The Ohio State University

Department of Horticulture and Crop Science

H&CS 521 Greenhouse Crop Production

Light

LIGHT!!!

Characteristics of Light as They Relate to Growing Plants

• Quantity (Intensity)– photosynthesis

• Quality (Wavelength - Color)– photomorphogenesis

• Duration– photoperiodism

What is Light ?

Energy in the form of Electromagnetic

Radiation (EMR) that produces a

visual sensation

The “Dual Nature” of Light

• Particle– light behaving like a “package” of energy

– PHOTONS - a particle of light• important for plants. Photons are what the plant

“sees” (senses)

– QUANTA- “packet” or amount of energy contained in a photon

The “Dual Nature” of Light

• Wave– EMR can have very short wavelengths very long

wavelengths

– Energy is inversely proportional to wavelength

– Shorter wavelength = higher energy

Relationship between Energy and Wavelength ()

Photosynthetic & Visible Light

Far-red

How is Light (EMR) Generated?

Everything with a temperature above absolute zero (-273C) is emitting EMR

• The amount of energy emitted depends on the temperature

• Increase in temperature = increase in total energy emitted

• Stephan-Boltzman Law

Temperature vs. Wavelength ()

• Temperature is inversely proportional to wavelength

• Wein’s Law: peak (wavelength) of EMR from an object is inversely proportional to temperature

can control color of light by controlling temperature of an object.

Pigments and Light Absorption

Objects absorb specific wavelengths

Pigments are the chemicals in an object that absorb specific wavelengths, giving that object its characteristic

COLOR

Biological Pigments

Green

Yellow Orange

Red

Pigment Light Absorbed

Chlorophyll violet, blue, red

Carotenoids blue

Zanthophyll blue

Melanin most visible light

Anthocyanins blue, UV

Phenolics UV

Black

Light Reflected

Yellow Orange

Measuring Light Quantity

• Photometric Method

• Radiometric Method

• Quantum Method

Photometric Method

• Based on the sensitivity of the human eye to detect electromagnetic radiation

• Very subjective

• Standard Unit = 1 foot candle (ftc)

• Amount of light given off from 1 candle at a distance of 1 foot

Light Absorption Human Eye vs. Leaf

Eyesight is not the best way to judge of the photosynthetic capability of a light source because the ability to detect colors by our eyes is the opposite of leaf absorption of colors for photosynthesis.

Radiometric Method

• Measures electromagnetic radiation in terms of total energy

• Standard Unit = W/m2

• Disadvantage– wavelength is irrelevant

Quantum Method• Measure of Photosynthetic Photon Flux (PPF) (400-

700nm)

• Not measuring all of entire spectrum, it is measuring the amount of photosynthetic light

• Standard Unit = mol = (6.02 x 1023) photons= mol = (6.02 x 1017) photons

• Best way to measure light in the greenhouse because plants are “counting” photons that they absorb.

Light IntensityIntensity Directly Effects:

1. Photosynthesis• plants are photon “counters”- photosynthetic yield is

directly related to photons absorbed

CO2 + H2O + Light Energy (CH2O) + O2

2. Height (stem growth)

3. Development (flowering)

What Limits Light Availability?

• Time of Year (season)*

• Latitude

• Time of Day

• Cloud cover (reduces availability 3-6x)*Also determines length of day which influences light availability

Sun angle is influenced by these factors and sun angle determines light availability

Angle of Light and IntensityLambert’s cosine Law

As you change the angle of incidence (), the intensity of a light beam will decrease as the angle of incidence decreases

The reduction carries over into the amount of light that passes through the greenhouse covering.

Angle of Incidence

90o is the angle at which transmission intensity occurs

As the angle of the sun hitting the greenhouse roof increases to 90o, light transmission into the greenhouse increases.

In general, the higher the sun is in the sky, the greater the transmission into the greenouse.

Low sun angle in the winter along with short days dramatically reduce light levels in the greenhouse during that time of the year.

