Almost all energy used by animals, fungi,
and everything else is the energy of sunlight
captured by photosynthesis in plants, algae,
and cyanobacteria.
Light intensity must be just right; in habitats
where sunlight is too intense (deserts, mountaintops),
plants often have hairs or spines that shade
the plant; if light is too dim (in shady forests
or deep in the ocean) not enough energy is
available to allow life.
I’m Dr. DeBusk and in this video, I’m
going to talk about the environmental factors
that affect photosynthesis such as light,
leaf structure, and water.
Photosynthesis is affected by the environment.
From a plant’s viewpoint, light has three
important properties: quality, quantity, and
duration.
Quality of sunlight refers to the colors or
wavelengths it contains.
Sunlight is pure white because it contains
the entire visible spectrum.
During sunset and sunrise, sunlight passes
through the atmosphere, and a large percentage
of the blue light is deflected upward; consequently,
light at ground level is enriched in red,
which is easily visible.
This period of red-enriched light lasts only
a few minutes and has little effect on photosynthesis.
At noon, sunlight passes nearly vertically
through the atmosphere and more blue light
is transmitted.
This is true of plants in deserts, grasslands,
and the canopy of a forest; however, herbs
and shrubs that grow near soil level in a
forest are understory plants, and the light
they receive has already passed through the
leaves of the canopy.
As light penetrates those leaves, red and
blue are absorbed by chlorophyll, so the dim
light received by understory plants is especially
depleted in these critical wavelengths.
They have extra amounts of accessory pigments
so that they can gather the wavelengths available
and pass the energy on to chlorophyll a.
Similarly, algae that grow near the surface
of lakes or oceans receive complete light,
but water absorbs red and violet.
Many kelps (brown algae) grow in deep ocean
water and receive mostly green and blue light.
Their accessory pigments absorb these wavelengths;
therefore, the kelps appear yellow-brown to
us.
Quantity of sunlight refers to light intensity
or brightness.
More light is available for photosynthesis
on a clear than on a cloudy day; understory
plants receive dim light; lower branches and
branches on the shaded side of a plant receive
less light.
Plants growing in the shadow of a mountain
or in deep canyons receive much less light
than plants that grow on slopes that face
the sun.
Plants growing near the equator receive intense
light because the sun is always more or less
directly overhead at noon, whereas plants
near the poles receive very little light.
Even during the summer the sun is low at noon,
and light is scattered by the atmosphere.
Think about how intensity of sunlight varies
during the day and affects photosynthesis.
Examine the solid line labeled “300 ppm
CO2.”
The rate of photosynthesis was measured for
plant grown under different intensities of
light, but all with 300 ppm carbon dioxide
in the air.
The light compensation point is the level
of light at which photosynthesis matches respiration.
Plants grown for a long time in conditions
below the light compensation point respire
faster than they photosynthesize and starve
to death.
Near point a, those plants that received dim
light absorbed less carbon dioxide, whereas
those grown in brighter light absorbed more
carbon dioxide.
Under normal levels of carbon dioxide, light
is the limiting factor.
At point b, plants that received more light
did not photosynthesize faster than those
that received slightly less.
In this case, limitation is carbon dioxide
since photosynthesis went faster at higher
carbon dioxide levels.
At point c, the light was too intense and
damaged the plant by overheating it and bleaching
the pigments.
These grape vines have abundant sunlight,
water, and fertilizer.
They could probably photosynthesize more rapidly
only if more carbon dioxide were present.
Light can become too intense, and some plants
in bright environments have developed adaptations
to reflect light, such as thick trichomes
or a heavy cuticle.
While young, the leaves of dusty miller
are completely obscured by trichomes, protecting
the leaf from strong sunlight and insects.
These cacti (Epithelantha) live in environments
where sunlight is extremely intense; their
spines are so abundant and closely spaced
that they shade the stem and prevent chlorophyll
from being damaged.
Plants of Dudleya brittonii are protected
from excess sunlight and water loss by a thick
layer of white wax; this particular wax strongly
reflects ultraviolet light.
Understory plants of forests are adapted to
low light.
If a roadway is cut into a forest and plants
adjacent to the cut are suddenly exposed to
full sunlight, the shock may cause them to
wilt and die.
The duration of light refers to the number
of hours per day that sunlight is available.
At the equator, days are 12 hours long throughout
the year.
Farther north or south, days become longer
in summer; maximum length occurs near the
poles, where the day is 24 hours long in midsummer,
and only night occurs in midwinter.
In middle latitudes, winter days are short
and sunlight is weak.
Most plants can survive by means of stored
nutrients.
Even evergreen plants are unable to undergo
very much photosynthesis, however, because
the temperatures are low, the plants are growing
little and have a low rate of respiration.
In many plants, longer days cause greater
amounts of photosynthesis.
In others, chloroplasts become so full of
starch that photosynthesis stops, even though
light is present.
At night, starch is converted to sugar.
The sugar is transported out of chloroplasts
and can be used for growth or stored.
By morning, leaf chloroplasts can resume photosynthesis.
Given all this talk about the problems with
light in a forest, how do wildflowers grow
on the forest floor in temperate regions in
early spring?
The quality, quantity, and duration of light
reaching the forest floor is highest in the
early spring than in the summer after the
leaves of the forest canopy have fully developed.
Many wildflowers take advantage of the abundant
light reaching the forest floor before they
are shaded by the canopy trees.
In hot, dry habitats, plant leaf cells are
packed closely without intercellular spaces.
The small internal surface area retards evaporation.
Cylindrical leaves reduce external surface
area.
In both cases, photosynthesis and growth slow
because carbon dioxide absorption slows.
These living stone plants of African deserts
conserve water in several ways.
They have only two leaves at a time; when
two new ones form, the old two die.
Not enough water is available for four leaves.
The leaves are fleshy and pressed together,
such that they form a cylinder with minimal
surface area through which water can be lost.
This is a plant of Haworthia cooperi.
Its short stem is located several centimeters
underground, and it produces many cylindrical
leaves that are just long enough to reach
the soil surface.
The tips of the leaves, visible here, have
transparent epidermis, and the mesophyll is
also transparent because it has no intercellular
spaces.
Consequently, light enters through the window-like
tip and then passes deep into the rest of
the underground leaf, where cells with chloroplasts
are located: photosynthesis actually occurs
underground.
Because most of each leaf is subterranean,
they stay cooler in summer, they are less
visible to animals, and the air in the soil
is richer in carbon dioxide than is air above
ground.
This transverse section of a leaf of Aloe
vera is so transparent you can read through
it.
It is excellent for transmitting light to
parts of the leaf that are underground.
The balance between water loss and photosynthesis
is critical.
Most plants keep their stomata open during
the day.
At night, carbon dioxide cannot by used, retaining
water within the plant.
If water stressed, stomata will close.
This prevents entry of CO2, and reduces photosynthetic
activity.
Light intensity exceeded the light compensation
point for these trees just after 6 AM, and
photosynthesis increased rapidly; however,
by early afternoon, photosynthesis dropped
even though light and temperature were adequate.
The problem was a lack of water (water stress),
and probably stomata had begun to close around
noon.
On a cloudy day, water stress never became
a problem and stomata remained open.
Even though there was less total light than
in (A), there was more photosynthesis for
the day.
In conclusion, light, leaf structure, and
water can greatly affect the photosynthesis
in a plant.
I hope you have a better understanding of
these factors.
