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