Plant Response to StimuliPlant Response to Stimuli

Stimuli and a Stationary LifeStimuli and a Stationary Life

Plants, being rooted to the groundPlants, being rooted to the ground• Must respond to whatever Must respond to whatever

environmental change comes their wayenvironmental change comes their way

For example, the bending of a grass For example, the bending of a grass seedling toward lightseedling toward light• Begins with the plant sensing the Begins with the plant sensing the

direction, quantity, and color of the lightdirection, quantity, and color of the light

A potato left growing in darknessA potato left growing in darkness• Will produce shoots that do not appear Will produce shoots that do not appear

healthy, and will lack elongated rootshealthy, and will lack elongated roots These are morphological adaptations These are morphological adaptations

for growing in darknessfor growing in darkness• Collectively referred to as etiolationCollectively referred to as etiolation

(a) Before exposure to light. Adark-grown potato has tall,spindly stems and nonexpandedleaves—morphologicaladaptations that enable theshoots to penetrate the soil. Theroots are short, but there is littleneed for water absorptionbecause little water is lost by theshoots.

After the potato is exposed to lightAfter the potato is exposed to light• The plant undergoes profound changes The plant undergoes profound changes

called de-etiolation, in which shoots and called de-etiolation, in which shoots and roots grow normallyroots grow normally

(b) After a week’s exposure tonatural daylight. The potatoplant begins to resemble a typical plant with broad greenleaves, short sturdy stems, andlong roots. This transformationbegins with the reception oflight by a specific pigment,phytochrome.

The Discovery of Plant The Discovery of Plant HormonesHormones

Any growth responseAny growth response• That results in curvatures of whole plant That results in curvatures of whole plant

organs toward or away from a stimulus organs toward or away from a stimulus is called a tropismis called a tropism

• Is often caused by hormonesIs often caused by hormones

A Survey of Plant HormonesA Survey of Plant Hormones

In general, hormones control plant In general, hormones control plant growth and developmentgrowth and development• By affecting the division, elongation, and By affecting the division, elongation, and

differentiation of cellsdifferentiation of cells Plant hormones are produced in very Plant hormones are produced in very

low concentrationslow concentrations• But a minute amount can have a But a minute amount can have a

profound effect on the growth and profound effect on the growth and development of a plant organdevelopment of a plant organ

Light ResponseLight Response

Responses to light are critical for plant Responses to light are critical for plant successsuccess

Light cues many key events in plant Light cues many key events in plant growth and developmentgrowth and development

Photomorphogenesis Photomorphogenesis • Is the effects of light on plant morphology Is the effects of light on plant morphology

(structure)(structure)

Plants not only detect the presence of Plants not only detect the presence of lightlight• But also its direction, intensity, and But also its direction, intensity, and

wavelength (color)wavelength (color) A graph called an action spectrumA graph called an action spectrum

• Depicts the relative response of a process Depicts the relative response of a process to different wavelengths of lightto different wavelengths of light

Action spectraAction spectra• Are useful in the study of any process that Are useful in the study of any process that

depends on lightdepends on light

Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light.

EXPERIMENT

The graph below shows phototropic effectiveness (curvature per photon) relative to effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light.

RESULTS

CONCLUSION The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.

Research on action spectra and Research on action spectra and absorption spectra of pigmentsabsorption spectra of pigments• Led to the identification of two major Led to the identification of two major

classes of light receptors: blue-light classes of light receptors: blue-light photoreceptors and phytochromesphotoreceptors and phytochromes

Blue-Light PhotoreceptorsBlue-Light Photoreceptors

Various blue-light photoreceptorsVarious blue-light photoreceptors• Control hypocotyl elongation, stomatal Control hypocotyl elongation, stomatal

opening, and phototropismopening, and phototropism

Phytochromes as Phytochromes as PhotoreceptorsPhotoreceptors

PhytochromesPhytochromes• Regulate many of a plant’s responses to Regulate many of a plant’s responses to

light throughout its lifelight throughout its life

The Effect of Light on the The Effect of Light on the Biological ClockBiological Clock

Phytochrome conversion marks Phytochrome conversion marks sunrise and sunsetsunrise and sunset• Providing the biological clock with Providing the biological clock with

environmental cuesenvironmental cues

Photoperiodism and Responses Photoperiodism and Responses to Seasonsto Seasons

Photoperiod, the relative lengths of Photoperiod, the relative lengths of night and daynight and day• Is the environmental stimulus plants use Is the environmental stimulus plants use

most often to detect the time of yearmost often to detect the time of year PhotoperiodismPhotoperiodism

• Is a physiological response to photoperiodIs a physiological response to photoperiod

Photoperiodism and Control of Photoperiodism and Control of FloweringFlowering

Some developmental processes, Some developmental processes, including flowering in many speciesincluding flowering in many species• Requires a certain photoperiodRequires a certain photoperiod

Critical Night LengthCritical Night LengthDuring the 1940s, researchers conducted experiments in which periods of

darkness were interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering in “short-day” and “long-day” plants.

EXPERIMENT

RESULTS

CONCLUSION The experiments indicated that flowering of each species was determined by a critical period of darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day” plants are really “short-night” plants.

24 h

ou

rs

Darkness

Flash oflight

Criticaldarkperiod

Light

(a) “Short-day” plantsflowered only if a period ofcontinuous darkness waslonger than a critical darkperiod for that particularspecies (13 hours in thisexample).

(b) “Long-day” plantsflowered only if aperiod of continuousdarkness was shorterthan a critical darkperiod for thatparticular species (13hours in this example).

GravityGravity

Response to gravityResponse to gravity• Is known as gravitropismIs known as gravitropism

Roots show positive gravitropismRoots show positive gravitropism• Grow with gravityGrow with gravity

Stems show negative gravitropismStems show negative gravitropism• Grow against gravityGrow against gravity

Plants may detect gravity by the Plants may detect gravity by the settling of statolithssettling of statoliths• Specialized plastids containing dense Specialized plastids containing dense

starch grainsstarch grains

Statoliths20 m

GRAVITY

Environmental StressesEnvironmental Stresses

Environmental stressesEnvironmental stresses• Have a potentially adverse effect on a Have a potentially adverse effect on a

plant’s survival, growth, and plant’s survival, growth, and reproductionreproduction

• Can have a devastating impact on crop Can have a devastating impact on crop yields in agricultureyields in agriculture

DroughtDrought

During droughtDuring drought• Plants respond to water deficit by Plants respond to water deficit by

reducing transpirationreducing transpiration• Deeper roots continue to growDeeper roots continue to grow

FloodingFlooding Enzymatic destruction of cells creates air Enzymatic destruction of cells creates air

tubes that help plants survive oxygen tubes that help plants survive oxygen deprivation during floodingdeprivation during flooding

Vascularcylinder

Air tubes

Epidermis

100 m 100 m(a) Control root (aerated) (b) Experimental root (nonaerated)

Salt StressSalt Stress

Plants respond to salt stress by Plants respond to salt stress by producing solutes tolerated at high producing solutes tolerated at high concentrationsconcentrations• Keeping the water potential of cells Keeping the water potential of cells

more negative than that of the soil more negative than that of the soil solutionsolution