Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 36.1 Adaptations for Acquiring Resources Were Key Steps in the Evolution of Vascular Plants
Vascular plants inhabit two worlds—above ground (shoot system) for capturing light and CO₂, and below ground (root system) for absorbing water and minerals. The success of land plants depended on structural and physiological adaptations for resource acquisition and transport, especially as competition intensified.
Evolution of Resource Acquisition
- Early nonvascular plants lived in shallow freshwater and absorbed resources directly.
- As plants evolved:
- Taller shoots improved light capture.
- Broader leaves boosted photosynthesis but increased water loss.
- Larger plants required better anchorage and internal transport systems.
- Key evolutionary innovations:
- Branched multicellular roots
- Xylem and phloem for long-distance transport
Shoot Architecture and Light Capture
- Stem traits (length, thickness, branching) influence light access.
- Branching improves light capture but competes with vertical growth.
- Leaf traits:
- Size: large in humid climates, small in dry/cold regions
- Phyllotaxy (leaf arrangement):
- Alternate (spiral): common in angiosperms
- Opposite/whorled: reduces sun exposure per leaf
- Self-pruning: shaded lower leaves are shed if they're unproductive
- Leaf area index: total leaf surface vs. ground area. Too high → shading issues
Leaf Orientation
- Horizontal leaves: capture more light in shaded conditions
- Vertical leaves: reduce overheating and allow light to reach lower leaves
- Plants optimize leaf angle based on sunlight intensity and habitat
Photosynthesis–Water Loss Compromise
- Open stomata allow CO₂ intake but lead to transpiration
- Over 90% of water loss occurs through stomatal pores
- Shoot structures represent a trade-off between photosynthesis and water conservation
Root Architecture and Nutrient Uptake
- Roots adjust growth to soil nutrient levels:
- Low nitrate: roots grow through with minimal branching
- High nitrate: roots branch and upregulate transporters
- Self vs. non-self root recognition: plants avoid competing with their own roots
- Mutualism with mycorrhizae: fungi increase root absorption area, especially for phosphate
In a Nutshell
Plants adapted to land by evolving structures to efficiently gather light above ground and water/minerals below ground. Shoot and root architectures, as well as vascular tissue, enabled taller, more complex plants. Leaves balance photosynthesis with water conservation, while roots adapt their form and function based on environmental cues and symbiotic relationships.