Transcript Plant Ecology - Chapter 3

Plant Ecology - Chapter 3
Water & Energy
Life on Land
Ancestors of
terrestrial plants
were aquatic
Dependent on water
for everything nutrient delivery to
reproduction
Life on Land
Evolution has involved
greater adaptation to
dry environments
Coverings to reduce
desiccation
Vascular tissues to
transport water,
nutrients
Changed reproduction,
development to survive
dry environment (pollen,
seed)
Water Potential
Plants need to acquire
water, move it through
their structures
Also lose water to the
environment
All these depend on
water potential of
various plant parts,
immediate environment
Water Potential
Water potential difference in potential
energy between pure
water and water in
some system
Represents sum of
osmotic, pressure,
matric, and gravitational
potentials
Water Potential
Water always moves from
larger to smaller water
potentials
Pure water has water
potential of 0
Soils, plant parts have
negative water potentials
Gradient in water potential
drives water from soil,
through plant, into
atmosphere
Water Potential
Energy is required to
move water upward
through plant into
atmosphere
Energy not expended by
plant itself
Soil to roots - osmotic
potential
Up through tree and out pressure potential
Sunlight provides energy
to convert liquid into vapor
Transpiration - Water Loss
Plants transpire huge
amounts of water
Far more than they use
for metabolism
Needled-leaved tree - 30
L/day
Temperate deciduous tree
- up to 140 L/day
Rainforest tree - up to
1000 L/day
Transpiration - Water Loss
Transpiration caused by
huge difference in water
potential between moist
soil and air
Huge surface area of
roots, leaves produce
much higher losses via
transpiration than
evaporative losses from
open body of water
Transpiration - Water Loss
Transpiration losses
controlled mostly by
stomata
High conductance of
water vapor when stomata
are open, low when
closed
Conductance to water
vapor, CO2 closely linked
stomata
Transpiration - Water Loss
Transpiration losses have
no negative effects on
plants when soil water is
freely available
Benefits plants because
process carries in
nutrients with no energy
expenditure
stomata
Transpiration - Water Loss
Problem develops when
soils dry
Stomata closed to
conserve water shuts out
CO2, ends photosynthesis
- starvation
Stomata open to allow
CO2 risks desiccation
stomata
Coping with Availability
stomata
Mesophytes - plants that
live in moderately moist
(mesic) soils
Experience only
infrequent mild water
shortages
Typically transpire when
soil water potentials are >1.5 MPa
Close stomata and wait
out drier conditions (hours
to days)
Coping with Availability
Common temperate
plants are mesophytes forest trees and
wildflowers, ag crops,
ornamental species
Drought-intolerant - begin
to die after days to weeks
of dry soils
stomata
Coping with Availability
Xerophytes are adapted for
living in dry (xeric) soils
Continue to transpire even
when soil water potentials
drop as low as -6 MPa
Can survive/recover from
low leaf water potentials that
would kill mesophytes
Water Use Efficiency
Ratio of carbon gain to
water loss during
photosynthesis (WUE)
Water loss greater than
CO2 uptake
Steeper gradient, smaller
molecules, shorter
pathway
Water Use Efficiency
CAM plants have highest
water use efficiencies decoupling of carbon uptake
and fixation
C4 plants more efficient than
C3 plants - efficiency of C4
step in capturing CO2
C3 WUE highest when
stomata partially open,
concentrations of
photosynthetic enzymes high
Whole-Plant Adaptations
Desert annuals - drought
avoidance
Carry out entire life cycle
during rainy season germinate, grow, flower,
set seed, die
Experience desert only as
a moist environment
during their brief life
Whole-Plant Adaptations
Desert trees and shrubs
- drought avoidance
Drought-deciduous lose leaves during dry
season, grow new
leaves when rains
return
Whole-Plant Adaptations
Herbaceous perennials in
xeric habitats (many
grasses) - drought
avoidance
Go dormant, die back to
ground level during dry
seasons
Major disadvantage - no
photosynthesis for
extended time periods
Whole-Plant Adaptations
True xerophytes - drought
tolerant
Physiology, morphology,
anatomy adapted for life in
dry conditions, continue to
live and grow
High root-to-shoot ratios take up more water and lose
less through transpiration
Succulents - store large
amounts of water
Physiological Adaptations
Series of physiological
events begin when soils dry
Hormones: signal changes in
plant functions
Cell growth, protein
synthesis slow, cease
Nutrients reallocated to
roots, shoots
Photosynthesis inhibited,
leaves wilt, older leaves may
die
Physiological Adaptations
Some plants synthesize
more soluble nitrate
compounds,
carbohydrates to lower