Time of Year

Cloud Cover

A cloudy day in May provides more photosynthetic light than a clear day in December, mostly because of the duration of the light period.

Plant Physiology Under Low Light Intensity

1. Longer internodes, increased stem elongation

2. Leaves have larger surface area

3. Thinner leaves and stems

4. Thinner cuticle

5. One layer of palisade cells

All are adaptations to maximize photosynthesis

Sun vs. Shade Leaf

Sun

Shade

Guess which plants haven’t seen the light yet.

Notice that both Easter lilies are flowering.

Light Quality

• Controls Photomorphogenesis (plant development and form)

• Mediated by phytochrome (protein pigment)– red light absorbing form (Pr)– FR light absorbing form (Pfr)– Forms are photoinconvertible, depending on the

which type of light is absorbed

Light Quality

• both forms induce plant responses

• response depends on which form is dominant

Pr PfrRED

FR

Hard to know that FR is present

Humans cannot sense it

FR

Red

FR box has 5X more energy than R box

Plant Growth Response to Low R:FR (R<FR generally < 1:1)

Low R:FR can result from increase in FR or reduction in Red and is indicated by:

1. Elongated internodes (stretching)

2. Reduced lateral branching

3. Elongated petioles4. Larger, thinner leaf blades

5. Smaller total leaf area (due to lower numbers of leaves

6. Reduced chlorophyll synthesis

Plant Growth Response to High R:FR (R>FR (generally > 1:1))

High R:FR can result from reduction in FR or increase in Red and is indicated by

1. Reduced internode length

2. Increased lateral branching

3. Shorter petioles

4. Thicker, smaller leaves

4. Greater total leaf area

5. Increased leaf chlorophyll (darker green)

R:FR is <1:1

Elongated internodes (stretching)

Reduced lateral branching

Elongated petioles

Larger, thinner

Smaller total leaf area (due to lower numbers of leaves)

Reduced chlorophyll synthesis

R:FR is >1:1

Reduced internode length (short stems)

Increased lateral branching

Shorter petioles

Thicker, smaller leaves

Greater total leaf area

Green (increased chlorophyll)

Which of the above would be the more sturdy, aesthetically pleasing

(desirable) plants?

Can you tell which plant in each picture was grown with R>FR?

Shade Avoidance Response

• Leaves strongly absorb red and blue light

• The closer plants are to a neighboring plant:

• less red light available for absorption• still have nearly all FR light present

because of FR is transmitted and reflected but not absorbed

Shade Avoidance Response

• Phytochrome responds to the quality of light within the canopy of crowded plants

• Mechanism by which plants can tell how close neighboring plants are and out-compete for available space

In dense canopies the dominant form of phytochrome is Pr (meaning it has absorbed FR)

Pr form elicits shade avoidance response

Effects of Leaves on Light Reflection, Absorption, and Transmission

Other Phytochrome Responses

End of Day Response

End of Day Response

• Plant response to the changes in the ratio of Red/FR light

• As day progresses, greater chance of scattering light in atmosphere because of lower sun angle

• Shorter have greater probability of scattering

• At end of day, lowest Red/FR ratio for the day– red light scattered much more than FR

End of Day (EOD) Response

EOD - important in timing of photoperiodic flowering

PhotoperiodismDuration of the Light Period

As a result of seasonal changes in daylength, plants have evolved systems to ensure viability of seeds:

- protection before winter- coincide with the rainy/ dry seasons

Photoperiodism - plant ability to detect and respond to day length

Photoperiodic Response

• Short Day Plant (SDP) - flower when the day length is less than the Critical Day Length

• Long Day Plant (LDP)- flower when the day length is greater than the Critical Day Length

• Day Neutral- flower without respect to day length

Photoperiodic Response

Photoperiodic RegulationPlants actually measuring NIGHT length

That means that during short day periods of the year by interrupting or splitting a long night with a relatively short photoperiod the plant perceives a short night and long day effect even though the natural day length has not changed

