Chapter 39
Chapter 39
Environmental Complexity
Overview: Most organisms exist in heterogeneous environments, requiring complex intercommunication and coordination among body systems.
Perception: Sensing environmental stimuli.
Transduction: Converting stimuli into a biological signal.
Response: Physiological action following transduction.
Plant Sensory Systems
Plant sensory systems and response mechanisms are generally simple, with slow signal transport methods.
Signal transport typically involves movement through the cortex parenchyma or phloem.
Environmental Inputs
Light
Light provides significant environmental information:
Direction of Light: Helps plants orient leaves towards the light source.
Duration of Light: Indicates the time of year, influencing growth cycles.
Gravity
Gravity influences plant orientation positively for optimal growth:
Roots: Grow downward to access water and minerals.
Shoots: Grow upward for better photosynthesis and pollination.
Touch
Plants respond to contact with objects, which guides their growth:
They may use objects for structural support.
Touch can influence direction; structures may fuse due to crowding.
Temperature
Temperature significantly impacts plant development and metabolism:
Cold temperatures are necessary for the normal flowering of biennials and perennials.
Winter cold can vernalize biennial plants, prompting reproductive states.
Low temperatures can induce dormancy in seeds and trees.
Water
Water plays a different role in plant signaling compared to other stimuli:
Roots do not specifically grow towards water, but those that grow in that direction do so rapidly due to favorable conditions.
Responses to water scarcity include:
Abscisic Acid: Triggers stomatal closure and inhibits new leaf production.
Abscission of leaves due to water stress.
Tropic Responses
Definitions:
Positive Tropism: Growth towards a stimulus.
Negative Tropism: Growth away from a stimulus.
Plagiotropism: Growth at an angle.
Types of Tropic Responses
Phototropism: Growth towards light.
Thigmotropism: Growth response to touch, common in tendrils.
Chemotropism: Positive chemotropism seen in pollen tubes during fertilization.
Nastic Responses
Definition: Stereotyped non-growth movements not directed by the stimulus.
Photonastic Response: Light presence or absence affects flower opening.
These movements result from turgor pressure changes rather than growth.
Morphogenic Responses
Affects plant metabolism or tissue state:
Photomorphogenic Responses: Include flower induction and seed germination.
Taxis
Definition: Movement towards (positive taxis) or away from (negative taxis) a stimulus.
Examples include:
Chemotaxis: Sperm cells swimming towards egg cells.
Phototaxis: Algae moving towards or away from light.
Perception and Transduction
Responses typically occur in different tissues than where the stimuli are sensed.
Statocytes: Specialized cells involved in gravity perception with statoliths as weight-sensing structures.
Presentation Time and Threshold
The time a stimulus must be present to trigger a response is termed presentation time.
Threshold: Minimum stimulus level necessary for perception and transduction to occur.
Response Types
All-or-None Response: Triggered if threshold conditions are met.
Dosage-Dependent Response: Amount/duration of stimulus influences response magnitude.
Chemical Messengers
Hormones: Key for plant communication:
Produced in one area and act on others, often at low concentrations.
Action involves binding to receptor molecules that trigger responses.
Other Compounds with Hormone-Like Activity
Brassinosteroids: Involved in growth and vascular differentiation.
Jasmonic Acid: Defense against biotic stressors.
Salicylic Acid: Resistance to pathogens.
Auxin Functions
Activation/Inhibition of Shoots
Auxin: Produced by shoot apical meristems, causes:
Cell elongation
Apical dominance
Differentiation of vascular tissues.
Auxin and Cell Elongation
Promotes elongation in young internodes but not in mature cells (lack receptors).
Auxin and Apical Dominance
Maintains dormancy in axillary buds, promoting a single active meristem per shoot tip.
Decrease in auxin concentration activates dormant buds, allowing branch growth.
Auxin and Vascular Differentiation
Stimulates dormant vascular cambium, promoting xylem differentiation during spring.
Hormonal Interactions in Shoots
Auxin and Cytokinins:
Apical dominance influenced by hormone interplay, where roots synthesize cytokinins that stimulate axillary buds.
Auxin and Gibberellin:
Auxin activates cambium; gibberellin differentiates new cells into phloem.
Environmental Factors and Hormones
Hormonal control influences plant responses to environmental stimuli:
Ethylene: Regulates auxin during fruit ripening and abscission.
Tropisms and Light
Blue light is most effective for phototropism, detected by phototropin proteins at different plant locations.
Higher auxin concentration on the darker side of stems causes bending towards light.
Gravitropism in Roots
Gravitropism: Detected by root caps; growth inhibitors moved to the lower side to slow down growth and adjust positioning.
Flowering Induction
Plants require maturation (adult phase) to flower, affected by environmental cues:
Cold temperatures trigger reproductive phase change in biennials.
Photoperiodicity in Flowering
Flowering can be induced by photoperiod (day length):
Short-day Plants: Flower when days are short.
Long-day Plants: Flower during long days.
Day-neutral Plants: Unaffected by day length.
Critical Night Length
Night length is crucial for flowering:
Each species has specific critical lengths that dictate flowering based on night duration.
Perception of Night Length
Young leaves sense night length; response requires chemical signaling to apical meristem.
Endogenous Rhythms in Plants
Plants exhibit endogenous rhythms, controlled internally:
These rhythms are typically around 24 hours in synchrony with natural light conditions.