Chapter 39: Control Systems in Plants. Question Do plants sense and respond to their environment ? ...
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Transcript of Chapter 39: Control Systems in Plants. Question Do plants sense and respond to their environment ? ...
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Chapter 39:Control Systems in Plants
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Question Do plants sense and respond to their
environment ? Yes - By adjusting their pattern of growth and
development.
In Dark In Light
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Comment Plants can’t “move” away from a stimulus,
but can change their growth response. Result – plant bodies are more “flexible” in
morphology than animals.
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Classical Example Phototropism - plant growth response to
unilateral light. Observation – plants bend or grow towards
the light.
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Phototropism Experimenters
Darwins: late 1800's. Boysen & Jenson: early 1900's. F.W. Went: 1926
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Went Experiments
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Mechanism of Phototropism Cells on the dark side elongate faster than the
cells on the light side. The uneven growth rate causes the bending of
the stem toward the light.
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Question What is the adaptive value of phototropism? It tilts the leaves toward the light source for
more efficient photosynthesis.
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Cause of Phototropism Chemical messenger from the tip caused the
growth response in the stem. The distribution of the chemical changes in
the unequal light, resulting in unequal cell elongation.
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Hormone Chemical signal produced in one location,
transported, has effect in another location. Phototropism is caused by a plant hormone.
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Plant Hormones Are produced in small quantities. Effects may reflect balance between several
hormones.
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Mechanism
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Plant Hormones1. Auxins
2. Cytokinins
3. Gibberellins
4. Abscisic Acid
5. Ethylene
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Auxins Named by Went in 1926. First plant hormone described. Ex: IAA (natural)
2,4-D (synthetic)
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Major Functions Stimulates cell elongation. Fruit development. Apical Dominance. Tropism responses.
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Apical Dominance
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Where Produced Apical Meristems. Young leaves. Embryos.
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Cytokinins Isolated from coconut "milk" (endosperm) in
the 1940’s. Named because they stimulate cell division. Ex: Zeatin
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Major Effects Stimulates cell division. Delays senescence. Root growth and differentiation. Where Produced - roots
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Auxin/Cytokinin Ratios Control shoot or root
differentiation in tissue cultures.
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Gibberellins Found from the "Foolish Seedling" disease in
rice. Ex: GA3
70 types known
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Foolish Seedlings
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Major Effects Internode elongation. Seed/Bud germination. Flowering (some species). Fruit development.
Extra GA3
No GA3
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Lack GA3
Have GA3
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Where Produced Apical Meristems. Young leaves. Embryos.
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Abscisic Acid Slows or inhibits plant growth. "Stress" hormone produced under unfavorable
conditions.
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Major Effects Inhibits growth Seed/Bud dormancy. Stomata closure. Leaf drop – produces abscission layer.
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Abscission Layer
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Where Produced Leaves Stems Green fruit
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Ethylene Gaseous hormone (fast diffusion rates). Often interacts with Auxin.
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Major Effects Fruit ripening. Accelerates Senescence. Stem/Root Elongation (+ or -).
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Where Produced Ripening fruits. Senescent tissue. Nodes.
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New Hormones Oligosaccharins – short chains of sugars released
from the cell wall. Function:
Pathogen responses Cell differentiation Flowering
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New Hormones Brassinosteroids – steroid hormones similar to
animal sex hormones. Function:
Needed for normal growth and development.
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Commercial Applications of Plant Hormones Weed killers Seedless fruit Rooting of cuttings Tissue culture
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Plant Movements1. Tropisms
2. Circadian Rhythms
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Tropisms Growth responses in response to external
stimuli. + toward a stimulus - away from stimulus
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Examples1. Phototropism
2. Gravitropism
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Phototropism Response to light (blue).
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Movie
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Gravitropism Response to gravity. Stems are – gravitropic and roots are +
gravitropic.
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Gravitropism - mechanism Statolith movement
may be the receptor for the stimulus.
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Thigmotropism Response to touch. A series of 5 genes are involved. Ex: Tendrils
Climbing stems Wind direction response of stems.
