Photobiology3rd Year Student of biophysics

Prepared ByProf. Dr. Mohammed Naguib Abd El-Ghany Hasaneen

Professor Of Plant Metabolism And Biotechnology

Academic Year2005 - 2006

ContentsIntroductionRadiationVisible lightUltraviolet lightUltraviolet light damagePhytochrome conceptDistribution and translocation of phytochromePhysiological effects of phytochrome

We see visible light (350-700 nm)

Plants sense Ultra violet (280) to Infrared (800)

Examples Seed germination - inhibited by light Stem elongation- inhibited by lightShade avoidance- mediated by far-red light

There are probably 4 photoreceptors in plants

We will deal with the best understood; PHYTOCHROMESLight in PlantsIntroduction

A Primer on Radiation

Some important plant responses to radiation

(light is only one form of radiation):

PhotosynthesisPhotomorphogenesis; Photropism; PhotoperiodismEnergy balance/temperaturerespirationenzyme activitytranspirationUV-responsesmutagenesis

(note that there is a much more detailed table and discussion of responses of plants to light in chapter 1 of Hart: Light and Plant Growth)

In what form does energy from the sun travel to Earth?Energy travels to Earth in the form of electromagnetic wavesElectromagnetic waves are classified according to wave lengthRadiation is the direct transfer of energy by electromagnetic waves

Most of the energy from the sun reaches Earth in the form ofVisible lightInfrared radiationA small amount of ultraviolet radiation

The different colors of light make up the visible spectrum.Red has the longest wave lengthViolet has the shortest wave length

Infrared radiation has the following properties:Wavelengths longer than red lightIt is not visibleIt can be felt as heatUsed to warm food or baby chicks in an incubator

Ultraviolet light has the following properties:Wave lengths shorter than violet lightCan cause skin damageCan cause eye problems

Radiation and radiation lawsThe way we describe and quantify radiation, and the units used, vary depending on the kind of process were interested inProperties of radiation that are important to plants include Quality, Quantity, Direction (including diffuse vs. direct) and Periodicity.

Radiation quality (or color, for visible light) is a function of its wavelength (or frequency) distributionThe symbol l is often used for wavelengthNote these two charts are arrayed in opposite directions one by increasing wavelength/decreasing energy and the other by increasing frequency/increasing energy

Radiation quantity is measured in one of three ways, depending on the application:

Quantum measurements (numbers of photons)Radiometric measurements (amount of energy)Photometric measurements (light intensity, based on human perception)

Radiation measurements

The amount of radiation is expressed as fluence (also known as density; quantity per area), rate (also known as flux; quantity per time) or fluence rate (also known as flux density; amount per area per time)

For studies of photosynthesis and photomorphogenesis, the quantity of radiation is usually measured in quantum units (quantum flux density; quantum fluence rate):

mmol m-2 s-1

usually, only the visible, or photosynthetically active part of the spectrum is measured, or in the case of photomorphogenesis, only specific wavelengthsPPFD = photosynthetically activephoton flux density

PAR = photosynthetically active radiation(400-700 nm)Note that mol refers to a mole of photons, and that 1 mol photons=1 Einstein. A quantum is one indivisible package of radiation, or one photon.

For energy balance studies, radiation is measured in radiometric units, for example:

Watts m-2(note: 1 Watt = 1 Joule s-1)

Radiometric and quantum units are interconverted based on the amount of energy in photons. See link from website to working with light or p. 28 of the handout by Hart or any good reference on radiation for more information on this conversion)

Because most light sources contain a wide range of wavelengths, it is difficult to convert precisely between quantum and radiometric units. Usually an approximation is used that assumes a typical distribution of wavelengths for a particular light source

All objects emit radiation (i.e., they radiate) as a function of their temperature (in addition to the emissivity of the material). Temperature affects both the amount and the quality (wavelength) of radiation emitted.

The bulk of solar radiation is shortwave (visible plus near infrared)

visible = 400-700 nm, about 45% of incident insolation solar IR = 700-5000 nm, about 46% of incident UV = 190-400 nm, about 9% of incident

Spectral Quality

Restating this as a rough rule of thumb:

When the sky is clear, the photosynthetically active part of the solar spectrum accounts for about HALF of the total solar energy, IR accounts for the other half

Radiance vs. Irradiance:

Radiance is the radiation that is emitted from an object

Irradiance is the radiation that impinges upon an object In this case, radiation is commonly described as a flux (rate), or amount per unit time. This could be either a radiant flux or a quantum fluxIn this case, radiation is commonly described as a flux density, or amount per unit time per unit area. Again, the flux could be quantified either with either radiometric or photometric units.