Classes of Photoperiodic Plants

• Obligate - plant that must absolutely meet the day length requirement to flower

• Facultative - plant that will flower under most photoperiods but will flower most readily when the photoperiodic requirement is met

Understanding Photoperidism

• Allows year-round production of photoperiodic plants

• Prior to discovery, mums only grown for fall sales

• Carnations only grown for spring & early summer

• Same thing for other SD and LD plants now grown year-round

Temperature InteractionCritical Daylength is Often Temperature Dependent

• SDP - as temp. increases, CDL decreases (requires shorter days than normal)

– Mums– Poinsettias

• LDP - as temp. decreases, CDL decreases (days don’t have to be as long as normal)

– Fuchsia– Spinach

Note: the concept of short/long day is not limited to 12 hrs. day/night.

The critical dark period for a short day plant may only be 8 hrs. (16 hrs.light), but if it does not flower when the night is any shorter than that, it is still a short day plant, even though it flowers when the day length is 16 hrs.

Light Manipulation to Control Plant Growth in the Greenhouse

Characteristics of Light

• Quantity (Intensity)– Photosynthesis

• Duration– Photoperiodism

• Quality (λ)– Photomorphogenesis

Maximizing Light Intensity:Depends On:

• Greenhouse Design

• Construction Materials Used

• Plant Spacing

• Other objects absorbing/reflecting light

Greenhouse Orientation(direction the ridge runs)

East-West• More light

interception• More permanent

shadows• More snow blown off

roof by wind (<40° N or S)

North-South• Less light interception• Less permanent shadows• Better natural ventilation

(<40° N or S)

Orientation Bottom line: 40° latitude

Higher latitudes…

Single-ridged → East-West

Multi-ridged → North-South

Incidence Angle of Light

• If light strikes roof at 90°, then have maximum light transmission

• If light strikes roof not at 90°, then less light transmitted

Using Roof Angle to Maximize Light Interception

• Winter months in Columbus, OH (~40°N), sun at low angle

• For light to strike at 90°, then roof angle would have to be >60°

Greenhouse Dimensions and Roof Slope

WidthPeak Height

@ 26° Slope

Peak Height

@ 63° Slope

21 ft 5.1ft 20.6ft

32 ft 7.8ft 31.4ft

49 ft 11.9ft 48.1ft!

Common Roof Angles

Width < 25 ft

32°

Width > 25 ft

26 °

Light transmission is affected by glazing materials and the maintenance of them

• Glazing Material (% light transmission)– Glass (low iron) (93%)– Exolite (double acrylic) (92%)– Lexan (double polycarbonate) = (78%)

• Cleaning glazing material– Several times a year (usually rainfall will do this)– Remove shading compound by mid-October

• Superstructure– ↑ superstructure, ↑ shading– Heavy glazing requires more superstructure– Frame 10-12%, sash bars 5-7% reductions– Supplemental lighting fixtures can shade

• Superstructure clean and painted– Aluminum = reflective– Wood - painted white and kept clean

Light transmission is affected by superstructure and its maintenance

LOTS of superstructure but it is white and clean so lots of reflection too.

Remove objects that shade• Adequate plant spacing

– Reduces shade avoidance response

– Don’t overdo with the numbers of hanging baskets

• Objects close to greenhouse (trees, buildings, etc)– Distance away = 2 x Height of

object

Greenhouse shadows can be a serious problem, especially if they don’t move during the day

Reducing Light Intensity

• Why shade?– Low light plants don’t like high light– Reduce temperature– Have reached light saturation point

• Shading methods– Shade cloth– Shading compounds

Internal and external shade systems

Automated shade system

Shade cloth Shading compounds

Advantages: Easily applied or Reduces air temps more

removed effectively

Known %

Disadvantages: Not as effective More or less permanent

at reducing (difficult to remove)

air temps Have to use specially formulated

compounds for both application

and removal.

