Plant adaptation
and response to
environmental
variability
• In the environment there are both spatial
and temporal variations in abiotic factors.
• All organisms including plants have ranges of
tolerance to various abiotic factors
• Because plants are sessile ( fixed in place)
they are unable to escape unfavourable
conditions and unable to move to areas of
favourable conditions .
• Rapid environmental changes in particular are
a problem for the survival of plants.
• Plants respond to environmental changes by:
– having their stages of life timed to coincide
with favourable (seasonal) conditions.
– their physiological adaptations.
– their patterns of activity.
• This requires plants are able to detect and
respond to environmental factors.
• Hormones coordinate and regulate plants.
• Hormones :
– are chemical messengers
– produced in plant tissues
– work by changing the permeability of cell
membranes resulting in the promotion or
inhibition of growth or the activity of cells
Seedlings, coleoptiles and apical
meristems
root growth
Tropic responses of plants to external stimuli can be
explained by the action of
hormones on the zone or region of elongation (cell
enlargement) of stem and root.
Phytohormones:
• Auxins:
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Master hormones
Promote cell expansion
Promote growth of shoots, ripening of fruit
Promote fall of leaves and fruit
The most common auxin found in plants is indoleacetic acid (IAA).
Auxins are usually released by the apical meristem of the stem.
Gibberelins
Cytokinenins
Absciscic acid
Ethylene
• Photo & geo tropism
Sessile plants are able to respond
to directional stimuli by growth:
Tropisms
Tropism is the involuntary response
of a plant, or part of a plant involving
orientation toward (positive tropism)
or away from (negative tropism) one
or more external stimuli.
Includes responses to:
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Light
Water
Gravity
Touch
Phototropism:
plant's response to light.
The plant stem exhibits positive phototropism ie. the growth of a plant stem
towards light.
http://plantphys.info/plant_physiology/auxin.shtml
Geotropism is plant response to gravity.
Roots of plants show positive geotropism, shoots show negative geotropism.
Geotropism was once thought a result of gravity influencing auxin concentration.
Thigmotropism is plant response to contact with a solid object.
Tendrils of vines warp around objects, allowing the vine to grow upward
The main effects of auxin on plant growth include:
•stem cell elongation – auxin increases the 'plasticity' of the primary cell wall. A
more flexible cell wall stretches more during cell expansion, while its protoplast
is swelling.
Ethylene:
Ethylene is a specific type of phytohormone.
Unlike the other four classes of phytohormones, ethylene (C2H4) is a gas at room
temperature. Ethylene gas diffuses easily through the air from one plant to another.
The saying "one bad apple spoils the barrel" has its basis in the effects of ethylene
gas.
One rotting apple will produce ethylene gas, which stimulates nearby apples to ripen
and eventually spoil because of over-ripening. Ethylene is released by ripening fruits
and dying leaves and flowers.
The main effects of ethylene on plant growth include:
fruit ripening – ethylene can stimulate fruit ripening in a number of species
(banana, honeydew melon, and tomato).
Ethylene helps to convert starch into sugars. Hence, the control of ethylene levels in
commercial storage facilities is of considerable commercial importance (i.e., ethylene can
be removed to promote storage life, or added to hasten or synchronize the ripening
process, as needed).
abscission – in some plant species, ethylene promotes abscission, which is the
detachment of leaves, flowers, or fruits from a plant.
For example, walnuts are harvested with mechanical tree shakers. Ethylene treatment
increases the number of nuts that fall to the ground when the trees are shaken.
Photoperiodism:
• The physiological response of plants to the length of
the day
• Phytochrome: a pigment that detects light.
• The ratio of
Inactive Phytochrome : Active Phytochrome
is a signal for:
• flowering
• germination
• dormancy
Shortday v longday plants
Mimosa
• Many plants have survival
techniques to cope with the
most extreme of
environments.
• Mangroves can cope with
extreme saltiness, they do
this by storing water in
separate areas, extensive
airspaces and salt excluding
pores.
• Other plants have the ability
to cope with fire. Their seeds
are covered with a tough shell
that is broken when they are
exposed to extreme heat
meaning despite
Plant Adaptations
http://www.youtube.com/watch?v=LwCi5y_QphY