Download Factors which influence plant growth • Environment • Plant Hormones

Factors which influence plant growth
• Environment
– Irradiation, Day-length,
Temperature, Water availability,
Gases
– Soil, Nutrients
• Plant Hormones
Light Intensity
Light Quality
Photomorphogenesis
Photoperiodism
• Day-length response in plants.
• Can affect:
– Flowering
– Bud dormancy in woody plants
– Formation of vegetables (i.e. storage organs like
rutabaga, potato, etc.)
Gases
• Atmospheric gasses are composed of
78% nitrogen (N2) = 780,000 ppm.
• 21% oxygen (O2) = 210,000 ppm.
• 0.035% carbon dioxide (CO2) = 350
ppm.
USA Temperature Change
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2006
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Precipitation and Temperature
Moscow, Idaho
Temperature (o C)
Temperature
• All plants have a minimum temperature
below which there is no plant growth –
often below 4oC (39oF).
• Plants also have a maximum
temperature limit whereby plants cease
to function – usually no higher than
50oC (122oF).
Temperature
• Low temperature damage is either chill
injury or freeze injury depending on cold
severity.
• Windmills in orchards and vineyards can
be used to mix air to prevent chill
damage.
• Low temperature damage can also be
prevented using (snow) mulches,
covering with plastics, and cold frames.
Beneficial Cold Treatment
• Stratification
• Vernilization
• Dormancy breaking
Growth
Hormones
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Auxins
Cytokinins
Gibberellins
Ethylene
Abscisic acid
Brassinosteroids
Jasmonic acid
Auxins
• The term auxin is derived from the Greek word
‘auxein’ which means to grow.
• Compounds are generally considered auxins if they
can be characterized by their ability to induce cell
elongation in stems and otherwise resemble
indoleacetic acid (the first auxin isolated) in
physiological activity.
• Auxins usually affect other processes in addition to
cell elongation of stem cells but this characteristic is
considered critical of all auxins and thus "helps"
define the hormone.
Auxins
• Stimulates cell elongation.
• Mediates the tropistic response of
bending in response to gravity and
light.
Indol-3-acetic acid - IAA
Phototropism
Auxin
• Charles Darwin was one of the first to examine
phototropism in plants.
• Studies showed that growth towards light is caused by
elongation of the cells at the side that is shielded from
the light.
• The phototropic reaction does not happen if the
coleoptile’s tip is removed, though it can be induced
again by the replacement of the tip. This indicates the
existence of a substance that is spread from tip to
bottom and that causes the elongation.
Phototropism
Gravatropism
Gravity causes localized
expression of auxin in
plant roots. Plants
grown on an angled
show increased
accumulation of growthpromoting auxin on the
lower side of the root
(green staining). Plant
roots will grow in the
direction of increased
auxin expression
Gravatropism
Auxin
No auxin
Auxins
Normal Fruit
(seed)
Development
Seed
Removed
Seed
Removed
auxin applied
2,4-Dichlorophenoxyacetic acid (2,4-D)
2,4-Dichlorophenoxyacetic acid (2,4-D)
•
•
•
2,4-D became the first successful selective
herbicide and allowed for greatly enhanced weed
control in cereal crops.
2,4-D was developed in Britain during World War
II to increase crop yields for a nation at war.
2,4-D is a synthetic auxin. It is absorbed through the
leaves and is translocated to the meristems of the
plant. Uncontrolled, unsustainable growth ensues
causing stem curl-over, leaf withering, and eventual
plant death.
Cytokinins
•Cytokinins are compounds with a structure
resembling adenine which promote cell
division and have other similar functions to
kinetin.
•Kinetin was the first cytokinin discovered and
so named because of the compounds ability to
promote cytokinesis (cell division).
•The most common form of naturally occurring
cytokinin in plants today is called zeatin which
was isolated from corn (Zea mays).
Cytokinins
•Cytokinin is generally found in higher
concentrations in meristematic regions and
growing tissues.
•They are believed to be synthesized in the roots
and translocated via the xylem to shoots.
