1. VCE Biology Unit 2 Area of Study 01 Adaptations of Organisms Chapter 14 Plant tropisms and hormonal control

2. Chapter 14.1 Environmental cues Plants must survive in the immediate environment They depend on the immediate environment for materials and energy They are relatively tolerant of environmental changes which they cannot escape

3. Chapter 14.1 Environmental cues Plant growth and reproduction are synchronised with seasonal changes and climate conditions Environment provides cues for many stages of plant growth. Flowering, ripening of fruit, seed germination – All which must occur in the ‘most ideal’ conditions Also respond to light and temperature

4. Chapter 14.1 Environmental cues Communication between cells in different parts of plant are required for direction and timing of growth Other plant responses are also co-ordinated Plants do not have a nervous system Internal conditions are controlled by hormones

5. Chapter 14.1 Environmental cues Hormonal systems A hormone is a chemical produced by cells in one part of an organism, transported throughout, accepted by specific receptor sites and affect specific cells Hormones act as intercellular messengers and regulate cell functions

6. Chapter 14.1 Environmental cues Hormonal systems (continued) Target cells possess specific receptors Involved in metabolic functions, rates of chemical reactions, transport of substances across cell membranes, secretion and cell growth Alter specific biochemical reactions – Production of enzymes – Change in membrane transport – Switching on of off of specific genes

7. Chapter 14.1 Environmental cues Hormonal systems (continued) Exert their affects directly by passing through cell membrane into cell or Indirectly by interacting with a receptor on the outside of the cell membrane Hormone-receptor associations trigger particular biochemical events Hormones are affective in low concentrations

8. Chapter 14.1 Environmental cues Specificity A particular stimulus will only affect a specific group of hormone-secreting cells. E.g. a sudden shock to humans causes the release of adrenaline Hormones are transported throughout the organism, but only the cells with specific receptors are able to respond to the hormone. This specificity is not only restricted to specific organs, but specific cells/structures within organs.

9. Chapter 14.1 Environmental cues Specificity (continued) Flower plants have fewer hormones than animals and they tend to affect most cells. Hormone producing cells in plants are not organised into glands. The cells receiving the environmental cue (stimulus) produce the required hormone. – E.g. a growing shoot tip of a plant receiving light.

10. Chapter 14.1 Environmental cues Specificity (continued) Plant response is specific. – E.g. growing tip of wheat, lettuce or spinach receives increasing daylight (i.e. after winter solstice) will be stimulated to produce flowers.

11. Chapter 14.1 Environmental cues Speed Hormonal response generally slower than nerve Effects last longer Affects cells widely distributed

12. Chapter 14.1 Environmental cues Speed (continued) Plant hormones slower in response than animals (why?) Transported through phloem and xylem Transported between cells Transported through air to other plants Transported through soil (inhibit root growth from neighbouring plants)

13. Chapter 14.1 Environmental cues Speed (continued) Movement of a plant hormone requires the expenditure of energy (ATP) Active transport is 10× faster than diffusion (passive) Still may be as slow as 1 cm/hour (1 cm  hour -1 )

14. Chapter 14.1 Environmental cues Hormones and responses of flowering plants Orientate the growth of roots, stems and leaves Timing of flowering, fruit ripening and seed germination

15. Chapter 14.1 Environmental cues Hormones and responses of flowering plants Response triggered by environmental factor Tropism – When the response is directed towards the environmental factor, this is called “Tropism” – Positive towards – Negative away

16. Chapter 14.1 Environmental cues Hormones and responses of flowering plants Hormones in plants are responsible for – Phototropism – growth in response to light – Geotropism – growth in response to gravity – Apical dominance – inhibition of lateral branches – Ripening of fruit – conversion of starches to sugars – Abscission – shedding of leaves and flowers

17. Chapter 14.1 Environmental cues Auxins Phototropism – reaching for light Growing plants bend and grow towards light Stimulated by chemical – auxin (“to grow”) Chemical diffuses downwards from growing tip (meristem)

18. Chapter 14.1 Environmental cues Auxins (continued) Produced continuously in meristem Diffuses through layers of cells Light interacts with receptors that control membrane permeability to auxin Auxin moves laterally away from light

19. Chapter 14.1 Environmental cues Auxins (continued) Higher concentration of auxin on dark side of tip Those cells grow faster (elongation not cell division) Softens cell walls allowing elongation under turgor pressure Cells outside growing area lack auxin receptors Auxins used as herbicides – excess auxin causes growth to the detriment of the plant (cells rupture, etc.)

