Download Lesson 24, 26: Light, Photosynthesis, Production

Lesson 24, 26: Light, Photosynthesis, Production
• Chapters 12, 14 and 15 in the textbook
• Brief review of solar radiation budgets,
temperature, leaf energy budgets, and variation
due to latitude, altitude, continentality, and
vegetation (Today’s lecture).
• Brief review of photosynthesis and production
(Fleur will cover this next Monday).
Visible light: the source of energy for most
plants and animals
•
•. Visible light (380750 nm) is the
energy
used in
photosynthesis
• Infrared radiation
is 756-100,000
nm is heat
Energy inputs to the top of the
arth’s
E atmosphere and surface
Top of Atmosphere
Earth Surface
• UV radiation 1380 nm is harmful
to life
FromLillisandandKiefer1987
Solar Constant
• Total short-wave radiation received at the top of the
Earth’s atmosphere in the wave bands from 250-4000 nm
is about 1.94 cal cm-2 min-1
• After transmission through the atmosphere, this is reduced
to about 1.3 cal cm-2 min-1 at sea level on a clear day.
Solar energy budget
S = R + C + G + Ps + LE
S = incoming solar radiation at the surface
R = reflected energy
C = convective heating of the air by radiation
G = heating of solids (e.g., soil) by conduction
Ps = energy used in photosynthesis
LE = latent heat of evaporation
Transfer of heat can occur in 3 ways
• Conduction: transfer of heat through solids
• Convection: transfer of heat by circulation (liquid or gas)
• Radiation: transfer of heat by electromagnetic waves
Albedo (R)
• Albedo is the reflectivity of a surface.
• R in the energy budget is the reflectivity of a material summed
over all wavelengths expressed as fraction of the total
incoming radiation.
• For example, in a tropical forest where the incoming net
radiation is 1.3 cal-2 min-1, a surface with an albedo of 0.16
would reflect 0.16 (1.3 cal cm-2 min-1) or 0.21 cal cm-2 min-1.
Annual net radiation balance for three sites
Leaf energy budget
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SOLAR ENERGY
(thin lines) Energy
may arrive at the
surface of the leaf
directly or indirectly,
as diffuse radiation
scattered by the
atmosphere (skylight,
Ssky) and by clouds
(cloud light, Scloud) or
as reflected from the
soil or other objects in
the habitat (rSdirect,
rSsky, rScloud)
HEAT (fat lines) (IR,
4000-80,000 nm).
This is heat radiated
or reradiated as a
function of
temperature. Leaves
are objects, so they
emit long-wave
radiation as a function
of their temperature.
From P.S. Nobel. 1983
The spectrum of light is different depending on its source:
Sunlight, skylight, cloud light, and sunlight penetrating a
stand of vegetation
• High proportion of blue light in
skylight.
• Low amount of red light in cloud
light.
• High absorption of light in the
visible portion by vegetation due to
the presence of chlorophylls,
carotenes, and xanthophylls.
• Most of the red light is absorbed by
vegetation, and much of the NIR is
transmitted or reflected.
Gates et al. 1965. By permission of the American Meteorological Society.
Transmission, absorption, and reflectance of
radiation by an idealized leaf
• Relatively
high reflectance
in the green and
NIR
wavelengths.
• Strong
absorption in
the blue and
red
wavelengths,
and also in
water
absorption
bands in the IR
region.
IR
Green
Water absorption bands
Red
Pyranometers
• Sensitive to radiation in the 3602500 nm range
• This includes a portion of the UV
region and quite a bit of IR
radiation.
• Filters can be added to restrict
the sensitivity to
photosynthetically active radiation
(PAR, 400-700 nm)
Photosynthetically active radiation (PAR) and
photosynthetic photon flux density (PPFD)
PAR
– Radiation in the wavelengths absorbed by chlorophyll and active in
the photosynthetic process (400-700 nm).
– Usually measured in Einsteins (Einstein = 6.02 x 1023 photons or
one mole of photons)
PPFD
– Flux of photosynthetically active photons per unit area.
– Usually measured in Einsteins m-2 s-1.
– In full sun this is about 2000 µmol m-2 sec-1
Daily potential flux of radiation as a function of latitude and
season
• The lower latitudes have low seasonal variation.
