Plant Responses to Environmental Stimuli. The Response to Light— The Action Spectrum.
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Transcript of Plant Responses to Environmental Stimuli. The Response to Light— The Action Spectrum.
Plant Responses to Environmental Stimuli
The Response to Light—The Action Spectrum
The Response to Light
Phytochrome—a photoreceptor—a protein bonded to a nonprotein part that functions as a chromophore, the light-absorbing part of the molecule.
The chromophore reverts between two forms (one absorbs red, Pr, one absorbs far red, Pfr)
Phytochrome Switching
Phytochrome Regulation of Seed Germination
Plants’ Biological Clock
Circadian Rhythm—Physiological cycles with a frequency of about 24 hours not directly paced by any known environmental variable
Light can contribute to the control of circadian rhythms
Photoperiodism
A physiological response (like flowering) to a photoperiod (cycle of light and dark)
Short-day plants—requires a light period shorter than a critical length to flower. Ex. Poinsettias, chyrsanthemums
Long-day plants—requires a light period longer than a critical length to flower. Ex. Spinach, radish, iris
Day-neutral plants—flower at a stage of maturity regardless of hours of light. Ex. Tomatoes, rice
Photoperiodism
Gravitropism
Roots exhibit positive gravitropism Stems exhibit negative
gravitropism
Thigmotropism
Directional growth because of touch
Climbing vines use thigmotropism to coil around supports
Responses to Drought
Guard cells lose turgor and close Increased synthesis & release of
abscisic acid resulting in closing of stomata
Young leaf growth inhibited Leaves roll in a shape to decrease
transpiration Growth of deeper roots
Other Responses
Flooding—stimulates production of ethylene for apoptosis; dead cell space is “snorkel”
Salt Stress—keep cell water potential lower than that in salty soil
Heat Stress—Synthesize heat shock proteins which protect enzyme shape
Cold Stress—Increase proportion of unsaturated lipids in plasma membrane to maintain fluidity