Irradiance usually has both direct and diffuse components:

The amount of energy in direct-beam irradiance is strongly affected by the angle between the surface and the beam

Solar angle and leaf angle can have a very big influence on irradiation, dramatically affecting photosynthesis, transpiration and leaf temperaturedefinitions:Heliotropic: sun trackingParaheliotropic: leaf stays parallel to direct beam of sunDiaheliotropic: leaf stays perpendicular to direct beam

Connections between matter and energyA short, painless review of simple organic chemistry to develop the connection between cycles of organic biomass and cycles of energy

(Inorganic; not a hydrocarbon.This is a highly oxidized form of carbon)

general deterioration of #4 green

general deterioration of #4 greenshade from trees and tower

general deterioration of #4 greenshade from trees and towerpoor air circulation from trees and shrubsconcentrated traffic between trap and green

general deterioration of #4 greenshade from trees and towerpoor internal and surface drainagepoor air circulation from trees and shrubsconcentrated traffic between trap and green

general deterioration of #4 greenshade from trees and towerpoor internal and surface drainagepoor air circulation from trees and shrubsconcentrated traffic between trap and green

general deterioration of #4 greenshade from trees and towerdelicate turfgrasspoor internal and surface drainageO2-deficient rootzonepoor air circulation from trees and shrubshot, humid microenvironmentconcentrated traffic between trap and green

Wavelength - ENERGYPhotons in short wavelengths pack a lot of energyVisible light (400-750nm): 1 mole of photons = 250kJ energyUltraviolet light (< 400 nm):1 mole of photons = 500 kJ energyPhotons in longer wavelengths do notInfrared radiation (>750 nm)1 mole of photons = 85 kJ energy

What happens when sunlight hits the wall of a building?

Some reflected back to space (no effect) (this depends upon the COLOR of the wall!)Most is absorbed. Then what?Absorption of radiation makes the temperature of the object riseHow hot?The hotter the more radiation emitted (as infrared)Heats until energy in = energy outOr energy absorbed = energy re-radiated

The Thermal EnvironmentEnergy is gained and lost through various pathways:radiation - all objects emit electromagnetic radiation and receive this from sunlight and from other objects in the environmentconduction - direct transfer of kinetic energy of heat to/from objects in direct contact with one anotherconvection - direct transfer of kinetic energy of heat to/from moving air and waterevaporation - heat loss as water is evaporated from organisms surface (2.43 kJ/g at 30oC)

change in heat content = metabolism - evaporation + radiation+ conduction + convection

Organisms must cope with temperature extremes.Unlike birds and mammals, most organisms do not regulate their body temperatures.All organisms, regardless of ability to thermoregulate, are subject to thermal constraints:most life processes occur within the temperature range of liquid water, 0o-100oCfew living things survive temperatures in excess of 45oCfreezing is generally harmful to cells and tissues

So how do organisms regulate temperature?Manipulating the energy balance equation!Net radiation Color, Orientation to sun, Minimizing/maximize IR losses (insulation)ConductionUse warm or cool surfacesConvection:Minimize or maximize exposure to wind or water (boundary layers, exposure, immersion)Evaporation:Minimize or maximize evaporation to control heat lossMetabolism: Generate or limit generation of heat!These can be morphological, physiological, or behavioral adaptations

Conserving Water in Hot Environments Animals of deserts may experience environmental temperatures in excess of body temperature:evaporative cooling is an option, but water is scarceanimals may also avoid high temperatures by:reducing activityseeking cool microclimatesmigrating seasonally to cooler climates

Conserving Water in Hot Environments Desert plants reduce heat loading in several ways already discussed. Plants may, in addition:orient leaves to minimize solar gainshed leaves and become inactive during stressful periods

The Kangaroo Rat - a Desert SpecialistThese small desert rodents perform well in a nearly waterless and extremely hot setting.kangaroo rats conserve water by:producing concentrated urineproducing nearly dry fecesminimizing evaporative losses from lungskangaroo rats avoid desert heat by:venturing above ground only at nightremaining in cool, humid burrow by day

