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Chapter Five Light

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Light as a driving force of life

Hours of light and dark influence

seasonal activities

terrestrial plants

shallow water plants


natural light is much more than what is seen


5.1 Solar radiation includes visible light

Wavelengths 400 - 700 nm = visible light

PAR = photosynthetically active radiation

include wavelengths plants use in photosynthesis

UV light

UV - A 315 - 380 nm

UV - B 280 - 315 nm


near infrared = 740 - 4000 nm

far infrared / thermal radiation = 4000 - 100,000 nm


Ozone layer

Upper atmosphere ( stratosphere )

absorbs nearly all wavelengths

especially: violets and blues of visible light

atmospheric gases scatter - Earth’s bluish shine

water vapor scatters - clouds whit appearance

dust scatters long wavelengths to produce reds and yellows in the sky - skylight

relative contributions of direct light and skylight - changes globally - latitude

high altitudes and sun’s low angle


Light intercepted by Earth

Light transmitted through objects




5.2 Plant cover intercepts considerable light

Quantity and position of leaves

Number of leaves above

not best measure

move down canopy - more leaves

light decreases

Leaf index - LAI

Beer’s Law and the Attenuation of Light

leaf angle


Forests and light

deciduous, pine rainforest

seasonal changes


Canopy and sunlight

Light decreases downward


70 - 80 % solar energy reaches floor

endure shadows to grow

1 - 5 % strikes canopy - deciduous forest

LAI = 3 - 5 summer

LAI = 2 - 4 , 10 - 15 % pine forest

LAI = 6 - 10, 0.25 - 2 - rainforest


5.3 The light a plant receives affects its photosynthetic activity

Light compensation point - CO 2 uptake = CO 2 loss due to respiration

Other factors: water, temperature, nutrients become limiting agents

Light saturation point



5.4 Species of plants are adapted to either high or low light

Shaded plants:

lower light compensation point

lower light saturation point

lower maximum rate of photosynthesis

Related to lower concentration rubisco

production of chlorophyll


Shade-intolerant species

Shade-tolerant species

5.5 Aquatic plants live in a shaded environment

Attenuation and water depth

light intensity most favorable to photosynthesis

water column canopy - light limited to lower plants

Phytoplankton and depth and amount of light

seasonal water temperatures, light and variation of visible light wavelengths

red, green, brown algae

caroteniod pigment, fucoxanthin - seaweeds of intermediate depths


5.6 Plants have evolved defenses against ultraviolet radiation

Solar UV - B radiation

decreases with latitude from the tropics

ozone layer thinnest

poles ozone layer the thickest

stratospheric ozone layer diminishing CFC’s

Humans - skin cancer

Plant DNA damage

epidermal cells absorb UV-B radiation

little known about these defenses

Back To: 349 Notebook