Plant Physiology & Development
Photomorphogenesis & Shade-Avoidance Responses
Plants use light quantity, direction, and spectral cues to shape architecture. Changes in red-to-far-red balance and local shading can influence internode elongation, leaf angle, branching, and allocation without being equivalent to simple light deficiency.
Key concepts
- Photomorphogenesis refers to developmental responses to light signals, not only photosynthetic energy capture
- Neighboring leaves alter both photon quantity and spectral composition
- Low red-to-far-red signals can contribute to shade-avoidance responses
- Architecture feeds back on future light interception
- Genotype strongly influences the magnitude and form of architectural responses
What to measure or observe
- Map internode length across canopy positions
- Compare branch angles and leaf orientation before and after crowding
- Measure PPFD rather than assuming shaded appearance equals low total light
- Document changes in plant spacing or canopy closure
- Track responses over several nodes rather than one internode
Common mistakes
- Calling all stretch a lack-of-light problem
- Ignoring spectral and neighbor cues
- Assuming one cultivar's architecture predicts another's
- Responding to elongation without checking temperature and developmental stage
- Using a single top-canopy light reading to explain lower-canopy morphology
Visuals this lesson still needs
- Sun versus canopy spectral comparison
- Shade-avoidance architecture sequence
- Red-to-far-red concept panel
- Canopy feedback loop diagram