Tolerance of FreezingFreezing disrupts life processes and ice crystals can damage delicate cell structures.Adaptations among organisms vary:maintain internal temperature well above freezingactivate mechanisms that resist freezingglycerol or glycoproteins lower freezing point effectively (the antifreeze solution)glycoproteins can also impede the development of ice crystals, permitting supercoolingactivate mechanisms that tolerate freezing

Organisms maintain a constant internal environment.An organisms ability to maintain constant internal conditions in the face of a varying environment is called homeostasis:homeostatic systems consist of sensors, effectors, and a condition maintained constantall homeostatic systems employ negative feedback -- when the system deviates from set point, various responses are activated to return system to set point

Temperature Regulation: an Example of HomeostasisPrincipal classes of regulation:homeotherms (warm-blooded animals) - maintain relatively constant internal temperaturespoikilotherms (cold-blooded animals) - tend to conform to external temperaturessome poikilotherms can regulate internal temperatures behaviorally, and are thus considered ectotherms, while homeotherms are endotherms

Homeostasis is costly.As the difference between internal and external conditions increases, the cost of maintaining constant internal conditions increases dramatically:in homeotherms, the metabolic rate required to maintain temperature is directly proportional to the difference between ambient and internal temperatures

Limits to HomeothermyHomeotherms are limited in the extent to which they can maintain conditions different from those in their surroundings:beyond some level of difference between ambient and internal, organisms capacity to return internal conditions to norm is exceededavailable energy may also be limiting, because regulation requires substantial energy output

Partial HomeostasisSome animals (and plants!) may only be homeothermic at certain times or in certain tissuespythons maintain high temperatures when incubating eggslarge fish may warm muscles or brainhummingbirds may reduce body temperature at night (torpor)

What are energy units?1. umoles of photons per meter squared per second:umol m-2 s-1Watts per meter squared: W m-2Sunny day in Colorado: solar input:2200 umol m-2 s-11100 W m-2Why no time unit for W? (W = 1 J s-1)Can you convert between the two units?Not quite since the conversion depends on wavelength

Infrared Light and the Greenhouse Effect 1All objects, including the earths surface, emit longwave (infrared) radiation (IR).Atmosphere is transparent to visible light, which warms the earths surface.

Infrared Light and the Greenhouse Effect 2Infrared light (IR) emitted by earth is absorbed in part by atmosphere, which is only partially transparent to IR.Substances like carbon dioxide and methane increase the absorptive capacity of the atmosphere to IR, resulting in atmospheric warming.

Greenhouse Effect - SummaryGreenhouse effect is essential to life on earth (we would freeze without it), but enhanced greenhouse effect (caused in part by forest clearing and burning fossil fuels) may lead to unwanted warming and serious consequences!

Ozone and Ultraviolet RadiationUV light has a high energy level and can damage exposed cells and tissues.Ozone in upper atmosphere absorbs strongly in ultraviolet portion of electromagnetic spectrum.Chlorofluorocarbons (formerly used as propellants and refrigerants) react with and chemically destroy ozone:ozone holes appeared in the atmosphereconcern over this phenomenon led to strict controls on CFCs and other substances depleting ozone

CloudsWhat happens on a cloudy day?Less radiation comes inWhat happens on a cloudy night?Less radiation goes out

The Absorption Spectra of PlantsVarious substances (pigments) in plants have different absorption spectra:chlorophyll in plants absorbs red and violet light, reflects green and bluewater absorbs strongly in red and IR, scatters violet and blue, leaving green at depthPlants Respond to Light

Photomorphogenesis, Phototropism, PhotoperiodismPhytochrome responses (red/far red)flowering and dormancy; branch patterns; root growthBlue light responsesstomatal opening; phototropism; chloroplast orientationPlants Respond to Light

Photomorphogenesis.

nondirectional, light-triggered developmentred light changes the shape of phytochrome and can trigger photomorphogenesis

PhototropismsPhototropic responses involve bending of growing stems toward light sources.Individual leaves may also display phototrophic responses.auxin most likely involved

Carbon vs. Energy Plants convert LIGHT energy into CHEMICAL energyThey use the chemical energy to take CO2 from the atmosphere, and turn it into glucose, and other C-structures.

