Light

1
Plant Environment Light

• Light

• Plants convert light energy into chemical energy

• photosynthesis

• 6H2O6CO2light --gt C6H12O6 6O2

• Photosynthesis provides plant with energy for
  growth

• Effects of light on plants important in
  horticulture

2
Plant Environment Light

• Light influences...

• Growth rates and habits

• Flowering and fruiting

• Seed germination

• Onset of dormancy

3
Plant Environment Light

• 3 characteristics of light important to plants

• Quality (color of light)

• Light made up of different colors R,O,Y,G,B,I,V

• Chlorophyll (main pigment that captures light in
  plants) absorbs red and blue light

• Accessory pigments capture different colors

• Quantity (amount of light)

• Photoperiod (duration of light)

4
Plant Environment Light

• Quality and quantity of light influences

• Spacing of plants

• Must prevent shading of neighboring plants

• Orientation of plants

• Taller plants to north and shorter plants to
  south to prevent shading

• e.g. squash and bean at south end of garden, corn
  to north

• Pruning strategies

• Fruit trees often pruned to remove middle
  branches admit more light to center of tree

• Hedges pruned so that base wider than top to
  prevent shading

5
Plant Environment Light

• Quality and quantity of light influences

• Etiolation of plants

• Light necessary for chlorophyll formation in most
  angiosperms

• Plants moved to location without light become
  blanched

• Blanching applied to several vegetables
  cauliflower, celery, scallions

• Potatoes should be covered completely to prevent
  production of chlorophyll produces green spots

• Etiolation development of plant in absence of
  light

• Whitish stems and leaves, elongated internodes

• Etiolation useful for cuttings etiolated stems
  produce adventitious roots better (light inhibits
  rooting hormones)

6
Plant Environment Light

• Quality and quantity of light influences

• Etiolation of plants

7
Plant Environment Light

• Quality and quantity of light influences

• Pigment production

• Chlorophyll breaks down when exposed to light

• Carotenoids (yellow-red accessory pigments)
  protect chlorophyll from photodestruction

• Normally chlorophyll masks carotenoids
  destruction of chlorophyll results in yellowing
  of leaves (chlorosis)

• Chlorosis also results from diseases and nutrient
  deficiencies (N, Mg, Fe, and other nutrients used
  to produce chlorophyll)

• Leaves turn color in fall because chlorophyll
  destruction unmasks carotenoids (tannins present
  produce browns)

• red leaves in some plants indicates high levels
  of carotenoids

8
Plant Environment Light

• Quality and quantity of light influences

• Seed dormancy

• Photoblastic seeds influenced by light

• Positive or negative photoblastic

• Many weeds are positive photoblastic tilling
  soil brings weeds seeds to surface and breaks
  dormancy

• Potato tubers are positive photoblastic (store in
  cool dark place)

• Some seeds require darkness to germinate (e.g.
  tomato)

9
Plant Environment Light

• Quality and quantity of light influences

• Branching and stem elongation

• Red light results in plants with short internodes
  and vigorous axillary growth

• Fluorescent lights high in red light

• Incandescent lights have little red light plants
  grown beneath them show etiolation

• Plants growing below tall plants get little red
  light (absorbed by leaves of tall plants) and
  have internode elongation

10
Plant Environment Light

• Quality and quantity of light influences

• Anthocyanin production

• High levels of light promotes anthocyanins (red
  pigment)

• Brightest fall leaves often on trees in full sun

• Rain causes leaching of anthocyanins dry falls
  produce most vivid colors

• Many fruits produce anthocyanins while ripening
  apples redder on outside of tree where exposed to
  more light

11
Plant Environment Light

• Effects of photoperiod on plants

• Photoperiod response to changes in day length

• Day length often triggers flowering

• Day-neutral plants plants that flower regardless
  of day length (e.g. dandelions, many houseplants)

• Long-day plants plants that flower when exposed
  to long day length (short nights)

• Spinach not planted in midsummer because it will
  bolt (flower)

• Short-day plants plants that flower as day
  length shortens (e.g. tomato)

• Flowering can be prevented by extending day
  length with artificial light or by night
  interruption

12
Plant Environment Light

• Effects of photoperiod on plants

• Shorter day length promotes formation of storage
  organs

• Short days stimulates formation of tubers and
  bulbs (modified stems) and tuberous roots
  (modified roots)

• Strawberry and spider plant stolons produced in
  presence of long day length

13
Plant Environment Light

• Acclimation to light

• Sudden change in lighting can cause damage to
  plants

• Annual flowers grown indoors need slow
  acclimatization to more intense sunlight outside

• Tropical plants grown outdoors must be
  acclimatized to low light in offices and homes

• Weeping figs particularly sensitive to changes in
  light will drop all leaves