Responses in Plants

Stimuli Plants respond to: Phototropism Geotropism Hydrotropism Chemotropism Thigmotropism Light Gravity Water Chemicals Touch

Tropisms A Tropism is a change in the growth of a plant in response to an eternal stimulus Negative tropism The growth is away from the stimulus Positive tropism The growth is towards the stimulus

Phototropism http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html

Geotropism

Responses in Plants Plants respond by Changes in their pattern of growth Flowering Production of chemicals

External Factors Regulating Growth Light Intensity Day Length Gravity Temperature

Internal Factors Regulating Growth A series of chemicals called growth regulators These chemicals are produced in meristematic regions Eg. Root tip and shoot tip

Growth Regulators Plant responses are due to growth regulators A growth regulator is a chemical that controls the growth of a plants Regulator that promotes growth Auxin = stimulates cell elongation Regulator that inhibits growth Ethene = inhibits cell elongation

Growth Regulators They are active in very small amounts They are produced in the meristems They are transported in the xylem and phloem Their effects are dependent on concentration A small amount of growth regulator can have great effect on growth

AUXINS production initiator production site(s), function, Objectives – What you will need to know from this section Study auxin as an example of a plant growth regulator under the headings of : production initiator production site(s), function, different effects.

AUXINS influence includes AUXINS -- HIGHER LEVEL Auxins affect virtually every aspect of plant development, and how they respond to environmental stimuli. AUXINS influence includes phototropism, geotropism, cell enlargement and growth, apical dominance, root growth, fruit development, vascular development, and senescence [aging].

AUXINS Promote cell enlargement and growth Are involved in phototropism—stem bends towards light. Are responsible for apical dominance, where the main bud inhibits the growth of buds lower down stem. Regulate the differentiation of the vascular tissue (xylem and phloem). Indole-3-acetic acid (IAA) is the most abundant naturally-occurring auxin,

IAA (Indoleacetic acid) is an auxin that is made in the meristems of shoots, buds and root, and in the tips of coleoptiles [of grasses].

Auxin travels backwards from the tip in the vascular tissue and causes cells to elongate (get longer). This makes the zone of elongation grow and so the tip is pushed further upwards (in shoots) and downwards (in roots). Movement is by active transport from cell to cell – this requires energy.

Auxins promote cell enlargement and growth, are involved in phototropism and apical dominance APICAL DOMINANCE PHOTOTROPISM

A tropism is a plant’s response to a stimulus coming from one direction, e.g. sunlight, gravity. Phototropism is a growth response of a stem towards light, so that it can receive the maximum amount of light for photosynthesis.

APICAL DOMINANCE Auxins are responsible for apical dominance, where the main bud inhibits the growth of buds lower down stem. This photograph shows side buds sprouting when the main stem is cut off [pruned].

Quicker growth here due to more hormones In shoots, light causes auxin to move down the shaded part of the stem, causing the shoot to grow towards the sunlight. This is called PHOTOTROPISM. Quicker growth here due to more hormones Artificial auxins can also be used to kill weeds, stimulate root formation in cuttings, …

Uses of plant regulators Artificial auxins are used in rooting powders to stimulate root formation in stem cuttings. A synthetic auxin [2,4-D] is used as a selective weedkiller, making the plants grow too fast. Fruit is transported green and unripe, and can then be quickly ripened by spraying it with ethene.

Growth Inhibitors Ethene is a gas. It is produced in: Stem nodes Ripe fruits Decaying leaves It causes: Fruits to ripen Leaves to fall in autumn Plants to age

Commercial Growth Regulators Ripen bananas Bananas are picked green – ethene is used to stimulate ripening just before distribution Rooting Powders Contain synthetic growth regulator NAA. It stimulates rapid root formation on stem cuttings

Adaptations for Protection in Plants Plants need to protect themselves from their environment They protect themselves against: Loss of water Overheating Infection from micro organisms Being eaten by herbivores

Plant protection Plants can adapt themselves for protection in two ways: Structural of anatomical adaptations Chemical adaptations

Structural Adaptations Bark/epidermis prevents entry of microbes and reduces loss of water Thick cuticle on stem or leaves in plants that live in dry places Thorns prevent plants from being eaten by herbivores Stinging cell in epidermis prevent plants been eaten Guard cells change shape when they lose water which causes stomata to close and this reduces water loss

Chemical Adaptations Production of Tannins to make the plant indigestible Production of toxic chemicals to prevent growth of insect larvae Production of strychnine and nicotine in legumes that damage nerves and muscles Production of heat shock proteins to protect enzymes when temperatures are high

Most important adaptation Plants can grow and regenerate throughout their life. They can produce new roots, stems, leaves.