14.1 plant tropisms and hormonal control

8/9/2019 14.1 Plant Tropisms and Hormonal Control

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AND HORMONAL

CONTROL


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Environmental Cues

Plants depend on their immediateenvironment for the materials and energythey require for survival.

Plants are relatively tolerant ofenvironmental changes from which theycannot escape.

Plant growth and reproduction aresynchronised with seasonal changes, andwith local physical and climateconditions.


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The environment provides the cues formany stages in plant growth (owering,ripening of fruit and seed germination) tooccur when conditions are ideal.

Plants also respond to gravity, light andtemperature.

Communication between cells in theplant is required so plant responses arecoordinated and controlled.

Plants control and coordinate theirresponse using hormones.


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Hormonal Systems

hormone is a chemical produced by aspeci!c cell(s) in an organism and movethroughout to stimulate other cells capable

of producing the desired response. "ormones act as messengers between

cells to regulate function. They control various metabolic functions

including rates of r#n$s, transport ofsubstances across membranes, secretion,and growth.


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They transmit their signal to speci!c cellsby altering speci!c biochemical reactionsin the cell (ie. Production of an en%yme,turning on or o& a gene).

The target cell has a speci!c receptor forthat hormone which causes it to be

a&ected. "ormones wor' by interacting with a

receptor on the cell membrane or bypassing through the membrane directly

into the cell to trigger a biochemical event. "ormones are e&ective even in low

concentrations.


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Specifcity

lthough hormones may pass through the entiresystem, only those cells with the speci!c receptor areable to respond to the hormone.

ew hormones a&ect the entire system, most target a

speci!c organ(s) and even only a speci!c type of cellwithin that organ.

Plants have fewer hormones than animals, and oftentheir hormones are simpler and a&ect the entiresystem.

This is because plants have no system for transportli'e the endocrine system. Plants have no glands andtheir hormones are often produced as a direct resultof environmental stimuli (light, temperature etc.).


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Spee

"ormonal e&ects are slower than nervoussystem responses but often last longer.

Plant hormones move much slower as they haveto be transported a fair distance to where theyta'e e&ect in stimulating things such asowering or causing stems to bend.

"ormones are often transported using #ylemand phloem, from cell to cell and occasionally by

air. ovement of a plant hormone requires energy

(TP molecules) for active transport which is*+# faster than by di&usion but still very slow.


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Hormones an Responses o!"lo#erin$ Plants

-n owering plants hormones orientate thegrowth of roots, stems and leaves and the timingof reproduction (owering), fruit ripening andseed germination.

growth response is triggered by environmentalfactors. hen the direction of growth response isrelated to the direction of the stimuli$s origin theresponse is called a Tropism.

-f a plant grows towards a stimulus it$s calledpositive tropism, growth away from the stimulusis a negative tropism.

Pg /01 "ormone responsibilities in plants.


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Au%ins& P'ototropism (Reac'in$ !or t'e li$'t

Plants often grow and bend towards a sourceof light (positive phototropism).

This growth is stimulated by a chemical

called u#in di&using downward from the tipof a growing grass shoot. u#in is produced continually in a growing tip

(meristem) and di&uses through layers of

cells as the tip has no vascular tissue. 2ight interacts with the receptors that control

membrane permeability to au#in, thus theau#in moves laterally away from the light.


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This causes higher concentrations of au#in on

the side opposite the light source, which thengrows faster and causes the plant tip toliterally bend towards the light.

u#ins promote growth by wor'ing with

receptors in the speci!c growth regioncausing a softening of cell walls allowing thecells to elongate more rapidly under turgorpressure (high internal uid pressure), cells

outside the growing region appear to lac'these receptors.


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)eotropism ( Responin$ to)ravity

u#ins are also involved in growth inresponse to the forces of gravity.

3eotropism is the response to gravity.

u#ins actually cause negativegeotropism (cause the plant to grow theopposite direction of gravity).

u#in concentrations are always lower atthe base causing the plant to growupwards.


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Apical Dominance ( One MainStem

u#ins also a&ect lateral growth in plants. u#in is produced in the apical tip moves

down the stem and inhibits the growth of

lateral buds. This is a phenomenon calledapical dominance.

This leads to a taller plant with fewer sidebranches.


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A*scisic Aci ,A-A.

These are the best 'nown growth5inhibiting hormones. They have theopposite e&ect of au#ins, gibberellins and

cyto'inins and are particularly importantin regulation of plant function.

They help plants to tolerate adverseconditions (drought, salinity, and low

temps) by promoting leaf drop, bud andseed dormancy and increasing frostresistance.


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4ynthesi%ed mainly in chloroplasts. The dropping of ripe fruit, and unfertili%ed

owers, and leaves in deciduous plants is'nown as abscission and occurs as a result ofthe disintegration of a special layers of cells atthe base of the organ being dropped due to

6. 7evelopment of seed dormancy and

vernalisation requires the presence of 6 tocause gene e#pression in nuclei.

6 also carries a message when too muchwater is being lost to close guard cells. 6 isincreased particularly in times of drought, hightemperature and waterlogging.


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Et'ylene

The gas ethylene or ethene (C/"8) is a smallmolecule, which is released by ripening fruit.

9ne of the e&ects is the stimulation of fruitripening. :ipening of fruit is accompanied by

colour change and softening of the esh. These changes help attract animals which

disperse the fruit and ma'e it easy for seeds tobe released from the fruit.

;thylene increases cellular respiration, brea'ingdown of starches and oils into sugars.

;thylene production is stimulated by au#ins and6.

14.1 plant tropisms and hormonal control

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