The Photosynthesis-Transpiration Compromise
I. Measurement of Transpiration
Know following terminology and basically what each system does
A. Lysimeter or Gravimetric Method:
1. Potted Plant sealed against water loss from soil while leaving
the shoot free to transpire
2. Evapotranspiration: the combination of evaporation from soil and
transpiration from plant.
B. Potometer: burette or capillary tube connected to a transpiring
plant or leaf.
C. Gas Exchange or Cuvette Method
1. Transpiration is calculated by measuring water vapor in a sealed
atmospheric chamber surrounding the leaf (cuvette)
2. Fick's First Law of Diffusion: flow is proportional to the
driving force and inversely proportional to the resistance.
D. Porometers: calculates transpiration from the absolute humidity
(relative humidity and air temperature) and the rate dry air is
introduced to maintain a constant humidity.
E. Stem-Flow Methods: thermocouples that surround the stem of a
plant. Heat water in the stem which is detected by sensors.
II. Leaves and Stomatal Openings:
A. Stomatal openings make up about 1% of the leaf surface area. The
leaf can transpire half as much water as would evaporate from a
wet filter paper of equivalent area. Evaporation is a diffusion
process from water surface to the atmosphere. Diffusion is
proportional to the driving force and the conductivity. The
driving force is the difference in vapor pressure (or density)
between the water surface (where the atmosphere is saturated with
vapor) and the atmosphere some distance away (were the atmosphere
is below saturation level).
B. Rate of evaporation depends on different diffusion conductivities.
1. Function of Area: distance in the atmosphere through with
water molecules must diffuse before their concentration reaches
that of the atmosphere. The shorter the distance, the higher
the conductivity.
2. Boundary Layer: much shorter above the stomata of a leaf than
above a free water surface. Molecules evaporating from the
free water are part of a dense column of molecules extending
some distance above the surface. Water molecules diffusing
through a stomata can go in any direction within an imaginary
hemisphere centered above the stoma. In this hemisphere, the
water molecule concentration drops rapidly with distance from
the stoma. Therefor the concentration gradient is very steep
because the boundary layer is very thin. If stoma are closer
together than the thickness of their boundary layers, their
hemispheres overlap and merge into a boundary layer.
III. Stomatal Anatomy
Be able to draw a picture of the stomatal apparatus and label the
following structures:
A. Structures involved in Stomatal and Leaf Anatomy
1. Cuticle
2. Guard Cells
3. stomate
4. Stomatal Apparatus
5. Stomatal pore
6. Accessory or Subsidiary Cells
7. Palisade Parenchyma and Spongy Parenchyma
8. Mesophyll
9. Intercellular spaces
10. Kranz Anatomy
11. Bundle-sheath Cells
12. Stomatal crypt and sunken stomata
B. Stomates occur mainly on lower leaf surfaces but can be found on
both surfaces.
C. Dicots have two kidney-shaped guard cells
D. Grass and sedge guard cells tend to be pre elongated.
E. Guard Cells contain chloroplasts while neighboring epidermal
cells usually do not. No plasmodesmata connect the protoplast of
guard cells and accessory cells. There may be plasmodesmata
between guard cells and the mesophyll cells below the stomata.
F. A square millimeter of leaf surface can have 100 stomates.
Stomatal densities are sensitive to CO2 concentration. Fewer
stomates per unit area as CO2 increases. Counts of a given
species as a function of increasing elevation. CO2 partial
pressures decrease, along with the other gases in the atmosphere,
with increasing elevation. Herbarium specimens that stomatal
densities have decreased by 40 percent over the past two
centuries as CO2 in the atmosphere has increased from 280 to over
350 mmol mol-1
IV. Environmental Effects on Stomates:
A. General Information:
1. Stomates open at sunrise and close in darkness. Opening
requires an hour, closing is gradual throughout the afternoon.
High irradience levels cause wider stomatal apertures.
Succulents act in an opposite manner.
2. Stomates respond to intercellular CO2 levels. Low
concentrations of CO2 in the leaves cause stomates to open.
High CO2 concentration in the leaves can cause the stomates to
partially close partially
B. Effects of light quality on stomatal opening and closing:
1. Low light levels: the intercellular CO2 concentration is a
major controlling factor
2. Light absorbed in the guard cells, rather than the mesophyll
cells, is primarily responsible for the effect.
