The Ohio State University Department of Horticulture and Crop Science H&CS 521 Greenhouse Crop Production Light - PowerPoint PPT Presentation

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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

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The Ohio State University Department of Horticulture and Crop Science H&CS 521 Greenhouse Crop Production Light

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The Ohio State University

Department of Horticulture and Crop Science

H&CS 521 Greenhouse Crop Production


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Characteristics of Light as They Relate to Growing Plants

  • Quantity (Intensity)

    • photosynthesis

  • Quality (Wavelength - Color)

    • photomorphogenesis

  • Duration

    • photoperiodism

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What is Light ?

Energy in the form of Electromagnetic Radiation (EMR) that produces a visual sensation

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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

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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

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Relationship between Energy and Wavelength ()

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Photosynthetic & Visible Light


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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

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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.

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Pigments and Light Absorption

Objects absorb specific wavelengths

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


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Light Reflected


Yellow Orange

Yellow Orange



Biological Pigments

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Measuring Light Quantity

  • Photometric Method

  • Radiometric Method

  • Quantum Method

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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

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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.

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Radiometric Method

  • Measures electromagnetic radiation in terms of total energy

  • Standard Unit = W/m2

  • Disadvantage

    • wavelength is irrelevant

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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.

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Light Intensity

Intensity 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)

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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

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Angle of Light and Intensity

Lambert’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.

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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.

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Time of Year

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Cloud Cover

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

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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

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Sun vs. Shade Leaf



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Guess which plants haven’t seen the light yet.

Notice that both Easter lilies are flowering.

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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

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Light Quality





  • both forms induce plant responses

  • response depends on which form is dominant

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Hard to know that FR is present

Humans cannot sense it

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FR box has 5X more energy than R box


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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 petioles

4. Larger, thinner leaf blades

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

6. Reduced chlorophyll synthesis

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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)

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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)

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

Which of the above would be the more sturdy, aesthetically pleasing (desirable) plants?

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

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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

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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

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

Pr form elicits shade avoidance response

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Effects of Leaves on Light Reflection, Absorption, and Transmission

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Other Phytochrome Responses

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End of Day Response

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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

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End of Day (EOD) Response

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EOD - important in timing of

photoperiodic flowering

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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

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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

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Photoperiodic Response

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Photoperiodic Regulation

Plants 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

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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

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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

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Temperature Interaction

Critical 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

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    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.

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    Light Manipulation to Control Plant Growth in the Greenhouse

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    Characteristics of Light

    • Quantity (Intensity)

      • Photosynthesis

    • Duration

      • Photoperiodism

    • Quality (λ)

      • Photomorphogenesis

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    Maximizing Light Intensity:Depends On:

    • Greenhouse Design

    • Construction Materials Used

    • Plant Spacing

    • Other objects absorbing/reflecting light

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    More light interception

    More permanent shadows

    More snow blown off roof by wind (<40° N or S)


    Less light interception

    Less permanent shadows

    Better natural ventilation (<40° N or S)

    Greenhouse Orientation(direction the ridge runs)

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    Orientation Bottom line: 40° latitude

    Higher latitudes…

    Single-ridged → East-West

    Multi-ridged → North-South

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    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

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    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°

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    Greenhouse Dimensions and Roof Slope

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    Width < 25 ft


    Width > 25 ft

    26 °

    Common Roof Angles

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    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

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    Light transmission is affected by superstructure and its maintenance

    • 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

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

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    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

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

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    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

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    Internal and external shade systems

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    Automated shade system

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    Shade cloth Shading compounds

    Advantages: Easily applied or Reduces air temps more


    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

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    Typical Light Intensities for Different Purposes

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

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    Manipulating Photoperiod

    • Control flowering stage of your crop

    • Vegetative vs. Reproductive

      • Artificial short days

        • Black cloth

      • Artificial long days

        • Daylength extension

        • Night breaks

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    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

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    Artificial Long Days

    • Daylength extension

      • Induce LDP to flower

      • Light (FR containing) for 3-6 hrs at end of day

      • Low intensity (1-3 mmol/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

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    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

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

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    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

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    Effects of FR-absorbing filters on stem elongation

    Darker color of filter indicates increasing


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    Supplemental Lighting for Photosynthesis

    • Law of Reciprocity

      • 500 mmol/m2/s for 1 hour = 100 mmol/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

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    Types of Supplemental Light Sources

    • Incandescent

    • Fluorescent

    • High Intensity Discharge (HID)

      • Metal Halide

      • Mercury vapor

      • Low pressure sodium

      • High pressure sodium

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    Energy consumption

    Ease of Installation

    Spectral characteristics (λ)

    Type of crop

    Power (wattage)

    Heat released


    Amount of electrical energy converted to light energy

    Life Expectancy

    Output Loss

    Weight of fixture

    Considerations for Lighting Choice

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    • 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

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    • 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

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    How well does fluorescent spectrum match plant needs?

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    High Intensity Discharge (HID) Metal Halide

    Best for photosynthetic light

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    Metal Halide Spectrum

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    Most light in narrow band around 589nm

    Bad for plants!!!!

    Low-Pressure Sodium (LPS) HID lamps

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    High Pressure Sodium (HPS)

    • Popular in US greenhouses

    • Like LPS, peak λ at 589nm but wider spectrum

    • Contain very little FR

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    Common HID Light Fixtures Found in Greenhouses

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

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    Representative spectra for sunlight and artificial light sources

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    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

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