Ability of plants to track time using light, sensing various wavelengths of sunlight to determine the time of day and year.
Phototropism
Directional growth response of plants towards (positive phototropism) or away from (negative phototropism) light.
Biological Clocks & Circadian Rhythms
Timing and duration of biological activities are controlled by a b biological clock , an oscillator toggling between two states approximately every 12 hours. This establishes circadian rhythms:
Period
Length of one cycle; is unaffected by temperature changes.
Amplitude
Magnitude of changes during the cycle; may decrease with cooler temperatures.
Persistence
Circadian rhythms can persist for days without external cues.
Rhythms can be entrained to light-dark cycles that do not precisely match 24 hours, aiding adaptation to varying day lengths through seasons.
Photomorphogenesis
Physiological and developmental responses driven by light, crucial for plant growth. Examples include:
Underground bean seedlings elongating upon emergence, triggering apical straightening, leaf unfolding, and chlorophyll production.
Lettuce seeds germinating only in response to light; even brief exposure can initiate germination.
Cocklebur plants flower only with prolonged darkness; exposure to light at night halts flowering.
This allows plants to optimize their utilization of light and space.
Aspects of Light for Plant Response
Quality
Wavelengths of light absorbed by plant molecules.
Quantity
Intensity and duration of light exposure.
Plant Photoreceptors and Their Functions
Light influences several plant processes:
Seed germination
Shoot elongation
Flower initiation
Classes of photoreceptors:
Phytochromes (Anaerobic) respond to red/far-red light (600-750 nm).