3. Blue light between 430 and 460 nm is 10X more effective as red
light between 630 and 680 nm. Inhibitors of photosynthesis
eliminated the response to red light. Red light response is
caused by light absorbed by chlorophyll. Blue light effect is
independent of photosynthesis. Blue light caused isolated
guard-cell protoplast to absorb K+.
C. Photosynthesis in Guard Cells: If photosynthesis occurs in guard
cells, it is in very low amounts.
D. Effects of environment on stomates
1. Stomates are sensitive to atmospheric humidity. Close when
difference between the vapor content of the air and the
intercellular spaces exceeds a critical level. Induce
oscillations with a periodicity of about 30 minutes. Steep
vapor gradients induce closing. When CO2 in the leaf is
depleted, the stomates open. Most rapid responses to lowered
humidity occurs under low irradiances.
2. Water potential within a leaf has a robust effect on stomatal
opening and closing. As water potential decreases (water
stress increases), the stomates close. Can override low CO2
and bright light.
3. High temperatures cause stomatal closing. Indirect response to
water stress, rise in respiration rate might cause an increase
in CO2. In some plants, high temperatures cause stomatal
opening.
4. Stomates partially close when the leaf is exposed to gentle
breezes. More CO2 is brought close to the stomates, increasing
its diffusion into the leaf. This increases transpiration,
leading to water stress and stomatal closure.
V. Stomatal Mechanics
A. Stomates open when guard cells absorb water and swell. Cellulose
microfibrils are arranged around the circumference of the
elongated guard cells radiating from a region at the
center of the stomate. The guard cell cannot increase in
diameter, but can increase in length, especially along the
outside walls. Pulls the inner wall which opens the stomate.
VI. Stomatal Control Mechanisms: changes in the osmotic potential in
guard cells that results in stomatal opening
A. Guard-Cell Absorption of Potassium Ions
1. Light causes a buildup of K+ in guard cells and in isolated
guard-cell protoplast. In the dark, K+ moves out of guard
cells into the surrounding cells, and stomates close.
2. Guard cells normally obtain K+ ions from adjacent epidermal
cells. Stomates can close in response to the application of
abscisic acid, a plant growth hormone which causes loss of K+
from the guard cells.
3. Mechanism of K+ movement: Starch is broken down to produce
PEP. This step is promoted by blue light. PEP combines with
CO2, producing oxaloacetic acid which is converted into malic
acid. H+ ions from the malic acid leaves the cell, balancing
the K+ ions that are entering.
4. Succulents have stomates that are open at night. CO2 combines
with PEP, producing malic acid in the dark in cells besides
guard cells. A decrease in CO2 concentration within the leaf
results in less dissolved CO2 in the guard cells, which causes
K+ absorption, and stomatal opening.
B. Effect of abscisic acid on stomatal opening.
1. Abscisic Acid causes stomates to close. Leaves under water
stress show a build up of ABA in their tissues. Suggests that
stomatal closure is a response to leaf water stress mediated by
ABA.
2. Stomates close in response to root water stress, suggesting
that the stomates are receiving a signal from the roots
(possibly ABA).
3. Three possible explanations
a. Water stress develops rapidly, water may evaporate from the
guard cells themselves.
b. ABA occurs in three pools.
Cytosol - where ABA is synthesized
Chloroplast - where ABA accumulates
Cell Walls - outside the protoplast
ABA could move from one pool to another before total ABA in
the leaf has a chance to build up.
c. ABA in guard cells represents only 0.15% of the total ABA in
the leaf. When the fresh weight of detached leaves decreases
by 10% through transpiration, guard-cell ABA increases 20
fold. ABA content of all leaf cells increases in response to
water stress.
4. Cytokinins: cause plant cell divisions, and stomatal opening.
C. Feedback Loops
1. When CO2 decreases in intercellular spaces and guard cells, K+
moves into guard cells and stomates open, allowing CO2 to
diffuse in, completing the first loop.
2. If water stress develops, ABA appears, the stomates close,
completing the second loop.
3. One feedback loop provides CO2 for photosynthesis; the other
protects against excessive water loss.
