Transcript Document

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
Pigment
Light Absorbed Light Reflected
Green
Chlorophyll
violet, blue, red
Carotenoids
blue
Yellow Orange
Zanthophyll
blue
Yellow Orange
Melanin
most visible light
Black
Anthocyanins
blue, UV
Red
Phenolics
UV
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 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?
• Time of Year (season)*
• Latitude
• Time of Day
Sun angle is
influenced by
these factors and
sun angle
determines light
availability
• Cloud cover (reduces availability 3-6x)
*Also determines length of day which influences light
availability
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
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
RED
Pr
Pfr
FR
• both forms induce plant responses
• response depends on which form is
dominant
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 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))
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
R:FR is >1:1
Elongated internodes
(stretching)
Reduced internode length
(short stems)
Reduced lateral branching
Increased lateral branching
Elongated petioles
Shorter petioles
Larger, thinner
Thicker, smaller leaves
Smaller total leaf area (due to
lower numbers of leaves)
Greater total leaf area
Reduced chlorophyll synthesis
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
Photoperiodism
Duration 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 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
• 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 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.
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
North-South
• Less light interception
• More light
interception
• Less permanent shadows
• More permanent
• Better natural ventilation
shadows
(<40° N or S)
• More snow blown off
roof by wind (<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
Width
Peak 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
Width > 25 ft
32°
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
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
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
Advantages:
Shade cloth
Shading compounds
Easily applied or
removed
Reduces air temps more
effectively
Known %
Disadvantages:
Not as effective
at reducing
air temps
More or less permanent
(difficult to remove)
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
Development
400-1200*
* 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 elongation
Darker 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 greenhouses
Like LPS, peak λ at 589nm
but wider spectrum
Contain 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