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
Energy in the form of Electromagnetic Radiation (EMR) that produces a visual sensation
Photosynthetic & Visible Light
Everything with a temperature above absolute zero (-273C) is emitting EMR
can control color of light by controlling temperature of an object.
Objects absorb specific wavelengths
Pigments are the chemicals in an object that absorb specific wavelengths, giving that object its characteristic
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.
= mol = (6.02 x 1017) photons
Intensity Directly Effects:
CO2 + H2O + Light Energy (CH2O) + O2
2. Height (stem growth)
3. Development (flowering)
*Also determines length of day which influences light availability
Sun angle is influenced by these factors and sun angle determines light availability
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.
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.
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.
Hard to know that FR is present
Humans cannot sense it
FR box has 5X more energy than R box
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
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
Thicker, smaller leaves
Greater total leaf area
Green (increased chlorophyll)
R:FR is <1:1
Elongated internodes (stretching)
Reduced lateral branching
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?
Can you tell which plant in each picture was grown with R>FR?
In dense canopies the dominant form of phytochrome is Pr (meaning it has absorbed FR)
Pr form elicits shade avoidance response
Other Phytochrome Responses
EOD - important in timing of
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
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
Critical Daylength is Often Temperature Dependent
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
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)
Single-ridged → East-West
Multi-ridged → North-South
Width < 25 ft
Width > 25 ft
Light transmission is affected by superstructure and its maintenance
LOTS of superstructure but it is white and clean so lots of reflection too.
Greenhouse shadows can be a serious problem, especially if they don’t move during the day
Internal and external shade systems
Automated shade system
Shade cloth Shading compounds
Advantages: Easily applied or Reduces air temps more
Disadvantages: Not as effective More or less permanent
at reducing(difficult to remove)
Have to use specially
for both application and removal.
Application not uniform
* Photosynthesis is a reciprocal process. Low intensity can be overcome by longer exposure.
Effects of FR-absorbing filters on stem elongation
Darker color of filter indicates increasing
Ease of Installation
Spectral characteristics (λ)
Type of crop
Amount of electrical energy converted to light energy
Weight of fixture
Best for photosynthetic light
Most light in narrow band around 589nm
Bad for plants!!!!
HID’s providing supplemental light for photosynthesis during low light conditions
Representative spectra for sunlight and artificial light sources
Fluorescent > Incandescent > HID
HID Incandescent Fluorescent
HID > Fluorescent* Incandescent
* Best source of photosynthetic light in germination rooms and coolers