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

Tropic responses of plants to external stimuli can be explained by the action ofhormones on the zone or region of elongation (cell enlargement) of stem and root.

root growth

Phytohormones:• Auxins:

• 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

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:

• Light • Water • Gravity • Touch

Sessile plants are able to respond to directional stimuli by growth:

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

Plant Survival Techniques• 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 Adaptationshttp://www.youtube.com/watch?v=LwCi5y_QphY