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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


characteristics of light as they relate to growing plants
Characteristics of Light as They Relate to Growing Plants
  • Quantity (Intensity)
    • photosynthesis
  • Quality (Wavelength - Color)
    • photomorphogenesis
  • Duration
    • photoperiodism
what is light
What is Light ?

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

the dual nature of light
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 light6
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
how is light emr generated
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 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
Pigments and Light Absorption

Objects absorb specific wavelengths

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


biological pigments

Light Reflected


Yellow Orange

Yellow Orange



Biological Pigments
measuring light quantity
Measuring Light Quantity
  • Photometric Method
  • Radiometric Method
  • Quantum Method
photometric 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
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
Radiometric Method
  • Measures electromagnetic radiation in terms of total energy
  • Standard Unit = W/m2
  • Disadvantage
    • wavelength is irrelevant
quantum method
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 intensity
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)

what limits light availability
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 intensity

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.

angle of incidence
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.


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
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


Guess which plants haven’t seen the light yet.

Notice that both Easter lilies are flowering.

light quality
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 quality29
Light Quality





  • both forms induce plant responses
  • response depends on which form is dominant

Hard to know that FR is present

Humans cannot sense it



FR box has 5X more energy than R box


plant growth response to low r fr r fr generally 1 1
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

plant growth response to high r fr r fr generally 1 1
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

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?

shade avoidance response
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 response37
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

end of day response42
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

EOD - important in timing of

photoperiodic flowering

photoperiodism duration of the light period
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
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 regulation
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

classes of photoperiodic plants
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
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 interaction
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

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.

characteristics of light
Characteristics of Light
  • Quantity (Intensity)
    • Photosynthesis
  • Duration
    • Photoperiodism
  • Quality (λ)
    • Photomorphogenesis
maximizing light intensity depends on
Maximizing Light Intensity:Depends On:
  • Greenhouse Design
  • Construction Materials Used
  • Plant Spacing
  • Other objects absorbing/reflecting light
greenhouse orientation direction the ridge runs

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)
orientation bottom line 40 latitude
Orientation Bottom line: 40° latitude

Higher latitudes…

Single-ridged → East-West

Multi-ridged → North-South

incidence angle of light

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
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°
common roof angles
Width < 25 ft


Width > 25 ft

26 °

Common Roof Angles
light transmission is affected by glazing materials and the maintenance of them
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

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
remove objects that shade
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
reducing light intensity
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

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
Typical Light Intensities for Different Purposes

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

manipulating photoperiod
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
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
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
manipulating r fr
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
other alternatives
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 elongation

Darker color of filter indicates increasing


supplemental lighting for photosynthesis
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
types of supplemental light sources
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



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
  • 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
  • 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
high intensity discharge hid metal halide
High Intensity Discharge (HID) Metal Halide

Best for photosynthetic light

low pressure sodium lps hid lamps

Most light in narrow band around 589nm

Bad for plants!!!!

Low-Pressure Sodium (LPS) HID lamps
high pressure sodium hps
High Pressure Sodium (HPS)
  • Popular in US greenhouses
  • Like LPS, peak λ at 589nm but wider spectrum
  • Contain very little FR
uses for 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