Lecture # 5 Light (Review of Plant Physiology ). Light is electromagnetic radiation, emitted when an...

Lecture # 5Lecture # 5LightLight

(Review of Plant Physiology(Review of Plant Physiology))

Light is electromagnetic radiation, emitted Light is electromagnetic radiation, emitted when an electrical dipole (a paired positive when an electrical dipole (a paired positive and negative charge, separated by a small and negative charge, separated by a small distance) in an atom oscillates and causes distance) in an atom oscillates and causes change in the field of force. The dipole change in the field of force. The dipole produces an electrical and a magnetic produces an electrical and a magnetic vector, which are in phase but at right vector, which are in phase but at right angles. Fluctuations in the field strength of angles. Fluctuations in the field strength of these vectors are perpendicular to the these vectors are perpendicular to the direction of travel of the wave, and hence, direction of travel of the wave, and hence, light is a transverse wave.light is a transverse wave.

What Is Light?What Is Light?

The Nature of LightThe Nature of Light

Characteristics of LightCharacteristics of Light

The electromagnetic wave The electromagnetic wave is characterized by both wavelength, is characterized by both wavelength, whichwhich is the distance between successive is the distance between successive positivepositive or negative maxima on the sine wave, or negative maxima on the sine wave, and byand by frequency, frequency, the number of oscillations the number of oscillations perper unit time. Wavelength and frequency areunit time. Wavelength and frequency are related by the velocity of propagation of related by the velocity of propagation of thethe

wave, wave,

The Dual Nature of LightThe Dual Nature of Light

Wave - wavelength, frequency, velocity.Wave - wavelength, frequency, velocity. Particle - “photon”; each photon containsParticle - “photon”; each photon contains

an amount of energy - quantum (pl. quanta).an amount of energy - quantum (pl. quanta).

The energy content of light is delivered inThe energy content of light is delivered in

discrete units, quanta. The photon has nodiscrete units, quanta. The photon has no

rest mass.rest mass.

Quantum and photon are distinct Quantum and photon are distinct concepts !concepts !

Quantum is the energy carried by a photon Quantum is the energy carried by a photon !!

Characteristics of LightCharacteristics of Light

The energy of a photon (The energy of a photon () depends on the ) depends on the frequencyfrequency of the electromagnetic wave, which is related to of the electromagnetic wave, which is related to ::

h h h*vh*vhh = Planck’s = Planck’s constantconstant The greater the frequency, and the smaller theThe greater the frequency, and the smaller thewavelength, the larger the energy of the photon.wavelength, the larger the energy of the photon.1 mole contains as many elementary particles as1 mole contains as many elementary particles as

there are carbon atoms in 0.012 kg of there are carbon atoms in 0.012 kg of 1212C, orC, orAvogadro’s number, 6.023 x 10Avogadro’s number, 6.023 x 102323, of particles. , of particles.

1 mole photons (mole of quanta) = 1 mole photons (mole of quanta) = = 1 Einstein = 6.023 x 10= 1 Einstein = 6.023 x 102323

WWHATHAT IS IS LLIGHT ?IGHT ? Light is an electromagnetic radiation coming Light is an electromagnetic radiation coming

from the sun. It has a wide spectrum – from cosmic from the sun. It has a wide spectrum – from cosmic rays (very short) to radio waves (very long).rays (very short) to radio waves (very long).

Light behaves as a wave and has a wavelength. Light behaves as a wave and has a wavelength. The shorter the wavelength, the more energy the The shorter the wavelength, the more energy the light carries.light carries.

Our eyes are sensitive to only a small part of Our eyes are sensitive to only a small part of the light spectrum – from 400 to 700 nanometers.the light spectrum – from 400 to 700 nanometers.

WWHATHAT ISIS LLIGHT ?IGHT ? Just like us, plants are sensitive to the same Just like us, plants are sensitive to the same

light spectrum range 400 - 700 nm, called a light spectrum range 400 - 700 nm, called a Photosynthetic Active Radiation (PAR). Photosynthetic Active Radiation (PAR).

Plants absorb blue and red light but reflect Plants absorb blue and red light but reflect green and this is why we see them colored green. green and this is why we see them colored green.

Actually, plants emit a lot more far-red light Actually, plants emit a lot more far-red light (800 nm) and if our eyes were sensitive to it, we (800 nm) and if our eyes were sensitive to it, we would be seeing all plants as very, very dark red!would be seeing all plants as very, very dark red!

A plant exposed to far red lightA plant exposed to far red lightgrows fast - in heightgrows fast - in height

Once it is past that other green fellow Once it is past that other green fellow it expands its green umbrella to revise its it expands its green umbrella to revise its

strategy for absorbing energy.strategy for absorbing energy.

Lars BjornLars Bjorn

Question: In what two basic ways does Question: In what two basic ways does the plant use light ?the plant use light ?

