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