Download External Factors and Plant Growth

External Factors and Plant Growth
Plant Growth Regulators
• The word hormone comes from the Greek word “To
excite” and hormones stimulate biochemical activities
that cause observable physiological responses by
organisms
• Plant hormones or plant growth regulators are rather
different from animal hormones in chemical structure,
mode of synthesis, and function. Higher animals
possess glands that are part of their endocrine system
that are specialized organs for production of
hormones
• Plant hormones on the other hand are synthesized in
the cells of general organs – the stems, leaves, roots,
and flowers
Tropisms
• A tropism is a growth response involving
bending or curving of a plant part toward or
away from an external stimulus that
determines the direction of movement
• Response toward the stimulus is positive, and
response away from the stimulus is negative
Phototropism
• Phototropism is the growth of stems of plants toward
light - it is probably the best known of the plant
tropisms - phototropism is caused by elongation of
the cells on the shaded part of the plant - so that entire
plant bends or curves toward the light
• This growth pattern is caused by the hormone auxin auxin migrates to the shaded part of the plant and
stimulates increased cell growth and elongation on
the shaded part of the plant
a, Grass seedlings have a sheath called the coleoptile that surrounds the first set of
leaves. Growth of the coleoptile depends on the tip, and removal of the tip stops
growth. Adding the tip back asymmetrically illustrates that the growth-promoting effect
travels downward and not laterally, causing the seedling to bend because one side is
growing faster than the other. Auxin can replace the tip for this effect. b, Stem cuttings
can be induced by auxin to produce roots. c, Strawberries depend on auxin produced
by their developing seeds for expansion and maturation. If the seeds are removed, little
growth occurs. Normal growth can be restored with auxin.
Gravitropism
• Gravitropism is a response to gravity - if a seedling is
placed on its side, the root will curve to grow
downward and the shoot will curve to grow upward
• Auxin may play a role in the response of shoots auxin-inducible genes exist and auxin induces
increased growth in cells in which the gene is
activated - auxin activates the transcription of certain
genes on the side of the shoot showing increased
growth - it is still not clear whether this
transcriptional activation is due to an increase in
auxin concentration or to an increase in sensitivity to
auxin already present in the shoot
Gravitropism in plants
More Gravitropism
• Calcium also plays a role in gravitropism calcium movements are mediated by the
calcium binding protein calmodulin - calcium
moves to upper surfaces of shoot cells before
the shoot actually curves upward and calcium
moves to bottom surfaces of root cells before
the root actually curves downward
More Gravitropism
• The perception of gravity is correlated with
sedimentation of amyloplasts (starch containing
plastids) within certain cells of the shoot and root such cells are found near the vascular bundles in
shoots and in the root cap of roots, particularly the
central column of the root cap
• Amyloplasts are sedimented to the transverse walls
(perpendicular to root surface) in the root cells - if the
root is placed in a horizontal position, these
amyloplasts slide downward and come to rest on what
were once vertically oriented walls - within hours the
root curves downward through growth of upper cells
and the amyloplasts return to their previous position
Thigmotropism
• Thigmotropism is response to touch or contact
with a solid object - this is most commonly
seen in tendrils, modified leaves or stems
depending on the species - the tendrils wrap
around any object they come in contact with
and so enable the plant to cling and climb - the
response can be rapid, a tendril can wrap
around a support one or more times in less
than an hour
Circadian Rhythms
• Many plant activities seem to occur on a daily
schedule - some plants open their leaves at
dawn and shut them at dusk - some plants may
open and shut flowers as the day changes from
dawn to dusk
• Photosynthesis, auxin production, and the rate
of cell division all have regular daily rhythms
and these rhythms continue even when
environmental conditions are kept constant
More Circadian Rhythms
• Entrainment occurs when a periodic repetition of
light and dark (or some other external cycle) causes a
biological clock to remain synchronized with the
same cycle as the entraining factor
• Light-dark cycles and temperature cycles are the
principle sources of entrainment
• The main advantage of a biological clock is that it
allows the plant to respond to the changes in seasons
by accurately measuring changing day length - thus
changes in the environment trigger responses that
result in adjustments of growth, reproduction, and
other activities of the organism
Photoperiodism
• Photoperiodism is a biological response to a
change in the proportions of light and dark in a
24-hour daily cycle - photoperiodism was first
discribed in plants and has since been found in
insects, worms, fish, birds and mammals
Plants have three basic
patterns of flowering
• Short-day plants will only flower if the light
period is shorter than a critical length - they
typically flower in spring or fall
• Long-day plants will only flower if the light
period is longer than a critical length - they
typically flower in the summer
• Day-neutral plants flower without respect to
day length
Temperature Control
• Often photoperiodism works in concert with
temperature to control plant growth and development
• For millennia, variations in day length with each
passing month have been as regular as the sun’s
shifting position in the sky and in temperate climates,
an annual winter’s chill can be reasonably expected to
occur.
• The consistent occurrence of such events makes them
conditions to which the physiology of plants has
become adapted, using them as external cues to turn
on internal processes
California redbud in bloom
Aging and Ripening
• When fruits ripen and
when leaves are
prepared for abscission
(separation from stems)
they undergo a process
of aging or senescence –
a process directed by
hormones, often in
response to external
cues
Plant Hormones and Senescence
• Once senescence is begun it is irreversible so plants
must be able to tightly control when senescence
begins. Some plant hormones inhibit senescence –
cytokinin (mainly promotes cell division), auxin, and
gibberellin all function to maintain normal function
and cell structure in plants.
• Two other plant growth regulators act to promote
senescence – ethylene a gas and abscisic acid –
named because it was thought to promote leaf
abscission in all plants. But now it appears that
ethylene is more commonly used for that.
Fall Foliage
Dormancy
• Dormancy is a special condition of arrested growth
• Plants do not grow at the same rate all the time during unfavorable seasons, they limit their growth or
cease to grow altogether - this allows plants to
survive water scarcity or low temperatures
• Normally, after a period of rest (no growth), growth
resumes when the temperature becomes milder or
water becomes available
• In contrast, a dormant seed or bud can be activated
only by a precise environmental cue or set of cues
Dormancy