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Turgor Movements Movement caused by turgor pressure
differences in certain cells.
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Types1. Rapid Leaf Movement
Ex: Mimosa2. Sleep Movements
Ex: Bean Leaves Prayer Plant
Day
Night
Sleep Movements
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Mimosa Rapid Leaf Movement
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Circadian Rhythms A physiological cycle about 24 hours long. Ex: Stomata opening
Sleep movements
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Causes Synthesis of a transcription factor protein that
regulates is own manufacturing through feedback control.
Gene is believed to be common in most eukaryotic organisms.
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Photoperiodism A physiological response to changing day
lengths. Used to detect and direct growth responses to
seasonal changes.
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Advantages Match growth responses to proper season. Ex: Leaf drop in fall
Flowering
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Flowering Types1. Short - Day Plants
2. Long - Day Plants
3. Day - Neutral Plants
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Short-Day Plants Flower when days are shorter than a critical
period (long nights). Ex: Mums
Poinsettias
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Long-Day Plants Flower when days are longer than a critical
period (short nights). Ex: Spinach
Iris Lettuce
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Day-Neutral Plants Flower whenever they have enough energy. Ex: Roses
African Violets
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Night Length Actually controls flowering response, not day
length. Proof – experiments show that if you interrupt
the dark period, you reset the “clock”.
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Comment Length of night not absolute, but relative for
the response to be triggered.
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Question
What detects day/night length changes? Phytochrome - plant pigment involved with
photoperiodism.
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Phytochrome Forms
Pr - responds to
660nm (red light).
Pfr - responds to
730nm (far red).
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Phytochrome Changes between the two forms.
Ratio or accumulation of enough Pfr triggers
the responses
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In Red light: Pr
Pfr Far-red light or darkness:
Pfr Pr
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Photoperiodism
Very sensitive (1 minute difference).
Sets clocks for plant responses.
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Other Effects Seed Germination Stomatal Opening Leaf Drop
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Lettuce Germination
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Responses to Stress Stress – an environmental condition that can
have an adverse effect on a plant’s growth, reproduction and survival.
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Plant Response1. Developmental changes
2. Physiological changes
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Water Deficit During high Ts, guard cells may close. Young leaves may slow expansion. Leaves may roll to reduce surface area.
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Oxygen Deprivation Common in roots in water-logged soils. Air tubes in roots may bring oxygen to the
cells.
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Salt Stress Damages the plant through unfavorable soil
water potentials and toxic ions. Some plants can concentrate and excrete salt
through salt glands (ex. halophytes).
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Heat and Cold Stress Heat - use heat-shock proteins to protect
other proteins from denaturing. Cold – lipid shifts to keep lipid bilayers
“liquid”. Cold – solute changes to lower freezing
point.
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Herbivores Plants have many physical and chemical
defenses against herbivores. Physical – thorns Chemical – crystals, tannins and other toxic
compounds.
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Herbivores Often trigger a plant to release chemicals to
attract predators or to warn other plants to increase their production of toxins.
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Pathogens First Defense – epidermis Second Defense – chemical events to restrict
or kill the invader.
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SAR
Systemic Acquired Resistance: chemicals that spread the “alarm” of an infection to other parts of the plant.
Possible Candidate: salicylic acid
![Page 80: Chapter 39: Control Systems in Plants. Question Do plants sense and respond to their environment ? Yes - By adjusting their pattern of growth and.](https://reader035.fdocuments.in/reader035/viewer/2022062422/56649ed05503460f94bdddbe/html5/thumbnails/80.jpg)
![Page 81: Chapter 39: Control Systems in Plants. Question Do plants sense and respond to their environment ? Yes - By adjusting their pattern of growth and.](https://reader035.fdocuments.in/reader035/viewer/2022062422/56649ed05503460f94bdddbe/html5/thumbnails/81.jpg)
Summary Know the general plant hormones and their
effects. Know tropisms. Know photoperiodism. Know general ideas about how plants respond
to stress.