VCE Biology Unit 2Area of Study 01

Adaptations of OrganismsChapter 14

Plant tropisms and hormonal control

Chapter 14.1 Environmental cuesPlants 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

Chapter 14.1 Environmental cuesPlant 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

Chapter 14.1 Environmental cuesCommunication 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

Chapter 14.1 Environmental cuesHormonal 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

Chapter 14.1 Environmental cuesHormonal 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

Chapter 14.1 Environmental cuesHormonal 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

Chapter 14.1 Environmental cuesSpecificity• 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.

Chapter 14.1 Environmental cuesSpecificity (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.

Chapter 14.1 Environmental cuesSpecificity (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.

Chapter 14.1 Environmental cuesSpeed• Hormonal response generally slower than

nerve• Effects last longer• Affects cells widely distributed

Chapter 14.1 Environmental cuesSpeed (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)

Chapter 14.1 Environmental cuesSpeed (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 cmhour-1)

Chapter 14.1 Environmental cuesHormones and responses of flowering plants• Orientate the growth of roots, stems and

leaves• Timing of flowering, fruit ripening and seed

germination

Chapter 14.1 Environmental cuesHormones 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

Chapter 14.1 Environmental cuesHormones 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

Chapter 14.1 Environmental cuesAuxins• Phototropism – reaching for light• Growing plants bend and grow towards light• Stimulated by chemical – auxin (“to grow”)• Chemical diffuses downwards from growing

tip (meristem)

Chapter 14.1 Environmental cuesAuxins (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

Chapter 14.1 Environmental cuesAuxins (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.)

Phototropism Experiment

Auxin causes elongation of cells on shaded side.

Chapter 14.1 Environmental cuesAuxinsGeototropism – 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

Negative Geotropism

Chapter 14.1 Environmental cuesAuxinsApical 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

Apical dominance

Chapter 14.1 Environmental cuesGibberellins• Promote cell elongation• Promote growth for whole plant• Promote cell division (mitosis and cytokinesis) – Flowering– Fruit enlargement– Seed germination

Chapter 14.1 Environmental cuesGibberellins• 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

Chapter 14.1 Environmental cuesCytokinins• 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

Chapter 14.1 Environmental cuesAbscisic acid• Opposite action to auxins• Involved in overall regulation of plant

functions• Little is known about them

Chapter 14.1 Environmental cuesAbscisic 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

Chapter 14.1 Environmental cuesAbscisic 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

Chapter 14.1 Environmental cuesAbscisic acid (ABA)• Development of seed dormancy and

vernalisation requires ABA• Appears to act on gene expression in nucleus

Chapter 14.1 Environmental cuesAbscisic acid (ABA)• Short term effects – stomatal movement– Plant loses too much water, ABA stimulates guard

cells to close• [ABA] increases under stressful conditions

Chapter 14.1 Environmental cuesEthylene (Ethene) (C2H4)• 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

Chapter 14.1 Environmental cuesEthylene (Ethene) (C2H4)• 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

Summary of plant hormonesHormone Where produced Effective site Action Visible effect

auxins Shoot tip (meristem) Growing region of shoot Cells elongate under turgor pressure

Tip bends towards light, apical dominance, used as herbicides, stimulate cuttings to root

gibberellin Fruits, seeds, growing buds, elongating stems

Roots, shoots, seeds, also found in fungi

Growth of cells Shoot elongation, germination of seeds, flowering fruit enlargement

cytokinins Roots 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 acid chloroplasts Gene expression in nuclei Growth inhibition Seed dormancy, vernalisation, drought-tolerance

ethylene Ripening fruits, flowers, seeds, leaves and roots

Cellular metabolism Fruit ripening and leaf drop

Fruit ripening, leaf and fruit drop, used commercially to ripen bananas and pineapples