plants
DESCRIPTION
Plants. Basic Structures. Apical meristems enable the plant to grow in length. -located in tips of roots and in the buds. Plants, being rooted to the ground Must respond to whatever environmental change comes their way. Plants have cellular receptors - PowerPoint PPT PresentationTRANSCRIPT
Plants
Basic Structures
Apical meristems enable the plant to grow in length. -located in tips of roots and in the buds.
Plants, being rooted to the groundMust respond to whatever
environmental change comes their way
Plant Hormones
• Plant hormones help coordinate growth, development, and responses to stimuli
• Hormones– Are chemical signals
that coordinate the different parts of an organism
•Auxin•Cytokins•Gibberelins•Abscisic Acid•Ethylene
• Plants have cellular receptors– That they use to detect important changes in their
environment• For a stimulus to elicit a response
– Certain cells must have an appropriate receptor
The Discovery of Plant Hormones
• Any growth response– Is often caused by hormones
• EX: results in curvatures of whole plant organs toward or away from a stimulus =tropism
– Shoots growing towards the light is positive phototropism– Shoots growing up is negative geotropism– Roots growing down is positive geotropism
Auxin– Is used for any chemical substance that promotes
cell elongation in different target tissues
• Auxin transporters– Move the hormone from
shoot tip to base and never in the reverse even if the plant is turned upside down.
• Auxin– Is involved in the formation and branching of roots– Inhibiting lateral bud growth
Other Effects of Auxin• Auxin affects secondary growth
– By inducing cell division in the vascular cambium and influencing differentiation of secondary xylem
• Developing seeds synthesize auxin• tomatoes grown in greenhouse conditions sprayed with auxin
induce fruit development without a need for pollination• This allows for seedless tomatoes
• Charles Darwin and his son Francis– Conducted some of the earliest experiments on phototropism, a plant’s
response to light, in the late 19th century
In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted.
EXPERIMENT
RESULTS
Control Darwin and Darwin (1880) Boysen-Jensen (1913)
Light
Shadedside ofcoleoptile
Illuminatedside ofcoleoptile
Light
Tipremoved
Tip coveredby opaquecap
Tipcoveredby trans-parentcap
Base coveredby opaqueshield
Light
Tip separatedby gelatinblock
Tip separatedby mica
In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin) but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical.
CONCLUSION
• In 1926, Frits Went– Extracted the
chemical messenger for phototropism, auxin, by removing the coleoptile tip & placed it on a block of agar. This allowed the chemical to travel through. Went concluded that a coleoptile curved toward light because its dark
side had a higher concentration of the growth-promoting chemical, which he named auxin.
The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark.
Excised tip placedon agar block
Growth-promotingchemical diffusesinto agar block
Agar blockwith chemicalstimulates growth
Control(agar blocklackingchemical)has noeffectControl
Offset blockscause curvature
RESULTS
CONCLUSION
In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others,he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side.
EXPERIMENT
Cytokinins
• Cytokinins (like cytokinesis)– Stimulate cell division
– Are produced in actively growing tissues such as roots, embryos, and fruits
– Makes the plant branch out.
Control of Apical Dominance• Cytokinins, auxin, and other factors interact in
the control of apical dominance– The ability of a terminal bud to suppress
development of axillary buds
Figure 39.9a
Axillary buds
Anti-Aging Effects
• Cytokinins retard the aging of some plant organs– By inhibiting protein breakdown, stimulating RNA
and protein synthesis, and mobilizing nutrients from surrounding tissues
Florists use cytokinins on their cut flowers to keep them fresh.
Gibberellins• Control the plant’s yearly cycle
– Tells plant when to go dormant, when to flower…
• Gibberellins stimulate growth of both leaves and stems
• In stems– Gibberellins stimulate cell elongation and cell
division
The release of gibberellins from the embryo of a seed signals the seed to break dormancy and germinate.
Fruit Growth• In many plants
– Both auxin and gibberellins must be present for fruit to set
• Gibberellins are used commercially– In the spraying of
Thompson seedless grapes making them grow larger.
• In the fall, when leaves change color. ABA basically cuts the leaves off the tree.
• Suppresses fruit formation• Stops transpiration
• Basically causes plants to go into dormancy– Preparing for winter– Drought tolerance
Abscisic Acid (opposite of gibberelins)
Ethylene• It’s a gas and travels
outside the plant• Is an example of positive
feedback loop
The Triple Response to Mechanical Stress• Ethylene induces the triple response
– Which allows a growing shoot to avoid obstacles
Figure 39.13 Ethylene induces the triple response in pea seedlings,with increased ethylene concentration causing increased response.CONCLUSION
Germinating pea seedlings were placed in thedark and exposed to varying ethylene concentrations. Their growthwas compared with a control seedling not treated with ethylene.
EXPERIMENT
All the treated seedlings exhibited the tripleresponse. Response was greater with increased concentration.RESULTS
0.00 0.10 0.20 0.40 0.80
Ethylene concentration (parts per million)
1. Slowing of stem elongation2. Thickening of the stem3. Curvature causing stem to grow horizontally.
Apoptosis: Programmed Cell Death• A burst of ethylene
– Is associated with the programmed destruction of cells, organs, or whole plants
Fruit Ripening• A burst of ethylene production in the fruit
– Triggers the ripening process
In animals, internal & external signals regulate a variety of physiological responses that
synchronize with environmental cycles and cues.
Circadian rhythms
Resets every day; is influence by internal & external signals such as light and dark; meal times; stress; exercise…
Plants are influenced by external factors as well.
Such as night length.This is called photoperiodism.
Phytochrome is an important macromolecule.
PHOTOPERIODISM
From the results above these plants can be more appropriately called “long night plants” and “short night plants”. Photoperiodism is controlled by the length of the night= critical night length
It is the leaves that give the cue that the night is long enough or short enough and signals the buds to flower.
PR PFR
IN SHORT DAY PLANTS
During the day, PR is converted to PFR During the night the PFR is converted back to PR
PR triggers flowering and PFR inhibits flowering
So the long nights give the plant plenty of time to get rid of the PFR that built up during the day.
IN LONG DAY PLANTS
PR inhibits flowering and PFR triggers flowering
So the long days give the plant plenty of time to build up the PFR & the nights are too short to get rid of all the PFR
Photoperiodism
• Day-neutral plants: light has no effect on their growth
Tomatoes
Rice
Dandelions
Photoperiodism
• Vernalization:– Some plants (winter wheat) need to be exposed to
cold temperatures for several weeks
Plants respond to a wide variety of stimuli other than light
Geotropism/Gravitropism
• Auxin plays a key role
Underground how does a seedling know which way to grow?
Thigmomorphogenesis
• Plants growing on a windy range– Have thicker trunks
• Touching leaves can alter plant growth– Climbing vines
5 ways plants defend themselves