphogenesis occurs when the

entire plant responds to the presence of environmental stimuli

 Wind

Coiling Response

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Plant Responses



 Do not involve growth and

 Are not dependent on the stimulus direction



 Mimosa pudica

 Venus flytrap



 Occur daily in response to light and dark changes

Turgor Movement

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After pulvinus vascular tissue

Before

cell retaining

turgor _{cell losing} turgor

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Plant Responses



Circadian rhythms:

 Biological rhythms with a 24-hour cycle

 Tend to be persistent

 Rhythm is maintained in the absence of environmental stimuli

Sleep

Movements

and

Circadian

Rhythms

a.

b.

Prayer plant (night)

Morning glory (night) Morning glory (morning)

flowers flowers open

Circadian Rhythm

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Plant Responses

•

Photoperiodism

:

– Any physiological response prompted

by changes in day or night length

– influences flowering in some plants

– requires participation of a biological

clock and a plant photoreceptor called

Plant Responses



Phytochrome is a blue-green leaf

pigment that alternately exists in two

forms

 Phytochrome red (P_r) is inactive

 Phytochrome far-red (P_fr) is active



Conversion of forms allows a plant to

detect photoperiod changes



Also promotes seed germination and

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Phytochrome Conversion Cycle

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inactive P_r active P_fr

light-sensitive region

red light

far-red light

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35

Plant Responses

•

Flowering

– Flowering plants can be divided into three

groups based on their flowering status.

• Short-day plants flower when the day length is shorter than a critical length

• Long-day plants flower when the day length is longer than a critical length

• Day-neutral plants are not dependent on day length for flowering

– Some plants may require a specific

Photoperiodism and Flowering

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1 2 3 4 5 6

Cocklebur Clover

critical length day

a. Short-day (long-night) plant b. Long-day (short-night) plant night

flash of light

flower

flower flower

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Plant Responses



• Plants are always under attack by herbivores and

parasites.

• Physical and Chemical Defenses

– Cuticle-covered epidermis and bark

– Secondary metabolites

• Tannins

• Alkaloids

• Cyanogenic glycosides

– Wound responses – proteinase inhibitors and systemin

– Hypersensitive response (HR) – initiates wound

response and seals wounded area

– Indirect defenses – prevent egg laying in insects

Plant Predators and Parasites

Apoptosis in plants

What is the evolutionary advantage of loss of leaves in autumn?



 response to hormones

 ethylene

 auxin

 death of annual plant after

flowering

 senescence

 differentiation of xylem

vessels

 loss of cytoplasm