2) convection - mass movement

3) radiation- radiant energy transferred as electromagnetic waves 

light - light is the layman's term for visible radiant energy in the 400 to 700 nm wavelength region of the spectrum. In other words, it is the form of radiant energy (i.e. radiation) that humans and most animals can see, that plants use in photosynthesis, and for most other reactions that require light. An exception would be far red light that humans cannot seem, but plants can.  In fact,different animals see many different color combinations.

2) quality - the wavelength or color of light

3) duration - determines the total amount of light energy received
                     total amount of light energy = quantity x # hours of light

4) photoperiod- the day length, or length of light in a 24 hour cycle, regardless of quantity. 

LIGHT CAN BE AFFECTED AS FOLLOWS
1) absorbed- when radiant energy (such as light) is absorbed it is converted primarily to heat energy.

re-radiation- heat energy is converted to radiant energy as long wavelengths in the infrared (IR)
                          region of the spectrum.

2) transmitted - when radiant energy (such as light) passes through an object unaffected, 
                         such as glass.

3) reflected or scattered - when radiant energy (such as light) is "bounced off" an object,
                                         such as a solid colored surface.

The color of an object is the color(s) of light that is transmitted or reflected. Your eyes see the color not absorbed. 

Expressed as:
a) foot-candle (ft-c) - 1 lumen per square foot

b) lux - 1 lumen per square meter 

    1 foot-candle = 10.76 lux

photosynthetically active radiation (PAR) - light intensity in the 400-700 nm
                                        wavelength band that is used by plants in photosynthesis.

Expressed as
a) microEinstein per second per square meter - mEs^-1m^-2 (400-700 nm)

b) watts per square meter - Wm^-2(400-700 nm)
 

4) spectral radiometer - measures the intensity at each wavelength (i.e. color spectrum
from a light).

1) phototropism - response of plants to light 
    a) plants bend towards areas of higher light intensity. 

2) photosynthesis  
    a) light reaction - increases with increasing light intensity 
    b) stomata - C3 and C4 plants: open in light; close in dark 
    c)  stomata - CAM plants: open in dark; close in light 

3) temperature - high light intensity increases temperature due to: 
    a) absorption of radiation, especially IR; greater with darker colors 
    b)  greenhouse effect  

4) transpiration - greater in high light intensity due to heat buildup, but transpiration may decrease 
                           if it gets too bright then too hot, which will cause the stomata to close.   

5) sun versus shade plants

a) leaf structure

sun grown leaf
- thicker, due to thicker palisade parenchyma layer
shade grown leaf
- thinner, due to thinner palisade parenchyma layer
- therefore, higher proportion of spongy mesophyll
- larger size
- softer and more pliable

b) optimum light intensity

shade plants: have a low optimum light intensity
sun plants: have a high optimum light intensity

6) photooxidation - destruction of chlorophyll by high light intensity.

7) etiolation - elongated, pale green to yellowish growth due to low light intensity.

8) blanching - lack of color development due to exclusion of light; (unblanched cauliflower)
                         - used on cauliflower, asparagus and celery.

9) light acclimatization - conditioning of plants to low light intensity interior environments.

Recommendation: 
Greenhouses are covered with shade cloth to yield the desired % shade.  In the Rio Grand Valley of south Texas, the greenhouses commonly are covered with 63-73% shade.  The plants are grown the entire time under these shaded conditions.  The production time is a bit longer than growing in higher light.  The plants are typically a bit taller with larger, deeper green leaves.  They can be used immediately indoors and in interiorscapes. 

In this method the plants are grown in a greenhouse at very bright light intensity, often full sun.  The plants grow fast, are short and compact with lighter green leaves.  If sun plants are placed directly indoors or in an interiorscape: 
a) growth stops
b) leaves turn yellow, especially the older leaves, and
c) leaves fall off, especially the older leaves.

So to acclimatize the plants, they are placed in a very heavily shaded greenhouse or a lighted warehouse in order to acclimatize them to low light intensities.

Notice on the graph below that each plant started out with a certain light compensation point.  For example, Dracaena was the highest at about 120 ft-c and Schefflera was the lowest at about 15 ft-c.  Over time in the acclimatization treatment, each plant's light compensation point decreased.  This is showing their rate of acclimatization.  Notice that all the species acclimatized, i.e. even very shade tolerant plants acclimatized to even lower light intensities.    After about 4 to 6 weeks, the light compensation point does not decrease much more, so the light acclimatization process is virtually complete.
 

2) growth responses - due to effect on photosynthesis
    a) colored coverings
         1) plant canopy - shade rich in green-yellow & far red, poor in blue & orange-red light
         2) fiberglass -
         3) tinted/shaded glass
         4) shade cloth or saran
    b) artificial light sources
         1) tungsten - rich in red and far red
         2) fluorescent - rich in blue and yellow-orange
         3) HID - varies

3) pigments
    a) anthocyanins- blue, red and purple in color
    b) carotenoids- orange and yellow in color; absorb 450-500 nm (blue and green);
                                   carotenoids can pass energy to chlorophyll to assist in photosynthesis
    c) phytochrome- absorbs red (660 nm) and far red (730 nm) light; involved in
                                      photomorphogenic and photoperiodic responses
 

  Any light that looks "white" to human eyes will act like red light.

• Red light, sunlight and any white causes germination (causes Pr to Pfr to be present)
• Far red light inhibits germination (causes Pfr to Pr form to be present)
• Seed germination responds to the last light seen - lettuce seed experiment

      Always very small seeds.
      Why would a light requirement be an advantage to small seeds?

• ultraviolet light - absorbed by ozone
• visible light - not selectively absorbed
• infrared light - absorbed by carbon dioxide and water vapor

long-day plant (LDP) - plants that exhibit their photoperiodic response when the 
                                           photoperiod is longer than a critical photoperiod.

day-neutral plant (DNP) - plants that are not affected by photoperiod. 

Plants can detect civil twilight, so it must be taken into account when determining the photoperiod that plants perceive.  Plants cannot detect moonlight, so it does not effect the photoperiod plants perceive.

photoperiod - the day length a plant perceives, which will be the absolute day length (time
                           from sunrise to sunset) plus 1 hour of civil twilight. 

critical photoperiod - the photoperiod (absolute day length + civil twilight) above or below 
                                          which the photoperiodic response is turned-on or turned-off.

Each species has its own unique critical photoperiod that it "looks" for.