Seed location?Red light from sun penetrates to seed.No light from sun to this deep seed.Seed germinates.No germination.Red light to seed = near surface

Sun Exposure and UV damageSunshine, essential for life, strikes the earth in rays of varying wavelengths. Long rays (infrared) are unseen but felt as heat. Intermediate length rays are visible as light. Shorter rays (ultraviolet) are also invisible and are further divided into the following groups: Ultraviolet (UVA) rays are beneficial in low doses, but may increase the chance of cancer in high doses. UVBs are primarily responsible for sunburn and cancer UVCs are the shortest and most dangerous UV rays contain enough energy to damage DNA in living skin and eye cells. DNA controls the ability of cells to heal and reproduce. The ozone layer allows life to flourish by passing the longer, beneficial wavelengths and effectively blocking almost all UVC, some UVB and a little UVA.

The Pigment That Controls Growth and Flowering In Many Plants

What Is Phytochrome ?Phytochrome is a pigment found in some plant cells that has been proven to control plant development. This pigment has two forms or phases in can exist in. P-red light sensitive (Pr) and P far red light sensitive (Pfr) forms. The actual plant response is very specific to each specie, and some plants do not respond at all.

Pr PfrThe structure of Phytochrome660 nm

730 nmBinds to membraneA dimer of a 1200 amino acid protein with several domains and 2 molecules of a chromophore. Chromophore

Signal Transduction of PhytochromePrPfrGaCa2+/CaMcGMPCAB, PS IIATPaseRubiscoFNRPS ICyt b/fCHSChloroplast biogenesisAnthocyanin synthesisMembraneG protein a subunitCalmodulinGuanylate cyclaseCyclic guanidine monophosphate

How Phytochrome Works

Promoter has 4 sequence motifs which participate in light regulation.If unit 1 is placed upstream of any transgene, it becomes light regulated.TranscriptionFactorsUnit 1Light-Regulated Elements (LREs)

e.g. the promotor of chalcone synthase-first enzyme in anthocyanin synthesis

There are at least 100 light responsive genes (e.g. photosynthesis)

There are many cis-acting, light responsive regulatory elements

7 or 8 types have been identified of which the two for CHS are examples

No light regulated gene has just 1.

Different elements in different combinations and contexts control the level of transcription

Trans-acting elements and post-transcriptional modifications are also involved.Light-Regulated Elements (LREs)

Which Wavelengths Are Photoperiodic?

The length of the night period plays a major role in determining which wavelength will be effective, as the phytochrome pigment tends to revert to Pr during long periods of darkness.

Thus the length of exposure to light in a building, or if outdoors, the seasonal light changes, affect how long the plants perceives each form of phytochrome. R FR

Photoperiodic Response:

Its all about Preferences!

Long Day Plants flower when there is adequate PR

Short Day Plants flower when there is adequate Pfr

Long-Day Plants Need Low Pr!

Long-Day Plants Need Low Pr!

Reproductive(Flowering)Short-Day Plant Need Low Pfr!

Short-Day Plant Need Low Pfr!

Short-Day Plants Need Low Pfr!

Black ClothShort-Day Plants Need Low Pfr!

Night lighting disrupts reversion to Prand maintains vegetative status!

24-hour day cycleCritical day lengthLight Interruption of Darkness Affects Short- and Long-Day Plants Differently

The Phytochrome System Works Within The Apical MeristemPhotoperiodicresponses are triggered in the meristem (both apical and axillary), long before the new branches develop.

We can control development !

To lengthen the night, plants are covered with a blackout shade cloth. Applied in late afternoon and removed in the morning (5 pm to 8 am)

Photoperiodic shade clothLight penetration through the shade cloth should not be more than 2 fc in order to prevent delay in flowering and/or disfigured flowers.

SUPPLEMENTAL LIGHTING Light sources. incandescent lamps emit large amounts of red light and are good for lighting mums (standard mum lighting) mums flower when the day length decreases to 13.5 hrs or less whenever the day length is longer than 14.5 hrs plants remain vegetative split each long night in two short nights with supplemental light to prevent flowering

The length of day has an effect on two plant processes time of flowering plant maturity This light-induced response is called photoperiodism, and plants that flower under only certain day-length conditions are called photoperiodic. DAILY DURATION OF LIGHT

Plants Respond to GravityGravitropism is the response of a plant to the earths gravitational field.present at germinationauxins play primary roleFour stepsgravity perceived by cellsignal formed that perceives gravitysignal transduced intra- and intercellularlydifferential cell elongation

The pigment phytochromeDetects R and FR lightProvides information about environmentAnswers 3 questions for plantAm I in the light?Do I have plants as neighbors or above me?Is it time to flower?