Ways a Plant Can Perceive LightWays a Plant Can Perceive Light

Light Quantity (photon counting).Light Quantity (photon counting). Light Quality (photon ratios).Light Quality (photon ratios). Light Duration (timing light / darkLight Duration (timing light / dark

transitions).transitions). Light Direction (photon gradients).Light Direction (photon gradients). Light Polarization (dichroic Light Polarization (dichroic photoreceptor).photoreceptor).

TERMS: TERMS: Photosynthesis Photosynthesis

PhotoperiodismPhotoperiodism

PhotomorphogenesisPhotomorphogenesis

PhototropismPhototropism

PhototaxisPhototaxis

Ways a Plant Can Perceive LightWays a Plant Can Perceive Light

Light Light TerminologyTerminology

PhotosynthesisPhotosynthesis : “making something : “making something using light”; continual input of energy using light”; continual input of energy isis required.required. PhotoperiodismPhotoperiodism : A nondirectional : A nondirectional responseresponse to a nondirectional to a nondirectional periodicperiodic stimulus. stimulus. PhotomorphogenesisPhotomorphogenesis : A nondirectional: A nondirectional developmental response to a developmental response to a nondirectionalnondirectional nonperiodicnonperiodic stimulus. stimulus.

Light Light TerminologyTerminology

PhototropismPhototropism : A directional growth: A directional growth

response to a directional stimulus.response to a directional stimulus. PhototaxisPhototaxis : A directional movement : A directional movement to ato a

nondirectional nondirectional periodicperiodic stimulus. stimulus.

Measurements of Measurements of LightLight

Physical.Physical. Measuring the Measuring the energyenergy of light. of light. Energy - J, cal, erg, ev (electron volts)Energy - J, cal, erg, ev (electron volts) Power - energy/timePower - energy/time Irradiance - energy/time/per unit Irradiance - energy/time/per unit spacespace (area, volume)(area, volume)


Psychophysical.Psychophysical. Measuring the energy of light but Measuring the energy of light but tailored to fit the human eye. tailored to fit the human eye. Ft-c, lumens, lux (metric term) - all Ft-c, lumens, lux (metric term) - all areare intensity terms.intensity terms. Brightness - measures reflection from Brightness - measures reflection from aa surface.surface.


PhytophysicalPhytophysical.. Measuring the light detected by a Measuring the light detected by a plant.plant. Photosynthesis; PhotomorphogeneticPhotosynthesis; Photomorphogenetic processesprocesses

PhytophysicalPhytophysical::

Photon Irradiance (400-700nm) - Photon Irradiance (400-700nm) - molmmolm--

22ss-1-1

Photon Fluence RatePhoton Fluence Rate (2(2 or 4or 4sensor)sensor)


22 sensor 4sensor 4sensor sensor

Plant ResponsePlant Response

Plant maintenancePlant maintenance: : The minimum photosynthetic irradiance The minimum photosynthetic irradiance for afor a foliage plant is that which permits the foliage plant is that which permits the plantplant to function at a level slightly exceeding to function at a level slightly exceeding thethe compensation point at which compensation point at which photosynthesisphotosynthesis is equal to respiration.is equal to respiration.

Native habitatNative habitat Production irradiance levelsProduction irradiance levels Degree of acclimatization.Degree of acclimatization.

Questions to PonderQuestions to Ponder

1. 1. What is Light Compensation Point What is Light Compensation Point (LCP) ?(LCP) ?

2. What is CO2. What is CO22 Compensation Point ? Compensation Point ?

3. How are the LCP and CO3. How are the LCP and CO22 CP related ? CP related ?

4. 4. Which is more likely to perform better in Which is more likely to perform better in

interior environment - Cinterior environment - C33, C, C44, CAM, or C, CAM, or C33 - - CC44

type of carbon fixation mechanism ?type of carbon fixation mechanism ?

CO

2 A

ssim

ilati

on

(m

ol C

O2

m-2 s

-1)

Photosynthetically active radiation (mol quanta m-2 s-

1)

C3 plant

C4 plant

50

0

200 1000 2000

5. Study5. Study carefully the graph. carefully the graph. (question follows)(question follows)

5. Question:5. Question: The COThe CO22 assimilation curves were assimilation curves were obtainedobtained in a normal atmosphere (21% Oin a normal atmosphere (21% O22). ). How would the two curves (CHow would the two curves (C33 and C and C44) ) changechange if the same experiment was conducted in if the same experiment was conducted in anan atmosphere containing only 1% Oatmosphere containing only 1% O22 ? ?

Light and Light and Photosynthesis in Photosynthesis in

the Intact Leafthe Intact Leaf

Only about 5% of the solar radiationOnly about 5% of the solar radiation reaching the earth can be converted reaching the earth can be converted intointo carbohydrates by a carbohydrates by a photosynthesizingphotosynthesizing leaf. This is because a major fraction leaf. This is because a major fraction ofof the incident light is of wavelengths the incident light is of wavelengths tootoo short or too long to be absorbed by short or too long to be absorbed by thethe photosynthetic pigments.photosynthetic pigments.

