Download Plant Ecology

Plant Ecology - Chapter 2
Photosynthesis & Light
Photosynthesis & Light
Functional ecology how the biochemistry
and physiology of
individual plants
determine their
responses to their
environment, within the
structural context of
their anatomy and
morphology
Photosynthesis & Light
Functional ecology closely related to
physiological ecology,
which focuses on
physiological
mechanisms underlying
whole-plant responses
to their environment
Photosynthesis & Light
Photosynthesis is a
“package deal”
How much light
Competitors
Limitations (pollution,
pathogens)
Herbivores
Plants must cope with
multiple items at same
time
Process of Photosynthesis
Biochemical process to
acquire energy from
sun, carbon from
atmosphere
2 parts
Capture of energy (light
reactions)
Storage of energy into
formed organic
molecules (carbon
fixation)
Process of Photosynthesis
Reactions take place in
chloroplasts
Light reactions on
thylakoid membranes
Carbon fixation (Calvin
cycle) within the stroma
Process of Photosynthesis
Light reactions involve
pigment molecules
Many forms of
chlorophyll
Accessory pigments
(carotenoids and
xanthophylls in
terrestrial plants)
Process of Photosynthesis
Pigment molecules
arranged into two
molecular complexes
Photosystems I and II
Capture energy (form
ATP, NADPH) plus
generate oxygen
Process of Photosynthesis
Energy captured from
light reactions powers
the Calvin cycle
Captured energy
ultimately stored in
chemical bonds of
carbohydrates, other
organic molecules
Rates of Photosynthesis
Gross photosynthesis total amount of carbon
captured
Cellular respiration organic compounds
broken down to release
energy
Net photosynthesis gross photosynthesis
minus respiration
Rates of Photosynthesis
Basic limiting factor - amount
of light energy reaching
thylakoid membranes
Darkness - loss of energy
due to respiration - giving off
CO2
Low light - respiration plus
some photosynthesis - giving
off and taking up CO2
Compensation point
Rates of Photosynthesis
Strong light - respiration
plus photosynthesis giving off and taking up
CO2, up to a point
Maximum rate of
photosynthesis, despite
further increase in light
energy
Rates of Photosynthesis
Different plants have
different photosynthetic
responses to same light
intensity
Some do better under low
light, others strong light
Habitat - shade vs. sun
Some can shift light
compensation point to deal
with changes in light
availability (lots in spring,
less in summer in shade)
Quality of Light
Light quality (availability of
different wavelengths) can
limit rate of
photosynthesis
Blue and red wavelengths
are captured preferentially
Green wavelengths are
discarded (green plants)
Global Light Availability
Tropical latitudes - day
and night equal
Polar latitudes continuously light at
midsummer, continuously
dark at midwinter
Maximum sunlight energy
greater in tropics than
polar regions
Global Light Availability
Maximum sunlight energy
greater at high altitudes
than at sea level
Damaging UV-B radiation
greater in tropics than
polar regions, high
elevations vs. low
elevations
Biochemical protection:
flavonoids to absorb,
antioxidant and DNA
repair enzymes
CO2 Uptake Limitations
CO2 diffusion from
surrounding air into
leaf and into
chloroplast
Leaf conductance rate at which CO2
flows into the leaf
Mostly under control
of stomata
CO2 Uptake Limitations
Stomata open, close to
maintain water balance
(seconds, minutes)
Stomata change as leaf
morphology, chemistry
change (days, months)
Natural selection
modifies (100s, 1000s
of years)
CO2 Uptake Limitations
Controlling water loss is
main reason why plants
restrict their CO2 uptake
Huge amount of air
required for
photosynthesis - 2500 L
air for each gram of
glucose produced
CO2 Uptake Limitations
Stomata can be very
dynamic, opening and
closing constantly to
regulate CO2 and water
loss
Much variation even
within same leaf
Patchy closure also
common in stressed
plants
Variation in Photosynthetic
Rates: Habitats
Photosynthetic rates
vary within and among
habitats
Correlated with species
composition, habitat
preferences, growth
rates
Variation in Photosynthetic
Rates: Habitats
Photosynthetic rates
may be unrelated to
species distributions,
populations processes
Other important
components of
photosynthesis: total
leaf area, length of time
leaves active,
maintained
Photosynthetic Pathways
Carbon fixation done
using 3 different
pathways
C3
C4
CAM (crassulacean
acid metabolism)
Photosynthetic Pathways
C3 and C4 named for 3carbon and 4-carbon
stable molecules first
formed in these
pathways
CAM named after plant
family Crassulaceae
where it was first
discovered
Photosynthetic Pathways
Most plants use C3
photosynthesis, and