Why bother?Seeds store materials to start growthMust reach light before running out of stored materialsSmall seedsNeed to be very near surfaceOften need light for germinationGerminating plants straighten & open leaves at surface, too

Plant neighbors?Far red reflected from other plants.Red absorbed by other plants.Far red enriched = neighbors

Why does this matter?Neighboring plants are threatsMight grow taller, shade youSolutionGrow at least as tall as neighborsNeed to know that you have neighborsIsolated plants typically shorter than crowded plantsOther reasons, too

Under other plants?Red absorbed by other plants.Far red reflected from other plants or transmitted.Far red enriched = understory

Why important?Best growth strategy for understory plants is different than for plants in openNeed to know whetherShaded by other plantsOR Just cloudyORLate in day (low light)

Right time to flower?Unreliable indicators of time of yearTemperature Moisture Light levels Reliable: length of day/night Varies with seasonVaries with latitudeDetected by phytochrome

Phytochrome has 2 formsRed-absorbing phytochrome

Far red absorbing phytochrome

InterconvertedTwo forms of the same compoundTotal amount same

In red lightPfrPrPfrPr absorbs red light, changes to Pfr form.Pfr doesnt absorb red light, stays the same.

In far red lightPrPfrPfr absorbs far red light, changes to Pr form.Pr doesnt absorb far red light, stays the same. Pr

In pure lightPrPfrIn pure far red light, all the phytochrome ends up in the Pr form.In pure red light, all the phytochrome ends up in the Pfr form.

SunlightMostly redA little far red

In sunlightIn sunlight most P gets converted to Pfr form.

Start of nightMost P in Pfr form.

In the darkPfr form changes gradually to Pr form.

After a short nightPfr still left.

LDP = SNPNeeds short nightNeeds Pfr still present at end of nightPfr promotes flowering for LDPs

Later in the nightMore Pfr changes to Pr.

After a long nightAll the Pfr is gone.

Day dawnsMost P gets converted to Pfr form again.

SDP = LNPNeeds long nightNeeds Pfr gone at end of nightPfr inhibits flowering for SDPs

LDP SDPLong day: Pfr left at end of short night.Pfr promotes flowering for LDPs.Pfr inhibits flowering for SDPs.

Short day: Pfr gone at end of long night.No Pfr to promote flowering for LDPs.No Pfr to inhibit flowering for SDPs.

Waiting for the right timePlants grow leaves until it is time to flowerLDPs wait until the day is long enoughReally night short enoughSome time before June 21SPDs wait until the day is short enoughReally night long enoughSome time after June 21Flower opening happens later

Day neutral plantsFlower when mature enoughMaybe other environmental signals (temp?)Day length (dark length) doesnt matter

Through the yearMayJuneJulyAugustSeptemberOctoberSpecific flowers at specific times.

Phytochrome tells plantsIf they are near the surfaceAbout their plant neighborsWhether it is time to flowerAnd lots more

Referenceshttp://www.abdn.ac.uk/sms/ugradteaching/GN3502/GN3502_0732005_1.ppthttp://www.warnercnr.colostate.edu/class_info/by220-indy/physical_environment/Physical%20Environment,%20part%202%202004.ppthttp://www.coe.unt.edu/ubms/documents/classnotes/Spring2006/256,1,Sensory Systems in Plantshttp://128.192.110.246/pthomas/Hort3140.web/Phytochrome%20lecture.ppthttp://fp.uni.edu/berg/pp/downloads/PhytochromeAction.ppthttp://www.fsl.orst.edu/~bond/fs561/lectures/radiation.ppthttp://www.coe.unt.edu/ubms/documents/classnotes/Spring2006/Plant%20Sensory%20Systems%201720_Chapter_40_2005.ppthttp://turfgrass.cas.psu.edu/education/turgeon/CaseStudy/BlueCourseGreen_01/Blue_Course_Green.ppthttp://siri.uvm.edu/ppt/warmweatherrinjuries/warmweatherrinjuries.ppthttp://www.cobb.k12.ga.us/~dickerson/ch%2016.ppt

Incandescent lamps are not desirable in greenhouse production because they have low efficiency and generate excess heat. In addition, their light spectrum consists of primarily red wavelengths, which causes excessive stem elongation in some plants.Fluorescent lamps are more efficient but are needed in large numbers to make up for their low power. They have a similar problem as the incandescent lights but they emit mostly blue wavelengths. Specially designed fluorescent lamps with better balanced spectrum also are available.