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H2O vapor
product of
photosynthesis
(sucrose)
H2O vapor
H2O vapor
H2O
mineral ions
H2O
Fig. 11-1, p. 164
Symplastic and apoplastic flow through roots
root
hair
plasmodesma
xylem
symplastic flow
apoplastic flow
cell wall
cytoplasm
epidermis
symplast
of endodermis
cortex
Casparian strip
of endodermis
stele
Fig. 11-7, p. 169
Control of Water Flow
• Environmental factors affecting rate of
transpiration
– Temperature
– Relative humidity of bulk air
– Wind speed
Control of Water Flow
• Transpiration
– Slow at night
– Increases after sun comes up
– Peaks middle of day
– Decreases to night level over afternoon
• Rate of transpiration directly related to
intensity of light on leaves
LIGHT
Events leading to
the opening of a
stoma:
The production of
malate and the
influx of K+ and Clpowered by the
electrical and pH
gradients produced
by the proton pump
increase the
concentration of
osmotically active
solutes in the guard
cells. As a result,
water flows into the
cells by osmosis.
starch
malic acid
malate–
plasma
membrane
ATP
H+
ADP
+ Pi
proton
pump

H+
+
K+
CI
H+
K+
CI
Fig. 11-8a, p. 170
cells connected
cellulose microfibrils
(radial micellation)
reinforced inner wall
How radial micellation
and reinforcement of
guard cell walls force an
expanding cell to bow
outward.
With increased
pressure, cell gets
longer. Because the
outer wall can
expand more readily,
cell bows
outward.
Fig. 11-9a, p. 170
Fig. 11-9b, p. 170
MINERAL UPTAKE AND
TRANSPORT
H2O vapor
product of
photosynthesis
(sucrose)
H2O vapor
H2O vapor
H2O
mineral ions
H2O
Fig. 11-1, p. 164
-P
-K
Effects of suboptimal concentrations of mineral elements on plant growth
-N
-S
- Mg
Fig. 11-10 (a-f), p. 173
Needed in large amounts
Needed in small amounts
Table 11-1, p. 171
Soil Formation
atmospheric gases:
CO2
SO2
N2O5
rock
acids:
H2CO3
H2SO3
HNO3
rain
wind and water
erode rocks
and soil
freeze-thaw
produces cracks
roots: crack rocks through
pressure, secrete acid
Fig. 11-11, p. 175
Soil Formation
• Lichens and small plants start to grow on this “soil solution”:
–
Rhizoids and roots enlarge fissures in rocks through turgor pressure and emit respiratory
CO2, which forms H2CO3, and thus more acid……
• Accelerated soil formation leading to invasion of
larger plants species:
– Larger roots and more respiratory CO2 , and so on……
?
Needed in large amounts
Needed in small amounts
Table 11-1, p. 171
Nitrogen Fixation and Symbiosis
• Clover root with root nodules that contain the nitrogen fixing
bacterium Rhizobium.
• Leguminous plants (pea, bean,…) benefit from the nitrogen-fixing
association while supplying the bacterial symbiont with
photosynthetic products (can be up to 20% of total photosynthesis
performed by the plant).
Nitrogen
•
Nitrogen predominantly exists as N2 gas in the atmosphere. Is not directly
available to plants.
•
Nitrogen becomes available after soil bacteria turn it into NH4+ or NO3-. This is
called nitrogen fixation.
•
However, fixed nitrogen is not stably present in soil:
- NH4+ (in equilibrium with NH3) is volatile.
- NO3- is very water soluble and easily leached from the soil.
•
Treatment with fertilizers that contain NH4+ or NO3- is very effective in increasing
crop yields, since it supplements the soil with an invariably scarce mineral
element.
Fertilizer use and food production
•
NH3 in water solution exists as NH4+.
•
NH3 is made industrially by the Haber-Bosch process:
Heat
N2(g) + 3H2(g) --------> 2NH3
pressure
•
H2 is made from light petroleum fractions or natural gas:
700 0C
CH4 + H2O(g) --------> CO(g) + 3H2(g)
•
Energy is needed to make H2 as well as to make NH3from H2 and N2.
Mineral uptake
H2O vapor
product of
photosynthesis
(sucrose)
H2O vapor
H2O vapor
H2O
mineral ions
H2O
Fig. 11-1, p. 164
Maintenance of Mineral
Supply
• All plant cells require minerals
Especially meristematic regions
• Four processes replenish mineral supply
– Bulk flow of water in response to transpiration
– Diffusion
– Active Uptake (requiring ATP)
– Growth
• As root grows, comes in contact with new soil region
and new supply of ions
PASSIVE
ACTIVE
Minerals can passively follow water flow until the
endodermis. From there on, active uptake is needed.
root
hair
plasmodesma
xylem
symplastic flow
apoplastic flow
cell wall
cytoplasm
epidermis
symplast
of endodermis
cortex
Casparian strip
of endodermis
stele
Fig. 11-7, p. 169
Active Uptake of Minerals
Into Root Cells
Fig. 11-12, p. 177
root
hair
After passing the endodermal cell membrane(s), nutrients
move into the vascular system to be transported throughout
the plant.
plasmodesma
xylem
Symplastic flow
Apoplastic flow
cell wall
cytoplasm
epidermis
symplast
of endodermis
cortex
Casparian strip
of endodermis
stele
Fig. 11-7, p. 169
Root pressure is generated by an osmotic pump
After passing the endodermis, mineral
nutrients accumulate in the stele of the
root. The endodermal cells provide the
differentially permeable membrane
needed for osmosis.
•Soil saturated with water
–Water tends to enter root and stele
–Builds up root pressure in xylem
–Forces xylem sap up into shoot
Fig. 11-13a, p. 178
Guttation: water forced out of hydathodes
by root pressure
Guttation on a California poppy leaf
Fig. 11-13b, p. 178
PHLOEM TRANSPORT
Phloem transport
H2O vapor
product of
photosynthesis
(sucrose)
H2O vapor
H2O vapor
H2O
mineral ions
H2O
Fig. 11-1, p. 164
Mechanism of Phloem Transport
high
pressure
low
pressure
sieve tube
sucrose
sucrose
H2O
H2O
sucrose
glucose
sucrose
H2O
glucose H2O
H2O
source
sink
sucrose
CO2 + H2O
parenchyma
H2O
Fig. 11-14, p. 179
parenchyma
Sucrose is actively transported into the sieve tubes at the food source region of the
plant (leaves or storage organs) and removed at the sink regions (regions of growth or
storage). Water follows by osmosis, increasing the hydrostatic pressure in the sieve
tubes at the source region and decreasing the pressure at the sink region. The sievetube contents flow en masse from high(source)- to low(sink)-pressure regions.
Phloem Transport
– Concentration gradient maintained by
• Continual pumping of sucrose at source
• Removal of sucrose at the sink
– Sink or source behavior of cells is controlled
by cell signaling mechanisms (developmental
and hormonal controls, see lectures on
hormone regulation).
– Change in signaling can abruptly switch a cell
or tissue from source to sink behavior.