Application not uniform

Typical Light Intensities for Different Purposes

Use mol/m2/s

Display 15

Photoperiod 10-12

Survival 100

Maintenance 200

Propagation 80

Photosynthesis for Growth and

Development400-1200*

* Photosynthesis is a reciprocal process. Low intensity can be overcome by longer exposure.

Manipulating Photoperiod

• Control flowering stage of your crop

• Vegetative vs. Reproductive

– Artificial short days• Black cloth

– Artificial long days• Daylength extension

• Night breaks

Artificial Short days• Pull black cloth

– Opaque material blocks all light

– SDP induced to flower

– Reflective to reduce heat delay

– Can be automated– Can ‘double’ as

thermal blanket to hold in heat on cold nights

Artificial Long Days

• Daylength extension– Induce LDP to flower– Light (FR containing) for 3-6 hrs at end of

day– Low intensity (1-3 mol/m2/s, 7-10 fc)

• Night Breaks– Prevent SDP from flowering– 2-4 hrs of low light during dark period– Want little FR in light

Manipulating R:FR

• Minimize shade avoidance response

• Remove excess vegetation from plants to prevent self-shading (e.g. geraniums)

• Prevent shading from other plants– Minimize # of hanging baskets over plants– Proper pot spacing

• Space visible between plants at least until plants are nearly ready to ship

Bench Cover and Pot-spacing Symbols (multi-lingual)

Other alternatives

• Spectral filters– Pigments in plastic film that absorb FR and

increase R:FR– Not all problems worked out yet

• Biotechnology– More phytochrome so plants “see” more red

light– Compact, darker green, more branching

Effects of FR-absorbing filters on stem elongationDarker color of filter indicates increasing

FR-absorbance

Supplemental Lighting for Photosynthesis

• Law of Reciprocity– 500 mol/m2/s for 1 hour = 100 mol/m2/s for

5 hours

• Use this law to your advantage, run relatively low intensity for several hours– Increasing intensity by adding additional

fixtures can be too expensive and cause too much shading

Types of Supplemental Light Sources

• Incandescent

• Fluorescent

• High Intensity Discharge (HID)– Metal Halide– Mercury vapor– Low pressure sodium– High pressure sodium

Considerations for Lighting Choice

• Cost– Fixture

– Installation

– Energy consumption

• Ease of Installation• Spectral

characteristics (λ)• Type of crop

• Power (wattage)• Heat released• Efficiency

– Amount of electrical energy converted to light energy

• Life Expectancy• Output Loss• Weight of fixture

Incandescent

• Easily installed• Low efficiency• Low intensity• Large amount of heat given off• Spectrum contains far-red (R:FR > 1:1)• OK for photoperiodic control

– Daylength extension

– Night break

Fluorescent• More efficient than incandescent• Low intensity• Less heat generated than incandescents• No far-red but some UV• Good for growth chambers, coolers, and photoperiod

(night break) use• More complicated to install (ballast) than incandescent• Different phosphors change spectrum

How well does fluorescent spectrum match plant needs?

High Intensity Discharge (HID) Metal Halide

Best for photosynthetic light

Metal Halide Spectrum

Low-Pressure Sodium (LPS) HID lamps

– Cheap

– Most light in narrow band around 589nm

– Bad for plants!!!!

High Pressure Sodium (HPS)

Popular in US greenhousesLike LPS, peak λ at 589nm but wider spectrumContain very little FR

Common HID Light Fixtures Found in Greenhouses

HID’s providing supplemental light for photosynthesis during low light conditions

Representative spectra for sunlight and artificial light

sources

Uses for Light Sources

• Night break

Fluorescent > Incandescent > HID• Daylength Extension for Photosynthesis

HID Incandescent Fluorescent• Supplemental Light Intensity for Photosynthesis

HID > Fluorescent* Incandescent* Best source of photosynthetic light in germination

rooms and coolers