Light and Light and Photosynthesis in the Photosynthesis in the

Intact LeafIntact Leaf

The Plant Leaf - An The Plant Leaf - An Overview of StructureOverview of Structure

The Architecture and Composition of a The Architecture and Composition of a Leaf Maximize Light AbsorptionLeaf Maximize Light Absorption

Palisade mesophyll cells: one to three Palisade mesophyll cells: one to three layerslayers

of columnar cells; they allow light to of columnar cells; they allow light to passpass

through.through. Spongy mesophyll cells: several layers ofSpongy mesophyll cells: several layers of

irregularly shaped cells; they trap lightirregularly shaped cells; they trap light

passing from the palisade layer. passing from the palisade layer.

Sieve effect: due to the fact that theSieve effect: due to the fact that the chlorophyll is not uniformly distributed in chlorophyll is not uniformly distributed in thethe cells but is confined to the chloroplasts. cells but is confined to the chloroplasts. This clustering of chloroplasts results inThis clustering of chloroplasts results in shading between the chlorophyll shading between the chlorophyll molecules, somolecules, so that the total absorption of light by a that the total absorption of light by a givengiven amount of chlorophyll in a chloroplast is amount of chlorophyll in a chloroplast is lessless than that of the same amount in a than that of the same amount in a solution.solution.


Light guide effect: channeling of some ofLight guide effect: channeling of some of the incident light through the the incident light through the intercellularintercellular spaces between the palisade cells, in aspaces between the palisade cells, in a manner analogous to transmission of manner analogous to transmission of light bylight by an optical fiber. an optical fiber.


The Photosynthetic Response to Light The Photosynthetic Response to Light in the Leaf Reflects Basic Properties of in the Leaf Reflects Basic Properties of

the Photosynthetic Apparatusthe Photosynthetic Apparatus

Light Compensation PointLight Compensation Point: : The photosynthetic irradiance at which The photosynthetic irradiance at which thethe

net COnet CO2 2 exchange in the leaf is zero. exchange in the leaf is zero. At this point the amount of COAt this point the amount of CO22 evolved evolved byby mitochondrial respiration is balanced by mitochondrial respiration is balanced by thethe amount of COamount of CO22 fixed by photosynthesis. fixed by photosynthesis.

LCP for sun plants - 10 - 20 LCP for sun plants - 10 - 20 molmmolm-2-2ss-1-1

LCP for shade plants - 1 - 5 LCP for shade plants - 1 - 5 molmmolm-2-2ss-1-1

Carbon Dioxide Compensation Point:Carbon Dioxide Compensation Point: The COThe CO22 concentration at which CO concentration at which CO22 assimilation is zero.assimilation is zero. At this point COAt this point CO22 fixation by fixation by photosynthesisphotosynthesis balances CObalances CO22 loss by respiration. loss by respiration.

The Photosynthetic Response to Light The Photosynthetic Response to Light in the Leaf Reflects Basic Properties of in the Leaf Reflects Basic Properties of

the Photosynthetic Apparatusthe Photosynthetic Apparatus

The COThe CO22 Compensation Point Compensation Point reflectsreflects

the balance between the balance between photosynthesisphotosynthesis

and respiration as a function of and respiration as a function of COCO2 2

concentration, and the Lightconcentration, and the LightCompensation Point reflects such Compensation Point reflects such

aabalance as a function ofbalance as a function of

photosynthetic irradiance.photosynthetic irradiance.

The relationship between photosynthesis and The relationship between photosynthesis and respiration in the daily cycle of a plant.respiration in the daily cycle of a plant.

Dark Increasing irradiance BrightDark Increasing irradiance Bright

Net

loss

of

CO

Net

loss

of

CO

22 N

et

up

take

of

Net

up

take

of

CO

CO

22Compensation PointCompensation Point rate of respirationrate of respiration

equals rate ofequals rate of photosynthesisphotosynthesis

PhotosynthesisPhotosynthesis exceeds respiration:exceeds respiration:

food storedfood stored

RespirationRespiration exceeds photosynthesis:exceeds photosynthesis:

food used upfood used up

Relative Light Intensity Relative Light Intensity (Increasing)(Increasing)

Light Compensation PointLight Compensation Point

Light Saturation pointLight Saturation point

More photosynthesisMore photosynthesis than respirationthan respiration

Net

gas

exch

an

ge

Net

gas

exch

an

ge

More respiration than photosynthesisMore respiration than photosynthesis

CO

CO

22 e

volu

tion

C

O e

volu

tion

C

O2

2

upta

keu

pta

ke

COCO22 evolution in darkness evolution in darkness

HighHigh

HighHigh

Photo

synth

esi

sPhoto

synth

esi

s

Leaf Age - monthsLeaf Age - months

DieffenbachiaDieffenbachia

PalmPalm

0 6 12 18 240 6 12 18 24

Lecture # 5 Light (Review of Plant Physiology ). Light is electromagnetic radiation, emitted when an...

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