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Responses in Flowering Plants

Responses in Flowering Plants. Tropisms and Growth Regulators. Factors affecting plant growth. External factors Temperature Light intensity Day length Water Carbon dioxide concentration pH Gravity Internal factors Growth promoters (auxins)

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Responses in Flowering Plants

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  1. Responses in Flowering Plants Tropisms and Growth Regulators

  2. Factors affecting plant growth • External factors • Temperature • Light intensity • Day length • Water • Carbon dioxide concentration • pH • Gravity • Internal factors • Growth promoters (auxins) • Growth inhibitors (e.g. Ethylene, abscisic acid)

  3. Tropisms • A stimulus is a change in the environment of a cell or organisms that results in a response • A response is a change in the cell or organism as a result of it receiving a stimulus • Plants often change their growth in response to a stimulus • A tropism is a growth response of a plant to an external stimulus

  4. Types of Tropisms

  5. Positive and Negative Tropisms • Positive tropism • growth is towards the stimulus • Negative tropism • growth is away from the stimulus

  6. Positive and Negative Tropisms • Stems • positively phototropic • negatively geotropic • this ensures the leaves get more light • Roots • negatively phototropic • positively geotropic and hydrotropic • this ensures the roots grow down into the soil and towards water

  7. Pollen Tube - Chemotropism • Pollen tubes are positively chemotropic to chemicals released by the ovule • They are also positively hydrotropic

  8. Positive thigmotropism • Tendrils of climbing plants (e.g. Ivy) • positively thigmotropic

  9. Plant Growth Regulators • A growth regulator is a chemical that controls the growth of a plant. • They are • produced in the meristem • transported to other parts of the plant • usually carried in the phloem (active transport) • often act on parts of the plant that are some distance from the site of production (plant hormones)

  10. Auxins • Plant Growth Promoter • Produced in meristem, young leaves and seeds • Functions and effects • stem elongation • root growth • fruit development • phototropism • apical dominance (inhibits side branching) • development of cells’ shape and structure • stimulates cell growth, elongation and division

  11. Giberellins and Cytokinins • Plant Growth Promoters • Giberellins • stem elongation • germination • Cytokinins • cell division • cell differentiation • lateral bud growth

  12. Combined Effect • Many growth process are influenced by a number of growth regulators • Auxin and Gibberellins promote stem growth, elongation • Auxin and Cytokinin: • Auxin promotes apical dominance and inhibits lateral bud growth at high concentrations • Cytokinin promotes lateral bud growth • When auxin is used in combination with cytokinin, apical dominance is broken, and the auxin promotes cell division generally (= growth)

  13. Ethylene • Gaseous plant growth regulator • Often an inhibitor • Produced in ripe fruit and decaying leaves • Causes fruit ripening, and leaf fall • Also causes production of more ethylene • Over-rips fruit causes surrounding fruit to ripen more quickly “one rotten apple rots the barrel”

  14. Commercial use of Plant Growth Regulators • Ethylene • Fruit ripening • Gibberellin • Seedless fruit • Auxin • Rooting powder, herbicide (e.g. 2,4-D) • Cytokinin • Micropropagation • Plant tissue culture

  15. Mechanism of a Plant Response- Phototropism • In unilateral light, auxin is produced in the apical meristem • Auxin moves down the stem • Auxin also moves laterally to the shaded side of the stem • Increased concentration on the shaded side causes cell elongation • Auxin on the illuminated side degenerates • The shaded side of the stem elongates. • Unequal growth causes the stem to bend towards the light

  16. Mechanism of Phototropism Unilateral light Increased auxin concentration on shaded side Lateral movement of auxin from the illuminated side Increased cell elongation on shaded side

  17. Phototropism in Unilateral Light

  18. Plant Adaptations for Protection

  19. Anatomical • A physical barrier consisting of epidermis or bark protects plants, these layers prevent the entry of pathogens and reduce water loss from the plant. • Sometimes a protective cuticle is also present. • In some plants the epidermis cells are adapted to form thorns e.g. roses or stinging hairs e.g. nettles. • A water shortage in plants causes the guard cells to shrivel, this closes the stomata and reduces any further water loss.

  20. Chemical • Excessive heat denatures plant enzymes, many plants can form special heat-shock proteins when temperatures exceed 40 ºC, these proteins surround other proteins especially enzymes and help to keep their shape. • Sometimes when infected by micro-organisms the plant may produce stress proteins (phytoalexins) which may: • Damage micro-organisms by attacking their walls. • Stimulate the formation of specialised plant cell walls that prevent the spread of the micro-organisms. • Stimulate nearby plant cells to respond to the micro-organisms.

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