Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 39.3 Responses to Light Are Critical for Plant Success
Light is more than just energy—it’s a key environmental signal. Plants detect light’s intensity, direction, and wavelength to regulate growth, development, flowering, and internal clocks. These responses are essential to plant survival and reproduction.
Light as a Developmental Signal
- Photomorphogenesis: Development triggered by light (e.g., seedling greening).
- Plants detect:
- Light direction
- Intensity
- Wavelength (color)
- Action spectra show which wavelengths cause specific responses—red and blue light are most important.
Blue-Light Photoreceptors
- Regulate:
- Phototropism (growth toward light)
- Stomatal opening
- Inhibition of hypocotyl elongation
- Types:
- Cryptochromes: Suppress stem elongation after emergence
- Phototropins: Control phototropism and chloroplast movement
Phytochromes and Red/Far-Red Light
- Phytochromes absorb red (660 nm) and far-red (730 nm) light.
- Two forms:
- Pr: Inactive, absorbs red → becomes Pfr
- Pfr: Active, absorbs far-red → reverts to Pr
- Pfr initiates:
- Seed germination
- Inhibition of stem elongation
- Branching stimulation
- Internal clock setting
- Flowering control
Seed Germination Example
- Red light: Promotes germination (Pr → Pfr)
- Far-red light: Inhibits germination (Pfr → Pr)
- The last light received determines the response—reversible effect
Shade Avoidance
- Canopy filters red light, allowing far-red to reach shaded plants
- Low Pfr triggers stem elongation to escape shade
- Direct light increases Pfr, promoting branching and halting elongation
Circadian Rhythms and Biological Clocks
- Plants have ~24-hour endogenous rhythms that regulate:
- Leaf movements
- Stomatal opening
- Photosynthetic protein production
- These rhythms persist in constant light/dark
- Light entrainment aligns the clock to daily cycles
- Phytochromes and blue-light receptors reset clocks at dawn
Photoperiodism: Measuring the Seasons
- Photoperiodism: Flowering response to night length
- Plant types:
- Short-day: Flower when nights are long
- Long-day: Flower when nights are short
- Day-neutral: Flower regardless of light length (e.g., tomatoes, rice)
A Flowering Hormone?
- Leaves sense photoperiod and signal shoot tips to flower
- Experiments suggest a mobile signal called florigen
- FT gene codes for the florigen protein, which travels to the shoot
- Phytochrome light input regulates FT expression
In a Nutshell
Light isn’t just for photosynthesis—it shapes nearly every aspect of plant life. Plants use specialized receptors to monitor light quality, direction, and timing, enabling precise responses that guide growth, flowering, and circadian rhythms. Through red/far-red and blue-light systems, they stay in tune with the changing environment.