• There is great seasonal variation at high latitudes with areas north of the Arctic Circle
receiving no radiation during at least a portion of the winter.
• Highest energy received in a day is during the summer at high latitudes!
Global solar radiation (kcal cm-2 yr-1) received at the ground
surface
Over the land areas is there is a gradual decrease in radiation toward the poles. The very
dark areas are over relatively cloudless portions of the globe.
The rates of decline are most marked north and south of 30˚ latitude because of
reduced daylight in the winter and lower angles of incident radiation.
Some equatorial regions also have low radiation because of consistent heavy cloud
cover.
Sun flecks: British Columbia douglas fir forest floor
Light intensity
(PPFD, mol m-2 s-1)
>0.5
0.2-0.5
0.05-0.2
TOTAL
Time (min)
In open
Beneath canopy
559.1
8.5
700.7
31.5
803.3
270.3
2063.1
310.3
Chen and Klinka 1997
Growth rates as well as photosynthetic rates of many forest species are highly
correlated with daily accumulative duration of sunflecks. For example, plants
receiving 60 minutes of sunflecks a day had a threefold to fivefold more rapid
growth rate than plants receiving 20 minutes a day.
Brief sunflecks lasting 5-10 seconds can be utilized with surprisingly high
efficiency.
Temperature measurement in the field
• Thermisters and data loggers
– Campbell data loggers
– Hobos
– i-buttons
Types of plant adaptations to varying levels of temperature
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Thermoperiodism: require a certain daily range of temperatures (e.g., 4˚ in
coastal redwoods, also see next figure for response in seeds of Carex otrubae,
Rumex sanguineus, and Rorippa islandica).
Dormancy: period of inactivity for plant organs that is broken only with
certain environmental requirements are met. Need for oxygen is a key feature
of dormancy.
Stratification: The requirement of cold temperatures before germination can
occur.
Vernalization: The requirement for cold temperatures after germination (e.g.,
winter wheat).
Sumorization: Heat cracking of the seed coat, either through fire (many
pines) or through extreme summer temperature (many desert annuals).
Germination response to various levels of thermoperiodism in
three species
Carex otrubae
Rumex sanguineus
Rorippa islanica
Sumorization for several species of desert annuals
Temperature as function of altitude: Lapse
rates
Lapse rate: The inverse relationship between temperature
and elevation due to less atmospheric pressure at higher
elevations..
– Dry adiabatic lapse rate (= -10˚C/1000 m): The maximum
amount of cooling that can occur in dry air with no moisture. There
is no heat gain due to condensation or cloud formation.
– Wet adiabatic lapse rate (= -5˚C/1000 m): The lapse rate in
saturated air.
– Ecological lapse rate (about -6.5˚C/1000 m): A rough global
average. It is higher over deserts, and lower over humid tropical
areas.
Influence of topography
• California community mosaic of
grasslands, chaparral and oak
woodland.
• The oak (Quercus douglasii)
woodlands cover slopes in the
foreground on the right, which faces
north.
• The chaparral (Adenostoma
fasciculata)-covered slope in the
foreground on the left faces south.
• Grasslands in the background which
are at a lower elevation, and face
south.
• The effects of slope aspect are very
familiar to us in Fairbanks. On northfacing slopes there is usually black
spruce forests. South-facing slopes
have deciduous or mixed forests. On
steep south facing bluffs along some
rivers there can even be unique
grassland communities.
Daily cycle of solar radiation on various slope angles and
aspects at 40˚N during three seasons
Winter solstice:
1. The north facing slopes receive no radiation.
Winter
solstice
2. Maximum radiation is on the 45˚ south
facing slope.
3. East slopes receive the maximum amount of
radiation in the morning. The opposite would
be true for west facing slopes.
Spring equinox:
Spring
equinox
1. North-facing 45˚ slopes receive some
radiation, but vertical ones do not
Summer solstice:
1. At this point the radiation on the horizontal
surface is higher than the south-facing 45˚
slope.
Summer
solstice
2. There is considerable radiation on the north
facing slope.
From BBPGS, Fig. 14-10. From Gates, D.M. 1972. Man and
his environment: Climate. Harper & Row.
