Natural Sun Light
By
Jay Holcomb
Terms to Understand
Radiant Energy--the form of energy which is propagated through space in the form of electromagnetic waves.
Radiation--the process by which radiant energy is generated and emitted by a source and propagated through space.
Photosynthetically Active Radiation--energy used for photosynthesis 400-700 nm
Terms to Understand-2
Irradiance--the radiant flux density incident on a surface. This is the amount of radiant energy that is received by an object.
Light--that portion of the electromagnetic spectrum with wave lengths between 380 and 780 nanometers.
Changes in Irradiance
Atmosphere can reduce irradiance 30%
Cloud cover reduces irradiance dramatically
Plant canopy reduces irradiance and shifts spectral quality
Changes in Irradiance-seasonal
Of the total irradiance available December has 3% while June and July have about 14%.
Solar radiation in December is about 1/3 of that in June at latitudes near 40°*
– *Giacomelli and Roberts, HortTech 3/93
Definition--Moles/day
Photosynthetically active radiation (PAR) is measured as µmoles/m2/sec.
This is an instantaneous measurement.
It must be summed to get moles/day
To determine moles/day multiply times 60 sec/min times 60 min/hour times 24 hr/day then divide by 1,000,000 to get moles/day.
Moles/day
For a day in June the mean would be 29 with a minimum of 18 and a maximum of 36
For a day in December the mean would be 9 with a minimum of 1 and a maximum of 16
January 6 February 12 March 16 April 20 May 24 June 24 July 26 August 22 Sept 16 October 12 Nov 6 Dec 6 For Grand Rapids by Faust from
GPN
Light Compensation and Saturation Compensation point-
point when Pn is equal to Pr
Saturation Point-point above which an increase in light will not cause an increase in carbon fixation
Statement
“A successful grower is one who can cause the plant to produce the maximum amount of fresh weight of acceptable quality from a given amount of dry weight.”
“Solar radiant energy and temperature determine the quantity of photosynthates available for plant growth, whereas nitrogen and moisture levels determine how they will be utilized by the plant.”
Plant Response to Irradiance
Increased irradiance increases yield Increased irradiance speeds up flowering Increased irradiance will reduce flower bud
senescence Increased irradiance will improve root growth Increased irradiance will permit greater post
harvest life
Methods of Affecting Irradiance
Angle of Incidence Greenhouse shape Greenhouse orientation Greenhouse cover Amount of structure
above the plants Upkeep of the
greenhouse interior
Shading compound on the roof. Applied about May1-15, wash off by mid-September
Changing plant density and planting patterns
Using reflective materials to reflect sun light to plant
Plant Response to Irradiance
Too High Light– Burned spots on
foliage
– Fading of flower color
– Light green leaves of orchids
Too Low Irradiance– Stretched plants
– Poor quality
– Reduced productivity
– Reduced yield
– Reduced post-harvest life
Light Quality-Phytochrome
Pr is phytochrome that absorbs red light
Pfr is phytochrome that absorbs far-red light. Converted in darkness
Light quality (amount of red and far-red light) will affect phytochrome
Phytochrome affects growth
Pfr/Ptotal controls plant growth
High Pfr/Ptotal produces compact growth (little stem elongation)
Low Pfr/Ptotal produces elongated growth (substantial stem elongation)
How to affect Phytochrome
The red/far-red ratio in sunlight is affected during the day.
During mid-day R/Fr ratio is between 1.05-1.25
During sunrise and set the ratio is 1.15-0.65
As light passes through a plant canopy there is a substantial increase in far-red light.
This has the effect of encouraging stem elongation in the under story plants
Photoperiodism-defined
Response of the plant to the relative length of the day and night.
The response can include flowering, tuber formation, bulb formation, others
Photoperiodism-terms
Critical day (night) length--the point where the response shifts from a long day to a short day response
Facultative (quantative) short day plant--flowers faster with short days
Obligate (qualitive) short day plant--must have short days to flower
Phytochrome affects flowering
P far-red inhibits flowering in Long Night Plants (short day plants)
P far-red promotes flowering in Short Night Plants (long day plants)
Controlling Photoperiod
Create short days by covering plants with blackcloth from 7pm to 8 am
Create long days by incandescent lamps at 10 footcandles from 10 pm to 2 am
Lamps for Lighting
Plant Lighting
Sole source--lamps can be used to provide the only source of energy for the plant
Supplementary--the plants receive some natural light but that is supplemented by light from lamps (Photosynthetic lighting)
Photoperiodic lighting--lamp light is provided for the purpose of providing long day conditions
Lamp Types-Incandescent
Energy provided in the red and far-red region
10% of electricity given off as light
Used for photopereiod control for flowering
Uncomplicated and easy to install
Lamp Types-Incandescent-1
Number of wattages are available
Economical to purchase and install
Can have internal or external reflectors
Lamp Types-Fluorescent
Low pressure gas discharge lamps
UV radiation absorbed by fluorescent powder
Light quality controlled by the phosphor
Use a ballast Efficiency is20%
Lamp Types-Fluorescent-2
Lamps can be located near the plants because operating temp is low
Used as sole source in growth rooms
Cool white plus warm white -- good combination
8’ VHO and HO hot
Lamp Types-HID
High intensity dis-charge (high pressure)
Light produced by electrons moving through gas
Sodium-yellow, mercury-blue-green
Operate with a ballast Efficiency 25%
Lamp Types-HID-2
Cost of lamps and installation is high
Wattages are between 400 and 1000
Most frequently used as supplementary light because high irradiance and small fixture
Supplementary light-plant response Irradiance level times
duration creates plant response
Cost of lighting is irradiance times duration