20. Phototropism Experiment

21. Auxin causes elongation of cells on shaded side.

22. Chapter 14.1 Environmental cues Auxins Geototropism – responding to gravity Auxins involved in negative geotropism (growing away from gravity) Plant laid on side in dark will bend and grow away from gravity Auxin concentration on lower side of lateral stems

23. Negative Geotropism

24. Chapter 14.1 Environmental cues Auxins Apical dominance – one main stem Auxin produced in the apical tip of a plant Diffuses down stem Inhibits lateral bud development Results in taller plants with less side branches Gardeners cut out apical tips to promote bushiness of cultivated plant Bushfires burn off the apical tips of eucalypts in the crown

25. Apical dominance

26. Chapter 14.1 Environmental cues Gibberellins Promote cell elongation Promote growth for whole plant Promote cell division (mitosis and cytokinesis) – Flowering – Fruit enlargement – Seed germination

27. Chapter 14.1 Environmental cues Gibberellins Synthesised in flowers, developing fruits, seeds, growing buds and elongating stems Important for triggering cereal grain germination – Produced in embryo – Moves to target cells in the endosperm – Induces the formation of enzymes that digest the endosperm to produce malt  Used in the brewing of malt whisky

28. Chapter 14.1 Environmental cues Cytokinins In the presence of auxins, cytokinins stimulates cell division and cell differentiation Ratio of auxins to cytokinins determines path of differentiation of new cells – Stems and leaves develop when more cytokinins than auxins – Roots develop when less cytokinins than auxins

29. Chapter 14.1 Environmental cues Abscisic acid Opposite action to auxins Involved in overall regulation of plant functions Little is known about them

30. Chapter 14.1 Environmental cues Abscisic acid Assist plants to tolerate or avoid adverse conditions – E.g. drought, salinity, low temperature – Promotes leaf drop, bud and seed dormancy and increased frost resistance

31. Chapter 14.1 Environmental cues Abscisic acid (ABA) Synthesised mainly in chloroplasts Dropping of ripe fruit, unfertilised flowers and falling of leaves in deciduous trees known as abscission Disintegration of special layer of cells at base of organ being dropped

32. Chapter 14.1 Environmental cues Abscisic acid (ABA) Development of seed dormancy and vernalisation requires ABA Appears to act on gene expression in nucleus

33. Chapter 14.1 Environmental cues Abscisic acid (ABA) Short term effects – stomatal movement – Plant loses too much water, ABA stimulates guard cells to close [ABA] increases under stressful conditions

34. Chapter 14.1 Environmental cues Ethylene (Ethene) (C 2 H 4 ) Released by ripening fruit (apples, pears, citrus, banana and avocados) Stimulates fruit to ripen Ripening of fleshy fruit shows colour changes and softening of flesh

35. Chapter 14.1 Environmental cues Ethylene (Ethene) (C 2 H 4 ) Wild plants use fruit to attract animals to eat them and disperse the seeds Ethylene increases rate of respiration and other processes of fruit ripening (e.g. breaking down starch and oils into sugars). Triggered by auxin and abscisic acid

36. Summary of plant hormones HormoneWhere producedEffective siteActionVisible effect auxinsShoot tip (meristem)Growing region of shootCells elongate under turgor pressure Tip bends towards light, apical dominance, used as herbicides, stimulate cuttings to root gibberellinFruits, seeds, growing buds, elongating stems Roots, shoots, seeds, also found in fungi Growth of cellsShoot elongation, germination of seeds, flowering fruit enlargement cytokininsRoots and developing fruits Branch and leaf buds, moves through phloem and xylem Stimulates cell division, cell elongation and tissue differentiation Growth of lateral branches, used to promote life of vegetables in storage Abscisic acidchloroplastsGene expression in nucleiGrowth inhibitionSeed dormancy, vernalisation, drought-tolerance ethyleneRipening fruits, flowers, seeds, leaves and roots Cellular metabolismFruit ripening and leaf drop Fruit ripening, leaf and fruit drop, used commercially to ripen bananas and pineapples