Influence of continentality: Average dates of killing
frosts
Spring
Fall
• The position of sites with respect to
large water bodies, particularly
oceans has a marked effect on daily
and seasonal temperatures.
• This figure shows the moderating
effects at coastal sites with regard to
the first and last days of frost.
Coastal sites generally have earlier
dates of last frost in the spring and
later dates of first frost in the fall.
Diurnal and seasonal effects of continentality
Effects of cloud cover
• Water vapor is opaque to some areas of infrared radiation so less total radiation.
• Re-radiation of longwave radiation from clouds back to
ground (particularly evident on cloudy nights in Fairbanks,
when it is usually much warmer).
• Stabilizing influence - less diurnal variation
Moderating effect of forest cover on microclimate
Effects of vegetation canopy on light regimes
A. The highest radiant flux reaches the forest floor
between the spring leafless phenoseason and early
summer, when the canopy is in full leaf. It is during
this period that sufficient light and adequate
temperature allow the spring bloom of geophytes
and herbaceous plants on the forest floor.
B. Approximate daily total of un-depleted solar
radiation received on a horizontal surface at 35˚N.
C. A comparison of the percentage of leaves in the
mature leaf stages of four taxa at various times of
the the year. Stippled band indicate expanding or
falling leaves in the canopy.
From Hutchison and Matt. 1977. The distribution of solar radiation within a
deciduous forest. Ecological Monographs 47: 185-207 and Mahall and
Bormann. 1978. Botanical Gazette 139: 467-481.
Observations of a farmer regarding climate effects of forest clearing in Ohio
“While the earth was defended from the rays of the summer sun, and protected from the cold blasts of
winter by an impenetrable covering of fallen leaves, and a thick growth of forest trees, there can be
no doubt of the winters being milder, and summers more temperate, than at present. It was especially
noticed in the summer nights, which were so cool as to render a blanket both a pleasant and desirable
covering to the sleeper. [T]he earth, when protected by the forest from the influence of cold winds,
and covered with a thick coat of fallen leaves, never froze; while in an adjacent cleared field it froze
to the depth of several inches. The warm vapour constantly rising from the earth, served to temper
the atmosphere and render it more mild than at present…. It is true there were some very cold winters
and deep snows, but they were not so changeable as now: nevertheless it yet remains certain, that the
winters are much more uniform, while a country is covered with forest, and not subject to such
sudden changes of temperatures as the are in an open region….The summers are as much changed as
the winters; fifty years since, they were more humid, and there was more generally that conditions of
the atmosphere which we call sultry, and now experience in warm weather after a heavy rain. This
constant humidity of the air was occasioned by the regular evaporation of moisture from the leaves of
the trees, shrubs, and plants, that clothed the face of the earth, and shut out the drying influence of the
sun and air.
The same causes kept the surface constantly moist, and afforded a regular supply of water to the springs,
during the summer as well as the winter, protecting the tender roots of the grasses and other plants
from the cold, caused them to vegetate early in the spring, and bring forth a plentiful supply of
herbage for the wild animals of the forest, and domestic cattle of the new settler.
Samual Hildreth, Pioneer History, 1857
Climate modelers are just now being able to account for the effects of land-use changes on regional and
local climates (e.g., studies of Roger Pielke).
Plant structural adaptations to low light levels are often really
adaptations to water balance
•
Sun leaves
Sun leaves and shade leaves: really an
adaptation to water balance. Sun leaves
are relatively xeromorphic. The
determining factor is the water status of
the the cytoplasm of the leaf priomordia
at the time of their formation.
Hydrature (water activity) of the
cytoplasm of sun branches is less than
that of shade branches.
Shade Leaf
Two sun leaves and a shade leaf of Robinia pseudacacia (after Walter
1931).
Light interception vs. leaf area and leaf
orientation
The extinction of light as it travels through a
vegetation canopy depends on the total leaf area/
per unit ground surface and leaf orientation.
Very little additional sunlight penetrates a
canopy of horizontal leaves with a LAI > 3, but
when more leaves have a vertical orientation,
more light penetrates canopies of higher LAI.
Some forest communities have LAI >8.
