Bio 100 Guide 27.

Transcription

1 Bio 100 Guide 27

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6 Experiments on how plants turn toward light led to the discovery of a plant hormone Phototropism is growth of a plant shoot in response to light Adaptive response, directing shoots and seedlings toward sunlight In experiments, cells on the darker side of a seedling elongated faster than those on the light side Different rates made plant bend toward light

7 Figure 33.1a

8 LE 33-01b Shaded side of shoot Light Illuminated side of shoot

9 Experiments demonstrating the mechanism of phototropism in grasses Darwin demonstrated that the tip of a seedling is responsible for detecting light Boysen-Jensen showed that a mobile chemical is the signal for phototropism Went isolated the specific chemical messenger (auxin) Experiments with grasses opened up research on plant hormones

10 LE 33-01c Light Control Tip removed Tip covered by opaque cap Tip covered by transparent cap Base covered by opaque shield Tip separated by gelatin block Tip separated by mica Darwin and Darwin (1880) Boysen-Jensen (1913)

11 LE 33-01d-4 Agar Shoot tip placed on agar block. Chemical (later called auxin) diffuses from shoot tip into agar. Control Block with chemical stimulates growth. Offset blocks with chemical stimulate curved growth. Other controls: Blocks with no chemical have no effect. No light

12 Five major types of hormones regulate plant growth and development Plant hormones exert control by affecting cell division, elongation, and differentiation Auxins Cytokinins Gibberellins Abscisic acid Ethylene

13 Table 33.2

14 Auxin stimulates the elongation of cells in young shoots Auxins are a class of chemicals that promote seedling elongation The apical meristem of shoot tips is a major source of IAA synthesis At different concentrations, auxin stimulates or inhibits the elongation of shoots and roots

15 LE 33-03b + Stems 0 Roots g/l Increasing auxin concentration (g/l)

16 LE 33-03c Plasma membrane Cell wall Vacuole H + H + H + pump (protein) Cell wall Cellulose molecule H 2 O Cell elongation Cytoplasm Enzyme Cellulose loosens; cell can elongate Cellulose molecule Cross-linking molecule

17 Auxin has a number of effects other than elongation Promotes growth in stem diameter Promotes growth of fruit Can induce fruit development without pollination

18 LE Terminal bud No terminal bud

19 Gibberellins affect stem elongation and have numerous other effects Gibberellins stimulate cell elongation in stems and leaves Enhance the action of auxin Gibberellin-auxin combinations can also influence fruit development Gibberellins are important in seed germination in many plants

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21 Figure 33.5a

22 Figure 33.5b

23 Abscisic acid inhibits many plant processes Abscisic acid (ABA) slows growth Inhibits germination during seed dormancy and maturation Some desert plant seeds remain dormant until rains wash out ABA Ratio of ABA to gibberellin can determine whether a seed will remain dormant or germinate ABA causes stomata to close in wilting plants

24 LE 33-07b Leaf stalk Stem (twig) Protective layer Stem Abscission layer Leaf stalk

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26 Ethylene triggers fruit ripening and other aging processes Ethylene production in fruit triggers ripening Includes enzymatic breakdown of cell walls Spreads from fruit to fruit as a gas Ethylene probably also plays a role in the autumnal changes in deciduous trees Leaf abscission is promoted by a change in the ethylene-auxin balance Triggered by shorter days

27 Figure 33.7a

28 Plant hormones have many agricultural uses Auxin is used both to delay and to promote fruit drop Gibberellins can be used to produce seedless fruits or promote seed production The weed killer 2,4-D is a synthetic auxin that disrupts hormones regulating plant growth The use of synthetic versus natural pesticides and herbicides is a highly debated question

29 Figure 33.8

30 The End