VII. Role of Transpiration:
A. Mineral Transport
1. Minerals absorbed by the roots move up through the plant in the
transpiration stream (the flow of water though the xylem caused
by transpiration). BUT, the transpiration stream is not
essential for this movement.
2. In the absence of transpiration, water in solution will
return to the assimilating organs through the xylem tissue.
Such a circulation has been demonstrated with radioactive
tracer. Transpiration is not essential for the movement of
minerals within the plant.
3. When transpiration occurs, it may aid mineral absorption from
the soil and transport in the plant.
B. Optimum turgidity verses water stress: cells function best with
some water deficit.
VIII. The Role of Transpiration: Energy Exchange
A. Leaf Temperature:
1. Evaporation of water is a powerful cooling process.
Latent Heat of Vaporization
2. Under steady-state condition, three factors influence leaf
temperature: radiation, convection, and transpiration
B. Radiation
1. If the leaf is absorbs more radiant energy than it radiates the
excess is dissipated by convection or transpiration, or both.
There are three important things when considering the net
radiation of a leaf;
a. the absorbance wavelengths,
b. the total spectrum of incoming radiation,
c. the amount of energy radiated by the leaf.
2. First: absorption spectrum. Some energy incident on the leaf
is transmitted, some is reflected, some is absorbed. Energy
absorbed depends on the spectrum. All the far-infrared or
thermal part of the spectrum is absorbed.
3. Second: radiation sources vary considerably. Radiation
absorbed by a plant is a function of the leaf's absorption
spectrum the spectrum of radiation impinging upon it.
4. Third: plants and all objects emit radiant energy in the far-
infrared part of the spectrum
C. Convection
1. Heat is conducted-convected from the leaf to the atmosphere in
response to a temperature difference between leaf and
atmosphere. The temperature difference is the driving force.
2. The resistance to covective heat transfer is expressed by the
thickness of the boundary layer.
3. Usually there is air movement around a leaf; more rapid the
air movement the thinner the boundary layer. The boundary
layer is thinnest next to the leading edge of the leaf.
4. Boundary layer are thinnest and offer the least resistance
to convective heat transfer for small leaves and high wind
velocities. Convective heat transfer is efficient under these
conditions. Small leaves have temperatures closer to air
temperature especially in a wind.
D. Transpiration:
1. Transpiration driving force is the gradient in water-vapor
density from within the leaf to the atmosphere beyond the
boundary layer.
2. Leaf resistance always occurs. Leaf resistance to transpiration
can vary over a wide range as environmental factors that
influence stomatal aperture.
3. Besides boundary-layer thickness, the vapor-density gradient is
influenced by two factors: (1) absolute humidity, and (2) leaf
temperature. Under most conditions vapor density is higher
inside the leaf even if the leaf is the same temperature as the
atmosphere beyond the boundary layer.
4. Transpiration can occur in an atmosphere with a relative
humidity of 100% because the leaf is usually warmer than the
air (a common phenomenon in sunlight). The energy source for
transpiration is solar radiation.
IX. Energy Exchanges of Plants in Ecosystems
A. The Desert
1. High temperatures, high radiation, low humidity, little
available water. Desert plants: have small leaves, producing
thin boundary layers and resulting in efficient convective heat
transfer. Temperatures are closely coupled to air temperature.
2. Sunken stomates. Light gray and reflect much of the sun's
radiation. Close stomates during the day and fix CO2 into
organic acids at night.
3. A few desert species achieve evaporative cooling
B. The Alpine Tundra
1. Cool, fairly humid, gusty air combines with solar radiation
levels that can be extremely high. Evaporative cooling is of
no advantage.
2. Alpine plants are small or finely divided leaves, grow in a
layer about 10 cm above the ground, wind velocities are
reduced. Cushion or rosette form results in a thick boundary
layer that is determined by the entire plant and nearby soil.
High leaf temperatures indicate that transpiration does not
provide much cooling under these conditions.
C. Effects of Wind
1. When radiation intensity is low the leaf resistance is low.
Transpiration is increased by wind. If leaf temperature is
below air temperature, increasing wind velocity will increase
transpiration
2. Transpiration can be decreased by wind when the radiation-heat
load is high, particularly if leaf resistance is high. Leaf
temperature can be above the air temperature, which causes a
high transpiration rate if stomates are open. Wind cools the
leaf by convection, which reduces transpiration.