Zeatin
Cytokinin
• Stimulates cell division.
• Stimulates morphogenesis (shoot
initiation/bud formation) in tissue
culture.
• Stimulates the growth of lateral
buds-release of apical dominance.
A transgenic tobacco plant (right) with a high content of
cytokinin shows fewer signs of senescence compared to a
wild type plant (left) in which all the lower leaves are dead.
The ratio of auxin to cytokinin in a
tissue dictates growth of axillary
meristems:
• High auxin / Low cytokinins =
meristem remains dormant;
• Low auxin / High cytokinins =
meristem starts to under go cell
division and starts to grow.
Auxin:Cytokinin Association
0.0
0.03
0.18
Auxin Concentration
3.0
Gibberellins
• Unlike the classification of auxins which are
classified on the basis of function, gibberellins
are classified on the basis of structure as well
as function.
• The gibberellins are named GA1....GAn in
order of discovery. Gibberellic acid, which was
the first gibberellin to be structurally
characterised , is GA3. There are currently 136
GA’s identified from plants, fungi and bacteria.
Giberellic acid
• Stimulate stem elongation by stimulating cell
division and elongation.
• Stimulates bolting/flowering in response to
long days.
• Breaks seed dormancy in some plants which
require stratification or light to induce
germination.
GA1
GA3
Control
Giberellic acid
•Stimulates enzyme production (α-amylase)
in germinating cereal grains for
mobilization of seed reserves.
•Induces maleness in dioecious flowers (sex
expression).
•Can cause parthenocarpic (seedless) fruit
development.
•Can delay senescence in leaves and citrus
fruits.
Gibberellins Inhibitors
Abscisic Acid
• Stimulates the closure of stomata (water stress
brings about an increase ).
• Inhibits shoot growth but will not have as
much affect on roots or may even promote
growth of roots.
• Induces seeds to synthesize storage proteins.
ABA
Ethylene
H2C=CH2
• Ethylene, unlike the rest of the plant hormone
compounds is a gaseous hormone.
• Like abscisic acid, it is the only member of its
class.
• Of all the known plant growth substance,
ethylene has the simplest structure. It is
produced in all higher plants and is usually
associated with fruit ripening.
Ethylene
• Ethylene has been used in practice since the
ancient Egyptians, who would gas figs in order
to stimulate ripening.
• The ancient Chinese would burn incense in
closed rooms to enhance the ripening of pears.
• In London in 1864, gas leaks from street lights
showed stunting of growth, twisting of plants,
and abnormal thickening of stems
Harvest-aids are used to manipulate the concentrations
of the plant hormones auxin and ethylene so all the
leaves are dropped at approximately the same time
Ethylene
Brassinosteroids
 Regulate cell expansion and
are one of the most important
hormones that regulate stature.
 Without them, plants are tiny
dwarves, with reduced
vasculature and roots, and are
infertile.
 They also regulate senescence
or aging.
The dx mutant of tomato is severely dwarfed
due to brassinosteroid deficiency.
However the fruits develop normally.
Jasmonic Acid
• Major functions is in regulating plant
growth including growth inhibition,
senescence, and leaf abscission.
• It has an important role in response to
wounding of plants plant resistance.
• When plants are attacked by insects,
they respond by releasing Jasmonic
acid, which inhibits the insects'
ability to digest protein.
• It is also responsible for tuber
formation in potatoes, yams, and
onions.
Jasmonic Acid
Jasmonate ZIM-domain
Protein Degradation
Growth Hormones
• Auxins: stimulates cell elongation, mediates tropism.
• Cytokinins: stimulates cell division, in ratio with
auxins regulate meristematic cell division.
• Gibberellins: breaks seed dormancy.
• Abscisic acid: stimulates the closure of stomata.
• Ethylene: is associated with fruit ripening.
• Brassinosteroids: regulate plant stature (height).
• Jasmonic acid: give a response to wounding of plants
and associated with pest resistance.
Photosynthesis: Conversion of Solar
Energy to Chemical Energy by Plants