Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 39.4 Plants Respond to a Wide Variety of Stimuli Other Than Light
Although they’re rooted in place, plants are remarkably responsive to non-light environmental cues like gravity, touch, drought, and temperature. These responses help plants survive and adapt to changing surroundings.
Gravitropism: Response to Gravity
- Roots grow downward (positive gravitropism); shoots grow upward (negative).
- Statoliths (dense starch granules) settle with gravity and signal growth direction.
- Calcium and auxin redistribute to slow growth on one side, bending the root/shoot.
- Even without statoliths, some plants sense gravity—possibly through cytoskeletal tension.
Thigmomorphogenesis: Mechanical Stimuli
- Mechanical stress causes altered growth:
- Windy conditions produce short, stocky plants
- Touching leaves can change their growth patterns
- Thigmomorphogenesis: Growth response to mechanical stimulation
- Thigmotropism: Directional growth in response to touch (e.g., tendrils)
- Touch-sensitive plants like Mimosa pudica fold leaves via:
- Turgor loss in pulvini cells (K⁺ and water exit)
- Action potentials (slow electrical signals) spread the response
Drought Response
- Drought triggers:
- Stomatal closure via abscisic acid (ABA)
- Leaf rolling, drop, or reduced growth
- Plants can detect chemical warnings from wilting neighbors
- Crop yield is limited due to trade-offs with photosynthesis
Flooding Response
- Flooded roots lack oxygen
- Ethylene triggers cell death in root cortex → forms air tubes (aerenchyma) to supply oxygen
Salt Stress
- Salinity reduces water uptake and causes ion toxicity
- Plants respond by:
- Producing compatible solutes to retain water
- Limiting sodium accumulation
- Halophytes have adaptations like salt glands to excrete excess salt
Heat Stress
- Heat causes:
- Protein denaturation
- Reduced photosynthesis and growth
- Plant responses include:
- Evaporative cooling through transpiration
- Heat-shock proteins to stabilize damaged proteins
Cold Stress
- Cold affects:
- Membrane fluidity → plants add unsaturated fats
- Water balance → extracellular ice draws water from cells
- Some plants make antifreeze proteins to stop ice crystal growth
- These proteins are found in diverse species (e.g., fish, rye)—a case of convergent evolution
In a Nutshell
Plants are far from passive—they respond dynamically to gravity, touch, drought, salinity, and temperature. Through cell signaling, hormones, and structural changes, plants survive and thrive in challenging environments—all while staying rooted in place.