From Campbell, G.S. 1977. An introduction to environmental biophysics.
Berlin: Springer Verlag.
Erectophyllic vs. planophyllic and Diaheliotropic vs. paraheliotropic
leaves
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Erectophyllic: Vertically oriented
leaves.
Planophyllic: Horizontally oriented
leaves.
Diaheliotropic: Leaf movements
that track the sun such that the leaf
is perpendicular to the sun’s rays.
This maximizes reception of solar
radiation
Paraheliotropic: Leaf movements
that track the sun such that the leaf
if parallel to the sun’s rays. This
minimizes transpiration rates and
leaf temperature.
Theoretical photon flux incident on three leaf types
over the course of a day
Modified from J. Ehlerlinger and I. Forsyth. Science 210: 1094-1098.
Examples of plant adaptations to control temperature:
(1) Desert Holly: Leaf temperature and transpiration rates of leaves with
different absorptance and leaf angles
• Experimental environmental
conditions are those of Death
Valley, midday, midsummer.
• Leaf reflectivity is controlled
by salt bladders, which expand
in the winter, and deflate in the
summer.
• The leaves are twice as big in
winter, and white in summer,
due to salt crystals.
• Leaf reflectance is inversely
related to leaf water content.
• Color and orientation both
affect the leaf temperature and
transipiration rates.
• Black leaves are typical desert holly (Atriplex hymenelytra) leaves in winter. Absorptance
(ratio of absorbed to incident radiation) = 0.5.
• White leaves are summer leaves with absorptance = 0.25.
Examples of plant adaptations to control temperature:
(2) Leaf pubescence on four alpine species of Puya
• Pubescence helps maintain favorable
flowering temperatures in tropical
alpine arborescent rosette plants by
insulating against radiant heat loss.
• Amount of pubescence is corrrelated
with elevation and, therefore,
decreasing temperatures.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
• Miller also removed the hairs from
the inflorescences, causing a
significant increase in radiant heat
loss.
Modified from Miller (1994). Functional significance of inflorescence pubescence in tropical alpine species
of Puya. In: Tropical alpine environments: Plant form and function. Rundel, P.W. et al. pp. 195-213.
Response to high UV-B radiation (280-320 nm)
(Effect of reduced ozone concentrations)
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QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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Increased UV at the earth surface is a
consequence of reduced ozone concentrations in
the atmosphere.
A significant thinning, or reduction in ozone
concentrations, which results in the destruction
of up to 70% of the ozone normally found over
Antarctica.
The causes of this decrease are complex
chemical reactions in the atmosphere related
increased levels of Chlorofluorocarbons,
methane, and nitrous oxide.
Ozone absorbs UV-B radiation and protects most
plants from UV-B damage. Most plants do not
reflect UV-B well, but many have pigments and
other compounds that absorb UV-B (e.g.,
anthocyanins, total phenols).
Effects of increased UV-B:
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Reduced photosynthesis
Reduced levels of epidermal flavinoids
Stunted growth
Reduction in protective compounds (anthocycanin
and phenols)
The long-term effects of exposure to high UV-B
are unknown.
Summary
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Conduction (transfer of heat through solids), convection (transfer of heat by circulation), radiation:
transfer of heat by electromagnetic waves
Solar Constant: About 1.94 cal cm-2 min-1
Solar energy budget: S = R + C + G + Ps + LE
Albedo; the reflectivity of a material summed over all wavelengths expressed as fraction of the total
incoming radiation.
Leaf energy budget
Pyranometers
Photosynthetically active radiation (PAR) and photosynthetic photon flux density (PPFD)
Temperature measurement: Thermisters and data loggers, sucrose for integrating temperature over
time
Types of plant adaptations to varying levels of temp.erature (Thermoperiodism, dormancy,
stratification, vernalization, and sumorization).
Temperature as a function of latitude, season, altitude (lapse rates), topography, cloud cover,
vegetation.
The effect of leaf orientation (Erectophyllic vs. planophyllic and Diaheliotropic vs.
paraheliotropic leaves) and floral pubescence
Effects of UV-B radiation (reduced photosynthesis, reduced levels of epidermal flavinoids, stunted
growth, reduction in protective compounds (anthocycanin and phenols)