osmotic potential of plant
cells
Allows continued inflow of
water via osmosis,
prevents turgor loss,
wilting
Resurrection Plants
Unusual adaptations to
survive complete,
extended desiccation
Many different kinds of
plants
Various parts of world, but
common in southern
Africa
Survive cellular
dehydration by
coordinated set of
processes
Resurrection Plants
Synthesize drought-stable
proteins
Add phospholipidstabilizing carbohydrates
into cell membranes
Cytoplasm may gel
Metabolism virtually
stopped
Rehydration also step-bystep
Flooding
Adaptation to flooding
needed in some habitats
Variations: depth,
frequency, season,
duration
Adapted to predictable
flooding
Not adapted to greater
frequency, severity
Flooding
Biggest problem - lack of
oxygen
Plant roots need oxygen
Waterlogged soils inhibit
oxygen diffusion
Toxic substances from
bacterial anaerobic
metabolism accumulate
Plants get stressed
Flooding
Plants have evolved
physiological, anatomical,
life history characteristics
to function in flooded
environments
E.g., some plants able to
use ethanol fermentation
to generate some energy
in absence of oxygen
Anatomical Adaptations
Most water regulation
done by stomata
Pore width controlled by
guard cells - continually
change shape
Movement controlled by
plant hormones
Respond to changes in
light, CO2 concentration,
water availability
Anatomical Adaptations
Light causes guard cells
to open in C3 and C4
plants
Close in response to high
CO2 inside leaf, open
when CO2 is low
CAM plants open stomata
at night as CO2 is used
up, close during day when
it builds up
Anatomical Adaptations
Declining water potential
in leaf will cause stomata
to close, overriding other
factors (light, CO2)
Protecting against
desiccation more
important than
maintaining
photosynthesis
Anatomical Adaptations
Mesophyte, xerophyte
stomata respond differently
to changing moisture
Mesophyte stomata close
during middle of day, or
whenever soil moisture
drops
Xerophyte stomata remain
open during dry, hot
conditions
Related to capacities for
maintaining different leaf
water potentials
Anatomical Adaptations
Xerophytes typically are
amphistomous stomata on both sides
of leaf
Also often isobilateral pallisade mesophyll on
both upper and lower
sides of leaf
Adaptation to high light
levels
Anatomical Adaptations
Xerophytes also have
more stomata per leaf
area, but less pore area
per leaf area
Allows tighter regulation
of water loss while
allowing CO2 the most
direct access to cells
Anatomical Adaptations
Xerophytes may have
sunken stomata,
increasing resistance to
water loss
Leaves may also have
thicker waxy cuticle
covering, to reduce
water loss when
stomata are closed
Anatomical Adaptations
Root systems vary
Fibrous root systems of
monocots (grasses)
especially good at
obtaining water from
large volume of soil
Taproots can extend
deep into soil, possible
store food
Anatomical Adaptations
Plants adapted to
growing in aquatic,
flooded habitats may
have aerenchyma
(aerated tissues)
Air channels (gas
lacunae) allow gases to
move into and out of
roots
Oxygen and CO2
Anatomical Adaptations
Water-conducting
vessels vary among
plants
Thin-walled, largediameter xylem vessels
best for conducting
water under normal
conditions
But problems under low
water conditions
Anatomical Adaptations
Thin walls collapse under
extreme negative
pressures in xerophytes
(need thick-walled, small
diameter)
Big vessels prone to
cavitation - break in water
column caused by air
bubbles (especially during
freezing, low water
conditions)
Energy Balance
Radiant heat gain from
sun is balanced by
conduction (transfer to
cooler object) and
convection (transport by
moving fluid or air) losses
and latent heat loss
(evaporation)
Energy Balance
Large leaves in bright
sunlight, still air, dry soils
face problem
Heat gained needs to be
balanced by heat loss, or
risk severe wilting, death
Light breeze would be
sufficient to cool leaf
properly with normal soil
moisture, stronger winds
in drier soils
Energy Balance
Plants can control latent
heat loss, and leaf
temperature, by
controlling transpiration
Adaptation to warm, dry
habitats often involves
developing smaller,
narrower leaves that can
remain close to air
temperature even when
stomata are closed
Energy Balance
Holding leaves at steep
angle reduces radiant
heat gain (leaves of the
desert shrub, jojoba)
Some plants can change
angle as leaf temperature
changes - steeper at
hotter temps.
Energy Balance
Leaves with pubescence
(hairs) or shiny, waxy
coatings reduce
absorption of radiant heat
from sun and keep leaves
from overheating
Also reduces rate of
photosynthesis
Energy Balance
Plants are not simply
passive receptors of heat
Can modify what they
“experience” via shortterm physiological
changes and long-term
adaptations