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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
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
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
Light Reflected Green Yellow Orange Yellow Orange Black Red Biological Pigments
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 • 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 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.
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 FR box has 5X more energy than R box Red
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 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 • 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
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
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 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