plants that use it are
found everywhere
C4 and CAM are
modifications of C3, and
evolved from it
Photosynthetic Pathways
C3: CO2 joined to 5carbon molecule with
assist from the enzyme
RuBP
carboxylase/oxygenase rubisco
Rubisco probably most
abundant protein on earth,
but does its job very
poorly
Photosynthetic Pathways
Rubisco inefficient at
capturing CO2
Also takes up O2 during
photorespiration
O2 uptake favored over CO2
uptake as temperatures
increase
Limits photosynthesis
Plants must have HUGE
amounts of rubisco,
especially those in warm,
bright habitats, to
compensate for poor
performance
Photosynthetic Pathways
Increases in
atmospheric CO2
concentrations should
allow C3 plants to
increase rates of
photosynthesis
Photosynthetic Pathways
C4 photosynthesis
contains additional step
used for initial CO2
capture
3-carbon PEP
(phosphoenol-pyruvate)
+ CO2 = 4-carbon OAA
(oxaloacetate)
Catalyzed by PEP
carboxylate
Photosynthetic Pathways
PEP carboxylate only
captures CO2
Higher affinity for CO2 than
rubisco
Not affected by warmer
temperatures
Decarboxylation (CO2
removal) process allows
standard Calvin cycle
(including rubisco)
Photosynthetic Pathways
C4 requires special leaf
anatomy
Spatial separation of C4
and C3 reactions
Rubisco exposed only
to CO2, not O2 in
atmosphere like in C3
plant
Photosynthetic Pathways
C4: Mesophyll cells for
carbon fixation, bundle
sheath cells for Calvin
cycle - keeps O2 away
from Calvin cycle
C3: Mesophyll cells for
carbon fixation and
Calvin cycle - allows O2
access to Calvin cycle
Photosynthetic Pathways
C4 plants generally have
higher maximum rates
of photosynthesis, and
have higher
temperature optima
Photosynthetic Pathways
C4 plants generally do
not become lightsaturated, even in full
sunlight
Also have better
nitrogen use and water
use efficiencies
because of reduced
needs for rubisco (1/3
to 1/6)
Photosynthetic Pathways
Requires additional
energy to run C4
pathway, but easily
compensated for by
photosynthetic gains at
high light levels
Very successful in
warm, full-light habitats,
e.g., deserts
Photosynthetic Pathways
CAM photosynthesis Crassulacean acid
metabolism
Uses basically same
biochemistry as C4, but
in very different way
Rubisco found in all
photosynthetic cells, not
just bundle sheath cells
Photosynthetic Pathways
CAM uses temporal
separation of light
capture, carbon fixation
rather than spatial
separation as in C4
CO2 captured at night,
converted into organic
acids
Photosynthetic Pathways
During daylight, organic
acids broken down to
release carbon, used
normally in Calvin cycle
Stomata remain closed
during day
Photosynthetic Pathways
CAM plants have thick,
succulent tissues to
allow for organic acid
storage overnight
Tremendous water use
efficiency (stomata
closed during heat of
day)
Photosynthetic Pathways
Some CAM plants not
obligated to just CAM
Can use C3
photosynthesis during day
if conditions are right, to
achieve higher rates of
photosynthesis
CAM can’t accumulate
carbon as fast as C3 or C4
plants, lowering rate of
photosynthesis
C3, C4, and CAM
C3 plants most abundant
(# of species, total
biomass)
More CAM species than
C4 species
CAM plants less abundant
than C4 in biomass,
worldwide distribution
C3, C4, and CAM
Half of grass species are
C4
Dominate warm grassland
ecosystems
Warm, bright conditions
where C4 is favored
C3, C4, and CAM
CAM plants typically are
succulents in desert
habitats, or……
C3, C4, and CAM
Epiphytes growing on
trees in tropics or
subtropics
Both types experience
severe water shortages
C3, C4, and CAM
Phenology - seasonal
timing of seasonal events
C3 plants typically more
springtime, vs. C4 plants
being mostly summer
C3, C4, and CAM
C4 grasses are most
common where summer
temperatures are warm in
N. America
C3, C4, and CAM
C3 grasses - cool, winter-moist
C4 grasses - warm, summer-moist
C3, C4, and CAM
Sun & Shade Leaves
Sun & Shade Leaves
Sun & Shade Leaves
Higher light saturation levels
Greater maximum photosynthetic rates
Species Adaptations-Sun
Solar tracking increases light availability
Species Adaptations-Shade
Velvety, satiny leaf surfaces,
blue iridescence on leaf enhance
available light
Species Adaptations-Shade
Shade species use brief sunflecks
with high efficiency: stomata open +
slow loss of photosynthetic induction
Species AdaptationsEcotypes?
Genetically distinct populations of same species
adapted to low- and high-light conditions?
Phenotypic plasticity
Daylength
Flowering, seed
dormancy, seed
germination, other
physiological responses of
plants controlled by
daylength (actually
nightlength)
More reliable predictor of
seasonal change than
temperature
Ratio of two forms of
phytochrome A controlled
by length of dark period